: Transmission Input Speed Sensor need help??

11-22-04, 05:41 PM
ok my check eng light pops on here and there im getting a code for Transmission Input Speed Sensor.. now i was told that if the wireing harness is touching a plug wire etc it can push up a false code . now the thing is its not a false code cuz its shifting weird but it goes on in the morning and when its wet out i start the car drive it goes on and then i make a turn it goes off im just wondering if it could be a bad wireing harness conection like loose or dirty or the water gets in there ?? can any one help me out befor i pull the tranny out

11-22-04, 09:17 PM
There are two speed sensors on the 4T80E. One is on the trans housing above the right output shaft. That one is the vehicle speed sensor (VSS) and is visible and accessable through the right wheel well. The other is the input speed sensor and is a $20 part located $2000 deep inside the tranny. CAn't help much more than that but it sure wouldn't hurt to see if any harnesses are near the plug wires first.

11-23-04, 03:17 AM
But on the bright side, maybe the sensor itself is ok. Maybe there is a problem with wiring harness or with the PCM itself...



11-23-04, 07:32 PM
well dose any one know where the wireing harness goes inb to the trany is it under the air box hose ?? gotta check it as far as the vss i can replace that and see what happens

11-24-04, 06:27 PM
any one knoiw witch speed sensor is the input speed sensor is that the one on the out side or the inside??? :banghead:

11-24-04, 10:22 PM
any one knoiw witch speed sensor is the input speed sensor is that the one on the out side or the inside??? :banghead:
The input speed sensor is the one inside the trans. The VSS is the one mounted outside on the case above the right output shaft.

11-25-04, 11:44 PM
ok thanks i was just wondering it seems like when the tranys cold it acts up but when its warmed up its fine you think mabe the filter being cloged would have some thing to do with it ?

04-08-05, 09:02 PM
I am noticing the same thing with my '94 Eldorado, which is a low 60,000 mile car, I checked the plug wires and dont think theyre affecting it any more, but I don't think it's the sensor itself. it is too random, I'm thinking harness connections maybe? but I'm not sure where to look.

04-08-05, 09:14 PM
I have been dealing with this sensor issue on the 96 aurora for about a year. When the code is generated the transmission responds by keeping the torque converter clutch from locking at all and raising the shift pressure. The light comes and goes, I had suspected that a waterpump leak was the culprit a while ago since the water leaks down the wires right into the connector.

That seemed to make thing better but is still is not fixed. I have heard that this problem is not uncommon on the early year transmissions. You will not break anything driving with it like this. I actually like the firmer shifts, but dislike the fact that is does not downshift as readily. The gas mileage is affected a smidge too.

I had purchased the shop manuals and a rebuild manual for the transmission and I plan to do the fix myself - if I ever have time. Apparantely you can get the side cover off without taking the powertrain module completely out of the vehicle. You can drop the driver side down enough to get access.

10-02-05, 02:51 PM
I have the same problem with my Eldo 95, my service Engine soon light comes on and goes off, I went to autozone they told me to replace the oxigen sensor and that was gonna take care of the proble but i did not think that was the problem since my codes are not related to it, my codes are P047, P056, P107 and those are related to the VSS, Im gonna replace it, I'll post an answer as soon as I find out if if works.

10-02-05, 06:41 PM
P047 (E047) ................................................. IPC - PCM Data Problem
P056 (E056) ........................................ Transmission Input Speed Sensor
P107 (E107) .............................................. PCM/BCM Data Link Problem

Ken Seliga
12-01-05, 12:20 PM
[Fools rush in where Angels fear to tread] Well guys,I too ,thought that the plug wire touching the harness fixed the dreaded P056 problem, but that was wishful thinking...First I eliminated all the other possiblities (other codes) and then I scrapped the insulation off the two wires at the PCM connecter (Do not cut the wires) and connected my VOM with alligator jumpers to the wires. Vel -cro held the meter to the dash while I drove around monitoring the AC voltage put out by the input speed sensor.The voltage should follow the RPM ( Frequency also) and not drop below 5 volts ac. If your voltage drops below 5 volts, first check the wiring ( the big thick harness behind the engine at the firewall that goes from the firewall above the heater hoses, to the connector below the lower radiator hose. I found my connector full of tranny oil, I cleaned it with electronic spray cleaner and rerouted it higher than the place where it exits the tranny. Alas ,that fixed nothing....Next I cut the two wires that carry the input speed sensor signal and routed two new wires out side the harness. I went around the connector in case the connector was the problem..I now had two Shielded(RG-59) wires going directly from the tranny to the firewall pass thru..The shields were firmly grounded..I drove around and monitored the speed sensor output..The signal still WENT AWAY completely, when the Tranny was HOT and the problem got progressively worse, to the point where it was impossible to ignore.. I used the procedure outlined in this forum and at www.caddyinfo.com (http://www.caddyinfo.com) to replace the sensor..I cut apart the failed sensor and found dis-coloration on the copper windings closest to the pick up end..This is a sure sign that the windings were overheating and probably opening ..NOTE>>>The replacement procedure is NOT for the faint of heart, it is involved , time consuming and requires, an engine hoist , a COMPLETE set of tools , including 1/2 drive with METRIC sockets..I took digital pictures of each step to document for re- assembly and found I needed to refer to the pictures often...The instructions found at this forum are under the PC0717 code error ( 1996 used a different OBD2 code system) but the same sensor was used from 1993 thru 1996..The replacement sensor from Cadillac appears to have been re-designed, so that should tell us something. PS>>An ohmmeter check when the sensor was cold revealed no failure , the sensor read 1420 ohms, but if your sensor reads an open or high resistance .Start by measuring it at the connector. if reads good to the sensor in the tranny, but high resistance at the PCM < make sure you check the wiring for strains , cuts, etc before you tackle changing the sensor...IT IS A B**** JOB NOT TO BE UNDERTAKEN LIGHTLY>>> I Needed my heated garage, Cadillac Manuals , the procedure from the PC0717, and I wrote down each step on a legal pad as I dis-connected an item..If you do the job , be carefull when you have the tranny torn down, ther are two thin Thrust bearings that will fall out ( 1 on each gear between the chain) and have to be held in place for re-assembly ...Read Ray Greulich's procedure....DO NOT USE BEARING GREASE>>> A big thank you to RAY GREULICH >>>GOOD LUCK>..[/quote]

