[81 BENY Biarritz]

Hi Peter, here is a picture of the one in my Eldo. Seville is dirtier but the same.

As for the Silver troubleshooting manual, I have found many mistakes in it before so I wouldn't be surprised at anything.

I also have a specific Cadillac Climate Control service training book so if you need anything checked in it, let me know.

Dave

 

[JayArr]

I checked my 1982 manual and it's the same.

Weird.

 

[Lucky Peter]

Hello,
I've just swapped the wires to their original position and everything is working just like it was before. I tried to use a chart to diagnose the blower motor, but since the wires and pins are not matching the manual it's a little bit more complicated. The chart says that there is either a short or an open wire, but after checking it out there isn't any. I decided to check it my way, so now I know that the 13V and 8V wires have a correct voltage, and the blower drive wire is correctly changing voltage when different mode is selected, so I guess that the power module have to be defective, but I think that I won't be able to test it out, so I will need to find a new one (remanufaftured I guess) in some time.

Another interesting thing is that I replaced the ISC motor and it's still not working. It looks like the ECM is not turning it on, because it's in some failsoft mode or something like that, so there is a chance that ISC was good, but now it doesn't matter.

The new ECM should be delivered this week, but I'm still a little bit worried about getting it damaged because of something in my car... I don't know if there is anything else left to check, definitely the MDDA needs to be unplugged because it is probably defective, but is there something else worth doing before installing the new ECM?

Dave, could You take a look at Your Cadillac Climate Control manual to see if there is anything related to my problem? I will be grateful.

- Peter

 

[81 BENY Biarritz]

Hi Peter, I was rechecking the list you made of the readings for the solenoids that are controlled by the ECM and the "Critical Overload Circuits" from the "1981-87 DFI, Cadillac Digital Fuel Injection" manual and I see that you missed the Fuel Pump Relay circuit. It drives the fuel pump relay coil (which must be working on your car) but it probably should be checked for resistance just in case.
Also something that I noticed is that the ECM will not control the ISC if it detects that the fuel pump feedback voltage is less than 10V or greater than 16V. It is monitored on circuit 120 on the P1 ECM connector. In the "1981 Digital Fuel Injection - Cadillac Service Training" manual that I sent you it indicates that if the ECM detects a fuel pump voltage less than approximately 10V it will actually boost the injector pulse rate to compensate for what it thinks is reduced fuel pump output. I think it is highly unlikely that this might explain your running rich issue but it might be worth checking?
Finally, I was thinking about your Coolant light turning on and then dying with the new ECM. I had a look at the Black Book on Page A-128 and it doesn't seem to treat it as a significant failure item but does confirm that a bad ECM will cause the issue, however it might be interesting to disconnect the ECM and test the bulb circuit with your test meter set for currently draw. I haven't read anywhere that a bulb circuit can damage the ECM but checking it would make sure that circuit is OK and the bulb lights again for you.
As for the power module, as you probably already know they are almost impossible to find NOS and people that repair them want significant money for them. From what I have read it is usually a defective blower motor than damages them by putting a high current load through the grounding side (which is what the module is actually controlling) when the motor burns out. I have tested the current load on my blower by removing the large tan coloured wire from the power module and putting a meter set to current draw between it and the battery ground post. Something you should try before installing a new one.
Have you tested the new ISC motor like the guy in the Cadillac video did to make sure you have a good one. Just be very careful about the wiring connections though as you can damage it by putting power to the wrong pins as he mentions.
Hope this helps.

Dave

 

[Lucky Peter]

Hello Dave,
It turns out that I probably forgot about the fuel pump relay circuit. I'll try to check it tomorrow, but I don't exactly know where to check this circuit resistance - I suppose that I need to check it on a blue ECM connector certain pins (relay drive and relay return) but I'm not sure about that.

Today I only managed to check the fuel pump feedback voltage while turning the ignition and it looks like to voltage goes up to 12V while I can hear it running so that's a good sign. Tomorrow I'll try to check the feedback voltage when the engine will be running to see what the values will be.

As for the coolant light, I don't think that there is a problem in this circuit, but it is a good idea to check it.

I tested the blower motor just as You've said and the current drain is about 15 amps. Also I removed the power module to check its resistance, so is there anybody that knows what it should be? I've measured the resistance of this longitudinal element and the round one on the bottom, but I don't remember what resistance was there. I think that the problem is in the power module, but maybe there are some procedures to verify it.

And lastly - Yes Dave, I've tested the new ISC motor before mounting it and it's working fine, I would even say that it sounds better than the old one, so it should be working seamlessly.

So that's it from me for today. Thank You for all ideas and suggestions, I'll be waiting for Your replies. Next tests are probably coming tomorrow.