12-07-05, 04:11 PM
my 1995 STS acts up as well and I need help BAD!!! First off she has about 189K on her and I paid $4040.00 exactly for it two years ago, when she had 162K on her, did a tune up and new exhaust when I first got it, was told that I needed new mounts then, never got around to doing, the dogbone style ones up top are showin some wear, My tranny was acting up reving hard into second, and always wanting to slam back down to like 1500rpms right when you let up of the gas, even at interstate speeds, Im pretty sure the tranny needs some work. About a year ago, I had a problem where it wouldnt start at all, I ripped apart the steering column thinkin it was the ignition swith, wasnt that, so I had it towed up to Cadillac where the said the transmission transponder switch was messed up and the car couldnt tell wheather it was in park or drive. That was $450 right there. Now present day im stuck with the same dilema, go to start it and nothing no wanting to start or even a turn over, it just clicks from under the hood, like the something is relieving presure, (have yet to get the recall fixed on the fuel rail) Sometime though about a month ago, it would go in and out, like go on a streak where it would start for like 3 or 4 days and then go on a 2 week streak of not starting, and now its been sitting for atleast 3 weeks on its longest sitting period to date. Im pretty sure but not positive its not the starter, not the alternator, replace the battery today, and Im clueless what it could be. I can pull up trouble codes but most of them arent on the online look-up thing that everyone on here uses. If anyone can share info that will help I would appreciate it. Someone also said something about the ignition coil?? INPUT NEEDED!!

12-08-05, 08:11 AM
Ken S,
Thanks so much for this; you have really detailed it nicely. I was wondering what your symptoms were? Did your signal go away when the trans was hot? I am looking at the PD72 data input and my signal is not there when it is cold and is restored when the car gets warm. Once it is warm I never get the code, i.e. everything is fine.

Ken Seliga
12-22-05, 12:20 PM
51 Magnum..Yes my Pd72 data was always good (monitoring the RPM) when the car was cold and idleing, PD72 data was good when PCM checked for data over 10mph and engine was cold. Data started dis-appearing when Tranny got hot (only happened once from a cold start) first at about 200 miles of driving , then progressively got worse until I couldn't go 10 miles without the P056 code popping up. When it was hot at the end it seemed to pop on when the car was slowing down and turning left. The removed sensor had what appeared to me to be a poorly encapsulated end cap (maybe hot oil could impregnate it) and the windings had discoloration from heat..I have only driven the car about 100 miles ( it is winter here) since the replacement of the input speed sensor but the p056 code hasn't shown up. Since your problem is when the sensor is cold , you should try measuring the resistance when the tranny is cold..You can do this at any spot along the wires, I found it easy to measure at the PCM module connector under the dash board on the passenger side..Just scrape the insulation and connect your VOM and measure ohms on the ohmeter scale. I am too lazy to go down to the garage right now, but I believe the resistance should be between 1100 and 1500 ohms..I don't think you need to worry about dis-connectin the connector at the PCM , the internal circuit has a dropping resistor and is built to handle 100 vollts AC when running. DO NOT LEAVE YOUR VOM ON OHMS AND START THE CAR. VOM should be on AC VOLTS and auto range or 100 voltrange when monitoring input data from the sensor to the PCM> Good Luck

12-23-05, 08:57 PM
Ken, thanks for responding. I have done a lot of investigating and I think this thread will be very helpful to people in the future. My problem is different than yours and it is nice to get it documented.
This is what I have found:
1. I disconnected the C129 connector which is right next to the front side exhaust manifold on the driver side. To my complete amazement I found the connector was completely saturated in ATF. Strange I know, but here are the pics.
Pics are attached.....
So I thought this was it for sure. I cleaned and cleaned and cleaned the connector and finally got it to where I thought there was not more fluid in it. When I used my scope and measured the frequency of the Input Speed Sensor when the car was running I got about 530Hz with about 5.56VAC p/p. This is what I would expect, and also tells me the Input Speed Sensor is good because I did this when the car was cold. Here is the pin out of the connector. It is pins S and V (Thanks to alldatadiy.com, highly recommend this site).
Pics are attached.....
So I put everything back together and looked at PD72 and it was reading 0. AHHHH:banghead:
BUT!!! the data came right back when I accelerated. So it was a definite improvement!
So today I decided to check the PCM end of things and just verify that I was not getting a signal when the car was cold. Meaning the connector was at fault. When I checked the frequency while monitoring PD72 I found that I was getting 530Hz at the PCM. BUT!!! I was only getting about 1.2VAC. So it is the connector!
Next logical step was to look at the connector and see if I could figure out why it is not making a good connection. When I unplugged it I found the connector was completely saturated in ATF again.:banghead: I had only driven the car for about a week from when I cleaned it last.
So this is where I am at right now. I need some help! As near as I can tell the ATF is running down the inside of the wire, i.e. between the insulation and the strands, I know…..weird, but the outside of the wires going into the transmission are dry. What can I do?? I am sure I am losing the signal at the connection because of the ATF in the plug. However, I would be open to other ideas.