- Peter

 

[81 BENY Biarritz]

Hi Peter, I was very curious to see if the ALDL connector could put the ECM into diagnostic mode so I got a jumper and with the key in the Run position (car not running) I first tested the diagnostics via the ECC and found that to get it to go to ".." you have to hold the Off and Warmer buttons about 3 secs and then once ".." displays it then very quickly goes to "88" and then after a brief pause to "70" (my car doesn't have any codes to display at the moment). I then reset everything and tried a jumper wire between the two terminals closest to the firewall (single black wire on rearmost terminal and two blacks on the next one) which I thought should ground out the diagnostics request line and put the ECM into diagnostics. As you have found out it did not. This didn't seem right to me so I tried grounding the second terminal (with the two blacks) to something close by under the dash, it did nothing also but I probably didn't actually have a good ground as my jumper was too short and I couldn't find anything with bare metal. As a final test I put my multimeter on the same terminal (with the two blacks) and found that with the ignition on I was getting 5.1 V. !!! I then tried the ECC Off and Warmer buttons and after about 3 secs (time it takes to generate the ".." display) the voltage dropped to 0.29V and stayed there until I touched any other button on the ECC (which cancels diagnostics mode) and then the voltage returned to 5.1 V. I didn't want to spend any more time with it but I think that grounding out that second terminal in the ALDL connector should be able to get the car into diagnostics mode. I will get a proper (long enough) jumper wire that I can connect to a good ground connection and test it again maybe this evening or if not then it will be early next week as we will be going away tomorrow for the weekend.
Thought that you would be interested in this so that you can check it on your car to see if the ECC is actually pulling the diagnostic request line down to almost 0 V. If it is then maybe you are only missing the digital (data) display and if the MPG buttons are working you could try getting to the Output Cycling function (see page A-8, A-9 in the Black Book) with your old ECM first to see if it might be able to test all the output devices before installing the new ECM. Just remember that you will need to wait for maybe a minute or a bit more once it enters diagnostic mode to get to Code 70. Just some thoughts for you.
As for the power module it is a frequent failure item on these cars so maybe try doing a Forum search to see if anyone has done any testing or repairs themselves. I don't know if checking resistances on a good unit will tell you much but I do have a NOS unit somewhere in one of my boxes. If the guy with the Seville in Calgary still has the one on his car maybe you could offer him a low price and take a chance that it still is OK, however I suspect he knows the value of a good one as he seems to be a bit of a collector also. I seem to remember that the current on mine was also pretty high (I thought) so probably your blower is OK.
One thing that I had suggested that was incorrect is that the 8V for the ECC control might come from the power module. In reading a bit about the Programmer it appears that it is the source of the 8V control voltage and sends it out to the power module and the ECC unit. FYI.

Dave

 

[1979 Eldorado]

Just one comment... The prior generation had the fuel pump relay inside what was then called the ECU. As cars got old, connections became suspect the fuel pump circuit could require more amperage. Bruce Roe told me once that many of the ECUs he gets have signs of burned fuel pump pins on the ECU (I believe he mentioned near 50%). To avoid that failure mode he created a wiring diagram to use the ECU to trigger an external relay. In doing that the high amperage never needs to go through the ECU (only the amperage for the trigger signal so next to nothing).

Fast forward to your 1981's. In reading this topic it sounds like you have the power for the cylinder deactivation solenoids as well as the fuel pumps going through the ECM. If you were to move the relays to external relays I suspect you could install a dedicated ECM fuse which is drastically lower amperage.

In the prior system, 76-79, they used a 20Amp fuse for the fuel pump circuit but that was the only fuse. The Fast Idle Valve, as an example, only uses about 1.1 to 1.2 Amps. My point is that if you have a 20Amp fuse for what is primarily low current circuitry you still have protection but as soon as something shorts to ground you have 20Amps racing around trying to find a ground. That's a formula for disaster because anything which cannot handle 20Amps, and on the path to the closest ground, will immediately be exposed to 20Amps.

 

[JayArr]

OK, Electronics 101...

My point is that if you have a 20Amp fuse for what is primarily low current circuitry you still have protection but as soon as something shorts to ground you have 20Amps racing around trying to find a ground. That's a formula for disaster because anything which cannot handle 20Amps, and on the path to the closest ground, will immediately be exposed to 20Amps.

That's not how electricity works.

A component or wire that shorts to ground becomes the path and the current goes through it ignoring all other paths to ground. Electrons always follow the path of least resistance and you can't get less than a short.

A short does not leave 20A racing around trying to find a ground, a short is a path to ground. If there is a short to ground all of the current will flow to ground through the short.

Fuses protect wiring not devices. They are there so that if a relay/solenoid/pump shorts to ground then the fuse blows before the wire catches fire and burns the car down.