Ken Seliga
12-24-05, 10:29 AM
Magnum 51...It i s good to meet another electronics guy,nice troubleshooting on the sensor signal....I thought the same as you that the ATF fluid was trans conducting the signal. I sprayed my connector with circuit cleaner and tugged on the wires to make sure the internal connector crimps were good. I also believed that the fluid was coming out of the tranny and running down the wires and when I dis-assembled the tranny I found that the only seal is around the outside of the harness bundle..So there really isn't any way to stop the flow..The first time I cut all the wire ties and raised the connector so it was higher than the tranny ouput side...This fixed nothing..Then I started trying to remember if Tranny oil was conductive at all..When you get inside the tranny you will find that the input speed sensor is connnected to the main harness with a simple two wire push together connector THAT IS NOT A GAS TIGHT CONNECTION. That means the connector internally is soaked in tranny fluid..so much for conduction theory. During my investigation phase , I cut the 2 wires coming out of the tranny on one side of C129 and cut the wires on the other side of C129 and jumpered my own 2 pin connector in the wires. You might try measuring the resistance of each wire from the c129 connector to the PCM , as you know the resistance had better be less than .1 ohms..I did also run two separate wires out side the harness all the way from the tranny wires to the PCM, which didn't help my problem a bit..But yours sounds different since it only fails when cold ( and easier to fix). My PD72 data tracks RPM at idle(in neutral or park) but goes away when you put the car in gear and slowly rises as car speed increases. I think the PCM only checks for the signal after 10 MPH..SO you shouldn't get any code until you are moving .If you get the code before that you might think about the other signals the PCM requires. Still it sounds like you are on the right track..Don't get too hung up on the ATF conducting theory, if it was true then a lot of folks would have P056 codes...

Ken Seliga
12-24-05, 10:47 AM
Magnum 51 Please let me know what you find the problem...Good luck Ken

02-17-06, 11:10 AM
I need your advice. I have finally got some time to look at the sensor again and have found that the signal at the PCM is 2.2vpp at .550kHz and 5.3vpp at 1.5kHz. My problem all along has been that I lose the signal when it is cold or low RPM and it comes back at warm or higher RPM. So I am thinking of designing a simple Schmitt trigger. I mean I will design a circuit that will take the signal from the sensor and buffer it. The output will be a constant 5vpp. Do you think this would work?


I have read up on the signal and it is supposed to be > 5vpp at the PCM. My only concern is what is the upper limit, 12vpp, 9vpp??? Any thoughts?


Ken Seliga
02-24-06, 10:06 AM
Sorry I took so long to reply, I was rebuilding my Harley clutch again.Old shovelheads never die.Yes I also thought that a simple AC amplifier might solve your problem since yours is happening when cold.First, tho we are talking " apples and oranges" here.All of the manual readings are RMS not P-P..so you definitly have a low voltage signal from the sensor. As an aside to others reading this post, your standard VOM (VOLT-OHM-Meter) as purchased from Radio Shack, Sears,etc measues AC voltage(alternating current) in RMS ( Root mean squared). And you normally don't care, just put the meter on the proper range and AC Volts scale.Almost all AC voltages are given by manuals as standard RMS readings. P-P ( peak to peak ) readings are a special case. That said , Magnum 51, yes an amp might work.My volts go up over a 100 at high RPM, so there must be a limiting resistor built into the PCM..but are sure that the in line resistance of each wire is normal?Did you measure right at the sensor output at the tranny connector and at the firewall? My gut feeling is that something besides the sensor is the problem..High resistance somewhere..We both know that copper wire has a positive temperature coefficent, but to get any significant increase in voltage due to copper wire heating up is doubtful. I don't know how we got off on the P-P measurements, but I am pretty sure the measurements are RMS, at least mine are in this post..I havent looked at your link and can't right now...let me know

02-24-06, 04:31 PM
my car has the same problem, right now it seems to be coming on every 3-4 days. awhile ago, it was once every week. then went to coming on almost every trip. but not its coming on every 3-4 days.

does this intermitent code coming on have anything to do with the problem? like is it not the $2,000 jobbie? i will have the check my plug wires to see if there is anything weird or touching.

stupid question, but by plug wires u mean the wires that go into the spark plugs? cant think of anything else called plug wires.

02-24-06, 05:01 PM
Ken, thanks for responding. Yes, we are talking apples and oranges. When I measure with my hand held o-scope I am getting p-p.

Measuring the sensor and at the PCM give me the same resistance. So I do think my sensor is getting weak. I did build a simple amp circuit and I hooked everything up and IT WORKS!!!! I have driven the car for a week now and no service engine soon light!

However, I am a little confused and still "wet behind the ears" as far as EE is concerned. When I measure the voltage from the sensor at the PCM with it disconnected, i.e. the wires just hanging under the dash but still connected to the sensor. I get about 1.5vac (RMS). But it is ~a sin wave with no DC offset (so the signal goes positive and negative, for you non-geeks). Using a simple LM741 op amp I created an amplifier with +vcc and -vcc. Here is my question. I would like to use the 12vdc from the car instead of the 4AA batteries I am using now. How in the heck can I get the -vcc? I know this is more of a circuits question but it would be really nice to have the sensor input and output and just hook up 12vdc to my amp.

If people are having problems with this and it looks like they are (over 1000 views of this short thread) then a simple circuit that can be made for less than $20 can fix a "$2 part buried $2000 deep" in the transmission.


02-24-06, 05:05 PM
my car has the same problem, right now it seems to be coming on every 3-4 days. awhile ago, it was once every week. then went to coming on almost every trip. but not its coming on every 3-4 days.

does this intermitent code coming on have anything to do with the problem? like is it not the $2,000 jobbie? i will have the check my plug wires to see if there is anything weird or touching.

stupid question, but by plug wires u mean the wires that go into the spark plugs? cant think of anything else called plug wires.

The "spark" plug wire thing is a good thought but I do not really think that is the problem for a lot of people. Can you describe your problem, i.e. does it come on when the car is hot or cold?

02-25-06, 08:29 AM
Ive been following this thread in my constant search for info(always require input for my thirst for knowledge thing.)
I think that tricking the input speed sensor is a temp fix.
I know that the input vs output is constantly monitored and PCM will freak if not within specs.
Heres how I discovered this.
I changed tire size on my Rora to a shorter tire and wanted to "correct"
the speedo.
I got a universal speed recalibrator that will dial in your ratio up or down to 1000%.
Since mine was only about 7% off I set the dip switches up for that.
It seemed to correct it fine and everything was cool until I tried a couple of WOT passes.
The converter flashed and engine overrevved like it was in neutral.
I had a made a custom harness with plug in connectors so I got under hood,disconnected the VSS to recalibrator,reconnected the VSS and tried it again.
Trans worked fine with only result being it was off in speed again(due to tire size).
I contacted the manufacturer as I had bought a few of these to use on various cars,and they were at a loss.
Their only suggestion was to find the speedo pulse wire inside the dash and go inline with that. Manufacturer says this is the first time they havent been sucessful modifiying the signal at VSS.
So my conclusion was that the input vs output sensor signals are closely monitored by PCM and even a 7% variance will freak it out.

Ken Seliga
02-25-06, 09:43 AM
Good on you magnum 51..I hope that your fix continues to work..There used to be a series of small blue books called something like a compilation of simple and usefull circuits. They had a series of DC to DC converter circuits that step down the voltage to adj 5 to 9volts. LM741's don't draw very much current so you could burn off the excessive voltage with a dropping resistor calculated to keep the in spec vcc from the alternator 12 volts to 15 volts. You might want to put a small cap n the vcc line to absorb start up surges. I'm guessing based on your findings that PCM relies more on Frequency than on A-D conversion . When I measured my good sensor output (with it still connected to the PCM) it started at 400 hz and 5.6 volts AC at 700 to 900 RPM and increased in Freq and Amplitude as the car speed increased...Sooo maybe amplitude isn't as important as Freqency...You might want to search this forum for my saga of finally changing the input spped sensor//I believe the search should be on"p056" ..for OBD1 and PC 0717 or whatever the equivalent code is for OBD2..I hope you never have to change the sensor cause it wasn't a real fun job..keep me informed of how this fix of yours is doing..maybe you can save a lot of folks some trouble if your buffing amp idea continues to work...

02-25-06, 03:21 PM
doesnt that input sensor have a replacement thats an improved version?

02-27-06, 09:10 PM
I would think that the input speed sensor would have a new design. However, I am not sure.

As far as the buffered amp goes I have driven over 250 miles under stop and go conditions. I live about 20 miles from work and have had the car cold and then warm up 10 to 15 times. So I am pretty confident that the amp works. I would like to compare with other people on the rpm. I have driven it at 65 and I am at about 2100 rpm. Is this where everyone else is at?

I really think the PCM is just a high speed counter input (so to speak). It is only looking at frequency and not amplitude. Of course it has a low level discriminator that is the problem. When the signal gets week the sensor is not putting out enough voltage for the PCM to see it. So the amp just makes the signal have more amplitue but does not affect frequency.

Ken Seliga
02-28-06, 10:08 AM
Magnum 51 . ..I just have time for a quick thought..I didn't make myself clear in my last reply..The Sine wave you observed is the result of the each gear tooth approaching and retreating from the sensor, changing its reluctance and creating the sine wave..since the PCM is biased with +12 dc only, there is good reason to suspect that the negative portion of the sine wave is clipped or re-clamped internally and not used in the calcultion of speed..Therefore you may not need an amp that swings thru zero (gnd)..Try rebiasing your amp to just swing positive and see if that works...2100 rpm at 65 sounds about correct..The TCC kicks in around 50 and you should see an RPM drop of a couple of hundred when it kicks in..Sounds to me like you are on the right track....good job..