Fuses "should" be matched to the wire size of the circuit. the fuse should be a value less than it takes to make the wire overheat and catch fire.

Fuses are not designed or installed to protect devices - they protect the car from failed devices.

If you want to move the fuel pump relay out of the ECU then use the same heavy gauge wire for the 20A fuse or reduce the wire size and reduce the fuse amperage to match. If you install light gauge wire in a modification of a 20A circuit the fuse no longer protects the car and a short will cause the wire you installed to catch fire before the fuse blows.

Be very careful with modifications!

 

[1979 Eldorado]

Jay,
I know, if there's a short to ground, the current follows the path of least resistance and it doesn't go racing around. I worded my reply above poorly combining two completely different examples I was thinking of. I was trying to explain there is risk, but explain it in a non technical shortcut way...

The separate examples are: A short downstream of a sensitive component in the fuel pump circuit. The current shorts immediately to ground destroying anything which cannot handle the amperage up to the fuse (That's why the fuel pump single 20 Amp fuse is dangerous). In that example, of the earlier fuel pump circuit, not everything can handle up to 20Amps.

The second situation I was thinking of was a problem I've heard of in some lighting systems on old corroded vehicles. A friend had an early 2000's TrailBlazer and when the rear light grounds started to get corroded actuating the lights would sometimes power other circuits. I don't know exactly how it was wired but the current found a path of least resistance through another circuit (obviously they had to be somehow connected but it happened). This is as well not "racing around" but that was the term I used to describe the current following the path of least resistance (seemingly randomly even though it's not random at all; rather the path of least resistance).

Scott

 

[81 BENY Biarritz]

Just as a bit of clarification about the two systems that were brought up above. On the 81's both the fuel pump relay and the Modulated Displacement Amplifier (MDA), also called the Modulated Displacement Driver Assembly (MDDA) in some literature, are separate stand alone devices that are driven by the ECM.
For anyone following this thread in future I have attached a scan from a GM training manual titled 1981-87 DFI, Cadillac Digital Fuel Injection. I had sent this to Peter directly earlier and using it we now believe that the MDA outputs on his original ECM are defective but he is not sure whether the MDA box itself caused the issue or might still be OK.

 

[Lucky Peter]

Hello,
Today I've finally managed to check voltage in the ALDL connector as Dave did a couple of days ago, and it looks like the ECC isn't sending a diagnostic request signal, because the second terminal in the ALDL connector is showing voltage of 5.2V all the time after turning the ignition, and even after the two dots ". ." on the ECC are displayed the voltage is still the same. I wonder if there might be some problem with the ECC control head and that's why I'm missing the diagnostic mode? Maybe I should look for an used one to test it out? What are Your thoughts about it?
- Peter

 

[81 BENY Biarritz]

Peter, this is a chart from a latter Cadillac Serviceman Bulletin regarding issues with repeated ECM failures and the way to test the solenoids that could have damaged yours. Posting it here for others to reference in future.

 

[Lucky Peter]

Dave,
I've just tested the solenoids using the latest chart that You've sent and it looks like there isn't a short anywhere.


Next interesting thing is that I found an interesting information in one of the service manuals that Dave sent me a little earlier today (Once again, many thanks to You, Dave). Maybe some of You remember what is a 'hard back-up' mode (I was talking about it much at the beggining), but for those who don't, I want to remind it: So when there is some trouble code set, the car might enter a failsoft mode, but when there is a problem related to the PROMs, the car enters a 'hard back-up' mode which can be easily recognized by these symptoms, which I will quote:

• the check engine light will be lit,
• no coolant light illumination st ignition on, (engine not running),
• no ISC operation,
• no MPG display,
• no diagnostic display (". ." displayed only)

As You can see all these symptoms are currently present in my Seville, so it is definitely pointing to a PROM related problem. Now there is a question what is defective? I think that somebody with some advanced electronic knowledge (I'm talking about You, Jay) will definitely say a few words about it.

I'm wondering if there is a possibility that the PROMs went bad by itself, or if they might damaged anything? Also there is a chance that there is a problem with PROMs circuit in the ECM, but can this problem be related to the new ECM failure? New ECM should arrive soon, but I'm already worried about mounting it before finding anything that can cause damage to it again. I don't know what to think about it, but the problem must be definitely related with the PROMs according to the service bulletin that I quoted above (link will be attached below). Maybe it would be a good idea to find an used ECM with PROMs included (definitely won't be easy) and mount them in my original ECM to see what will happen?

DFI ECM CHANGE AND PROM DIAGNOSIS. Cadillac Seville 1981

gm.saturn.team

Let me know what do You think about my latest idea. Have a nice Tuesday.

- Peter