03-01-06, 11:37 AM
I have the same problem with my '94 Eldorado. The light(P056) comes and goes. The weird thing about it, which is making me think it's not the sensor, is that when I reset the PCM codes, the light goes away and the car shifts fine!:rolleyes: Could it be the PCM? The same thing seemed to happen with an A047 code(low refrigirant), after I reset the codes the AC started working again and the current situation went away. Does anybody else have similar situation? Would a bad PCM cause this to happen?

03-01-06, 06:53 PM
sometimes i think its my PCM that is bad too. stupid computers :P

03-02-06, 10:21 PM
Does your transmission shift hard/jerks when the light is on, shifting from first to second? That's what mine does when the code comes on. Still, when I reset the PCM codes it goes away. Magnum 51 did your car do what I described with the code. I think you posting your solution on how to build a circuit may save folks some big bucks and lots of worries. :thumbsup: Does anybody have any input if the PCM(the computer) could cause this, if so how do you troubleshoot it? Where's the best place to get one if it's bad?

03-03-06, 11:46 AM
eurumbaev, the answer is kind of. My trans would shift hard. However, my light would come on and go off by itself. What year is yours? Maybe they changed the code to turn off on the key cycle on your year.

Troubleshooting this code is really pretty simple. It takes a Digital Volt Meter (DVM) to measure the voltage at the input pins on the PCM. You can back probe the sensor leads and read the AC (RMS..... for you Ken) voltage. This should be above 5VAC, this comes from GM. Now, to tell if it is your sensor or the PCM, might be a little more difficult. You would have to cut the wires and measure just the sensor when it is not hooked up to the PCM.

Another thing to check it the PD072 data input. This is the data from the input speed sensor. Mine would be 0 rpm when the car was cold and would show actual rpm when it was warm (read above).

Let me know what you find. I would be happy to put an amp together for you to try. Mine is working perfectly. I am going to try and make the circuit a little better but it really has fixed my problem.

03-04-06, 11:02 PM
Thanks for the reply, Magnum 51. I have a '94 Eldorado ETC. I don't know if I'll be able to do the testing you suggested on my own. I'll have to have my friend help me with that sometime. Now from what I understand, your simptom was that when the car was cold, the light would be on, but when the car warmed up, the light went off? Well, mine seems to be the opposite, when the car gets warm it sometimes comes on. It's strange, 'cause sometimes I can take really long trips and the light would never come on. I really don't want to change the sensor!

03-08-06, 12:30 PM
Hi, new to the forum, new Caddy owner. Just bought a 1995 Eldo with 50K miles. Guess what code it has??? P056. Mine is also intermittent. Only throws the SES light when hot and not all the time, sometimes goes off while driving. Trans shifts fine except one time shifted hard to second, then the light went off. Any truth to the plug wire thing, and would that cause a hard shift if it's a false code?

03-08-06, 02:44 PM
Welcome to the forum. Some of the smartest Cadillac people are here. :worship:

I really do not think there is any truth to the plug wire thing. The shifting thing is strange to me. Mine did it every once in a while also. But I was able to fix mine with the little amplifier. However, my symptoms were different. On when cold and Off when hot. You can look at the PD072 parameter and see what the reading is when the car is showing the P056 code.

I would really like anyone’s help who is willing to throw a DVM on the wires going to the PCM. That is, anyone with the symptom of - Off when cold On when hot. I would like to know what the sensor is doing. I would suspect it is breaking down with heat but it would be nice to see exactly what the voltage and frequency is. If the sensor is just getting weak then an amplifier would work. However, if the sensor is shorting out or something like that then it might be the $2 part $2000 deep scenario.

03-10-06, 03:51 PM
Can you provide precise instructions on how to do this? I'd be willing to do it sometime when I get a chance. You can email me at eldar_moscow@hotmail.com, if you like, to give me the directions and we can disscuss this if I have any problems.

03-14-06, 12:41 PM
Any updates on this problem? Mine is still intermittent. But only happens when hot. When the SES light is on, I get firm shifts. Today when I got home the light was still on so I checked PD72 and it read 0. I never get the light on a cold engine only after it's fully warmed up.Then again some days the light won't come on at all.

03-14-06, 12:55 PM
Hey guys, I've been reading up on this post somewhat here and there...my query is the same of course, yet I can't come to a conclusion as to when the sensor acts this way (i.e. conditions at that given time)...I would like to know how to try you're method Magnum...I just thought that right now, that was too much to read haha

03-14-06, 01:04 PM
OK, update on mine. When I got home before and the light was on I checked PD72 and it was 0,the engine temp was 199 deg. I let the car sit awile and checked it again at 175 deg engine temp and PD72 was 650-700 and no light. It seems to be temp related on mine.

03-20-06, 08:15 AM
P056 – Transaxle Input Speed Sensor ($2 part buried $2000 deep)

Fault Conditions:
1. Service Engine Soon Light comes on when car is cold and goes off when car warms up.
2. Service Engine Soon Light comes on when car is warm and goes off when car cools down.

To test the fault condition look at pins 1B3(Red/Blk) and 1B4(Dk Blu/Wht) on the PCM Red Connector; located under the dash on the passenger side. Set the digital Volt Meter to read AC voltage and connect it to the pins (does not matter which probe is on which pin). Leave the connector attached to the PCM and just back probe the connector. Of course, this should be done when the code is present and the car running. For your own curiosity also check when the code is not present and notice the difference.

Testing Results:
1. If you read less than 5VAC but greater than 0VAC then the sensor signal is getting weak.
2. If you read 0 volts then your sensor is shorting out and you will have to replace it.

1. You can build a simple amplifier circuit for less than $20.
2. You will need to follow the procedure outlined in the “Cadillac Tech Tips” under “Input Speed Sensor (P0717) Replacement”- Thanks Ray Greulich!

Additional Testing:
1. Verify that when the code is set that PD72 is reading 0 and when it is not set it reads approximately the current RPM.
2. Disconnect connector C129 and measure the frequency between terminals V and S. It should be greater than 0 Hz. This sounds easy to do but is actually a real pain because of where the sensor is located.

My Results:
Fault Condition = 1.
Test Results = 1 ~ 2.5VAC.
Fixes = Simple amplifier circuit has been working for almost a month (1000 miles) with no fault code.

I hope this helps summarize this post and gets people to do some checking before spending the $$$$ to replace the sensor.

03-21-06, 10:24 AM
Thanks for your input Magnum. My 2000 STS just started doing this three days ago. It only has 88,000 miles. In mine after a long drive the transmission is revving so high that when you put it in drive or reverse, it lunges so hard that I know something else will break if I don't get this fixed

03-21-06, 05:09 PM
Do you mean it revs really high when u are in park? Like when its idling?

03-21-06, 05:29 PM
That sounds liek it could be a bad Throttle Position Sensor. The transmission Input Speed sensor would cause a high idle, but a bad TPS would. Mine went on be abotu 10k miles ago. The wiper arm would get hung up on a loose contact inside the the sensor and it would basically stick at around 1/4 throttle. Eventually, it woul throw a codes, and just run off the MAP Sensor along with some other various sensors (can't remember which ones).

If you want to test it, it's pretty easy if you can get your hands on a digital multimeter. You basically just hood the two lead up to the ground and Signal return wires (i.e. backprobe with two paperclips,..ect). The move the Throttle all the way from idle, to WOT looking to see if the voltage drops to 0 or becomes irratic at any point. It shoudd range anywhere between .45v at idle, to around 4.95v at WOT give or take a millivolt or two. This is a tell tell sign of a bad TPS.

03-24-06, 01:54 PM
I think I know what he means, by "reving high", since my car has the input speed sensor problem. My car does the same thing when the check engine light comes on with the P056 code. The transmission shifts pretty hard(drive and reverse), I don't think the revs go up necessarily, but it just shifts really hard. Once the light is off, though, it shifts as smooth as can be. It also shifts very rough from first to second, when code is present.

03-25-06, 09:56 AM
eurumbaev has my problem down exactly. Is this a expensive fix. It only does it after a long hard freeway drive.

03-25-06, 03:27 PM
This problem really affects performance in my car. The tranny and gears don't respond as well if I decide to WOT from a roll. They act very very sluggish n slow. looks like I have both symptoms. it can be on if its cold and can be on when its hot. its too random to tell...


Ok. Just by observation I've noticed two things. Ok, when we talk about the code coming on when "hot" or "cold", are we talking about the temperature outside? OR the temperature of the engine, underhood temp...etc?

For the temperature outside scenario...

My car has had the code when it has been around 40 degrees F outside when I turned it on...

MY car has also had the code when its been from 80-95 degrees F outside.

For the underhood/engine temp scenario...

The car has had the code when ir has not been used for a while and the engine temp is neutral so to speak, not entirely hot and not cold...

My car has had the code when the underhood/engine temp has been warm...

Can I come to a conclusion??

03-25-06, 10:14 PM
When I talk temperature I am talking just engine temperature not ambient temperature.

If I read your response correctly you say the code is present when the car is cold (engine temp) and warm? If that is the case does the code ever go away? If it does then we have a new category!

Can someone look at the PCM connectors when the code is present? I would be willing to make an amplifier circuit for whomever to try and fix the code in another car. Like I said before, over 1000 miles and no code or hard shifts from 2nd to 3rd.

03-26-06, 12:50 AM
51Magnum, I would greatly appreciate if you can make an amplifier circuit for my car please!! The weird thing is that the other day i FOUND a book and I flipped it open and the first thing I flipped it to said "Amplifier Circuits". I was like yes!! I need this, so I was trying to read and learn all this by myself, but I would not mind any help.

I just want to know if the procedure for installing the amp circuit different for the newer OBD-2 cars? I see you have a 94 which is OBD1...

What I meant is that no matter what engine temp the car has, the code is very random, and YES does go away sometimes.

I've started the car when its been sitting there not used (where engine temp. is pretty cool) in situations where its been both cool and warm outside and the code is still there. In my situation, the code has NEVER EVER turned off when I drive. Once, I turned on the car in the morning to go to school (first time using the car that day)and the code came on, and actually went off 1-2 min after sitting there in the car.

So for my car, engine temp does not matter most of the time.

Ken Seliga
03-30-06, 07:34 AM
I do believe that the PCM only checks for the input speed sensor signal when the car is moving over 10MPH..if your code comes on with the car just sitting there...you may have a PCM, wiring or other signal problem...Magnum 51 thats good news that your fix is still working...

03-30-06, 07:52 AM
Nice to see you are still alive Ken........

SL1CK, I will look at my amplifier this weekend and make sure I remember exactly what gain I used. I will post the schematic for all to use. Can you back probe the PCM connector and tell me what voltage you are seeing with the code present?

03-30-06, 08:04 AM
Ok, I have noticed a pattern in the behavior of my P056 code. The light comes on after driving for a while, whether it's temp related or not I don't know. But it does seem to come on when I'm comming to a stop from driving a while and stays on unless I clear PCM codes(sometimes doesn't help). But! When I accelorate really fast:thumbsup:, the light will go out and not come back on untill I come to a stop(not always though, either, sometimes it will stay off a long time.) But one thing that does make the light go out every time is punching it! What would cause that? I thought of a couple of things. Please let me know if this makes sense at all, or it's outlandish: 1)maybe the hot trany fluid is pushed back from the speed sensor when accelerate. 2)there's a voltage boost with acceleration which gives the sensor the needed 5 volts that 51 Magnum got fixed(you go dude!). 3)acceleration causes some wiring to be moved back. Does anyone else experience this behavior?

03-30-06, 10:42 AM
Actually if my light is off and I nail it it will come on. It also only comes on after being at operating temp. Sometimes right away other days I can go on a 40 mile round trip and no light. Another thing I tried was shifting from OD to third and back. The light went off and came back on.

04-01-06, 09:34 AM
Ok I finally got time to get everything.

I have attached 5 pictures. Two show the actual sensor in the car and the other three shows the schematic and the wave forms of a good sensor and the "fixed" sensor. You can buy the parts at radio shack any day of the week. You will need the following components:

1 UA741 op amp
1 circuit board
1 1k ohm resistor (Brown, Black, Red)
1 100k ohm resistor (Brown, black, Yellow)
2 battery holders
2 wire connectors

All of this should be less than $15. If you want me to build it for you I can.

SL1CK and eurumbaev, I really think this will fix yours. The random pattern you have sounded like mine.

clarkz71, I am not sure what is causing yours, but I would sure try this if I were you.

04-01-06, 11:28 AM
Thank you so much Magnum. I'd rather spend 15 bucks first before I ever spend 2000$ on the repair. THANKS!!!

04-01-06, 04:32 PM
I had a Cadillac dealer change my sensor for $1000 after I told them about dropping down just one side of the trans. They were going to take the trans out of the car before I told them what Ray G. did.

Just another option for you guys.

04-02-06, 03:08 PM
Magnum, I have one more question. Since you have a 94 STS, which is OBD-I, and I have a 96STS, which is OBD-II, will this change/affect anything? Is the process still the same. I looked under the passenger side dash and I see that there are wires running under there, so I'm just going to take off the cover and see for myself.

04-03-06, 07:16 PM
Just repaced a PCM on a 95 rora that had tranny acting up fiercely.
Put the old PCM in another car and tranny acted like a spoiled child.
Car shifts fine with a donor PCM.
If anyone has any spare 95 PCMs or memcals LMK I know of someone
on Aurora Club board that may need a turbine sensor spoof also.

04-29-06, 08:56 PM
anyone know if the input speed sensor was upgraded in 97?

04-30-06, 10:42 PM
I have noticed a pattern recently. The light only come on when I turn, which I think means that a wiring harness may be triggering the code when and only when I turn. And its more like, firm turns, not hard at all, just firm.

05-01-06, 08:02 AM
Could be a harness problem.

I started driving to work on Thursday and sure enough the dang ses light came on; checked the code and it was the P056. I had the sinking feeling again because the hard shifts were back and I got the P039 code after driving on the freeway for awhile. So I thought either my sensor finally went or something else happened. I then remembered that I never hooked my amplifier circuit up to the 12vdc system of the car (still using AA batteries)Well, I put new batteries in and problem fixed again. Just thought I would post an update.

05-02-06, 07:05 AM
I first misread your post on the amp.
I was thinking you had no signal and was trying to fabricate one.
Now I see you had a weak signal:thats a completely differnt animal.
Still looking for someone to explain what the differences are between
early 96-5 and later ones.

05-02-06, 11:43 AM
51magnum, I'm interested in your amp for my Eldo. I can't PM you, not enough posts. Can you e-mail me to discuss this? clarkz28racer@msn.com

05-29-06, 07:28 PM

About a month ago I got the dreaded P056 code. It's been okay since the transaxle electrical connector was reseated, but I feel a need to prepare for the worst.

Is there some reason I couldn't build a similar circuit, but AC coupled with a single ended vehicle-battery-derived power supply?

I'm thinking about constructing a "potted" module with appropriately color coded wires and asking my auto technician to do an under-hood installation near the transaxle connector. A little cutting, a little splicing . . . .


05-31-06, 10:26 AM
I guess making the circuit AC coupled would work. However, I do not remember seeing a DC component when I looked at it on my scope. The big thing that needs to be done is doing a vehicle powered amplifier. I have not had time to mess with it but right now how the circuit works is that you have a + and - power supply to generate the negative signal.

The PCM might be looking only at the positive signal and frequency however I do not know for sure. That is why I tried to just amplify the original signal.

If you have a better idea PLEASE share it. I have made a board for Clarkz71 and will find out if it fixes his problem soon.

The board is shipping to you today clarkz71!

05-31-06, 11:15 AM
Thanks 51magnum. I'm looking foward to getting rid of the SES light. I'll install the amp and test for a week. I'll post the results after I'm sure it's fixed. A week will be more than enough as I've never gone more than a day without the light coming on.

06-19-06, 08:06 PM
I too have developed the "dreaded PO56 " code , I was going to try the op amp trick, but in checking the wiring, connector C-129, and the sensor,
I have found that the wires are ok , the connector is ok except for the fluid
in it, but the sensor is open, checked with an ohmmeter.

I am going to try to repair this myself, and read on the forum that someone
indicated that an improved version of the sensor is availible, would anyone
have the part # for this item.

Thanks, Dave

06-20-06, 05:56 PM
You will need to remove the tranny to get to it.
Not an easy job.
Have you dropped the pan to check wiring?

06-20-06, 07:10 PM
I know its a big job, I did put an O'Scope at the pcm connections and sometimes got a good signal and sometimes a weak signal, the wiring from the pcm to c-129 is good but I can not read the 1400-1500 ohms across
terminals V & S on the trans. side of c-129 , in fact I dont read anything.

I wasn't aware that the input speed sensor could be accessed from the pan.

I have a printout from caddyinfo.com that pretty well explains the procedure
and am going to try it if all else fails.

09-17-12, 08:12 PM
Thanks to everyone for all your help and feedback with this issue. I saved my transmission and $2000 because of this information.
Here's a video to clear the P056 code - good luck!


09-18-12, 12:21 PM
Man I wish I was that lucky, it being an interference issue.

FWIW, I had the P0717 code on a 1996 DeVille, which is Transaxle Input Speed Sensor Volts low. Ligth would come one after car warmed up completely, hard shifts, reluctance to downshift, and when it downshifted it was really noticeable.

Took it in to AAMCO in Cheyenne, WY at the end of August. First inspection found that, like other posters in the thread, the Harness plug outside of the transmission was soaked in ATF, and the shop said it was pouring out ATF fron the plug. Originally estimated $1300 to fix, but the harness ended up being $400, a couple other things needed replacing, and labor was slightly longer than what was expected.

$2200 bucks and I was back on the road. Good thing is, all normal function returned, and I got exceptional mileage on the way back :D

FWIW, when inspecting my transmission when they had it out, they said everything looked fine, and there was no signs of abnormal wear (i.e. there weren't any shavings in the fluid, fluid looked good).

My 2 cents, my experience.

09-24-12, 06:46 PM
Can the input sensor be done with the Trans still in the car while repairing the TCC Solenoid? Are they both under the side cover? Thanks for the info..

09-24-12, 08:52 PM
Both under the side cover. Not sure if it can be done, but it's real close to the TCC solenoid.

09-25-12, 05:21 AM
I don't think ISS replacement can be done in the car. It's well buried under the case cover, next to the drive chain. You'll have to remove the side cover, oil pumps, upper valve body and the case cover to reach to the ISS.

Make sure to check everything else before tackling this job though, see if your 20pin harness is contaminated, use electrical contact cleaner.


09-25-12, 09:50 AM
The ISS cannot be replaced with the transmission in the car. I just watched my neighbor, Charlie The Transmission Man, do one a week ago.

09-25-12, 01:16 PM
There was some guy over at Caddy info who DID do it with the cradle still in the car, albeit tilted.



:doh: If only I had a shop, I would have DONE IT MYSELF. One day, I will have my epic shop. One day. :D

09-25-12, 02:32 PM
I remember that thread. Unfortunately he lost a HG not too long after that and sent it off to the junkyard.

09-25-12, 07:32 PM
:helpless: Shoot, if he put that much effort into fixing it, why did he not just do the HGs?

09-25-12, 08:50 PM
The safest route would've been to remove the drivetrain, repair the block with inserts/studs, (repair the halfcase leak if needed), replace TCC solenoid and seals, replace ISS, and inspect for third clutch housing support bushing wear.

Weird though, it seems like 95-96 model year vehicles are mostly affected of this P056/PC0717/P0717 issue.

09-25-12, 09:39 PM
:helpless: Shoot, if he put that much effort into fixing it, why did he not just do the HGs?
He was pretty knowledgeable and a competent mechanic, but I don't think he wanted to get into that big of a job. I don't blame him. That trans job was big enough.

09-25-12, 09:40 PM
Weird though, it seems like 95-96 model year vehicles are mostly affected of this P056/PC0717/P0717 issue.
Yeah, just like the P0741 on the 2000-2005s.

09-25-12, 09:51 PM
I need to inject that Charlie used the GM engine/drivetrain hanger jig and, with the car up on a 4-point hoist, lowered the drivetrain enough to entirely disassemble the driver's side drive components and get to the transmission to move it away from the engine................ it was a 3 day job, and I kick myself for not taking pics........... there WILL be a next time.......... there are at least 5 dozen FWD Seville/Deville/Eldorado cars within 20 miles of here, and Charlie is one hell of a good transmission man. He'll get one if it goes belly up.