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Discussion Starter · #1 ·
Hello,
I’m in the process of an engine wire harness swap. I am almost finished with the swap but I have ran into some issues. The issues relate to the gas pedal and throttlebody. The engine is running rough but is running. I have the codes and will leave them in an attachment for others to look at. Currently the battery in the vehicle is dead but I have been jumping it off with my power stroke and letting my power stroke supply it with enough juice to run. I’m wondering if this may be a grounds issue or if the throttlebody itself is bad and needs to be replace. The car does not respond to any pressure on the gas pedal sorry unable to rev the vehicle.

Vehicle: 2008 Cadillac STS, 4.6L RWD

thanks,
John
 

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Discussion Starter · #3 ·
It looks like a wiring problem. I'd be wringing it out end to end, pin to pin against the wiring diagram.
Thanks for the reply, I will note that I don’t have the grounds, bolted down, just in contact with metal of the chassis. (The grounds that come off the battery 2x and connect to what I assume would be the firewall. I do have the engine grounded well. Could the non-consistent ground for the chassis be an issue regarding this? I will look over my wires tomorrow, but the harness appeared to be in very good condition. I’ll remove the throttle position sensor plug and inspect its pins. Do you recommend me starting anywhere specific with I start looking over everything tomorrow?

Coming from the DSM car community I’m not so familiar with so many electronics on an engine.
 

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Discussion Starter · #4 ·
If you have a source for the engine harness diagram and all the grounds of the vehicle that would be great. (Normally I would of documented as I went, but I purchased the vehicle the harness was already pretty much removed and botched, the previous owner wrecked the vehicle in a ditch and the harness was ripped out and they could not figure it out.) From what I can see there only appears to be 3 grounds? 2 coming off the battery which I assume like stated before are chassis, and one that grounds the engine.
 

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2006 STS V8 AWD, '95 Ford Ranger
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If you have a source for the engine harness diagram and all the grounds of the vehicle that would be great. (Normally I would of documented as I went, but I purchased the vehicle the harness was already pretty much removed and botched, the previous owner wrecked the vehicle in a ditch and the harness was ripped out and they could not figure it out.) From what I can see there only appears to be 3 grounds? 2 coming off the battery which I assume like stated before are chassis, and one that grounds the engine.
Throttle body problems are almost unheard of and you're trying to correct harness problems. I'd be going over the harness and all connections inch by inch.
 

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08 STS 1SG HUD ACC F55 AWD, 08 STS 1SG Platinum HUD ACC F55 RWD
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A little corrosion is enough to cause problems with the ground connections. Go ahead and bolt them down. Good Luck.
Also dk is correct. Check the harness and connections. Inch by inch and connections with a magnifying glass.
 

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Discussion Starter · #7 ·
A little corrosion is enough to cause problems with the ground connections. Go ahead and bolt them down. Good Luck.
Also dk is correct. Check the harness and connections. Inch by inch and connections with a magnifying glass.
Thanks for the response, I see you own a 08 STS as well, do you know where the battery to body grounds attach? Currently I have the engine ground running to the battery off a throttle body bolt (curious as to if this could cause my TPS issues as well) so if you know where the original ground for that goes as well that would be great.

Below is taken a picture of my current grounds. The red wire is a temporary ground for the engine to battery off the throttle body bolt. The blue circled ground would be the original engine ground I’m assuming, but I have no idea where it goes. I have two grounds also on the back fire wall hanging onto a metal bolt coming off the firewall, no idea if it’s a sufficient ground location or metal.
 

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08 STS 1SG HUD ACC F55 AWD, 08 STS 1SG Platinum HUD ACC F55 RWD
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Now, cars are much too complicated to just bolt on a separate ground. Some of the computers monitor both the power side and the ground side of a circuit. That is in both control circuits and sensor circuits. The computers sometimes even compare the relative resistance of different parts of the circuit. What you are doing used to work back in the 60s and 70s but not now. Believe me, it was a high hurdle for me to jump over also. I am 71 and remembering the "good old days" also. Wire it right or it will not work unless you are very lucky.
 

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Discussion Starter · #10 ·
I understand what you’re saying. The blue circled wire that I assumed was a engine ground looks to be the strut tower ground. As for the other two on the fire wall, I assume the bolt onto the engine side or rear somewhere. I will try to find a diagram to see where they hook onto.
 

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Discussion Starter · #11 ·
Update

I have the vehicle grounded well now car starts up every time now before it would say shift into park and not try to start. I do not have the alternator plugged in (since I can’t recall doing it nor which plug it is). I hooked up the belt for the alternator. The vehicle is tripping into power saving mode and states system is not charging reading 10 volts throughout the car on the dash. No longer have high current voltage codes like before. All codes now are low current. Still no response to pedal. Going to assume the electric system doesn’t have the power to send the signals like it needs. I plan to figure out what hooks up the alternator soon.


I’m now down to 2 (3 really) CEL codes.
P2122, P2127 (and P0118 this is due to a broken clip on the water temp sensor)

P2122, P2135, P2127, P0223

-P2122
Accelerator Pedal Position
(APP) Sensor 1 Circuit Low
Voltage
-P2127
Accelerator Pedal Position (APP)
Sensor 2 Circuit. Low Voltage
———————————————
-P0223
Throttle Position Sensor 2 Circuit
High Voltage
-P2135
Throttle Position Sensors 1-2 Not
Plausible
 

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Fully charge the battery before any further troubleshooting. Any voltage below 11.9 will start making the computers go crazy with false codes popping up. You actually want 12.5 or above without the charging system. When you get the charging system working, you need to see 14.3 or 14.4 part of the time. Clear the codes after you get the battery right and see what comes back after running a bit. This car is very sensitive to low voltage/weak batteries. Good Luck.
 
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Discussion Starter · #13 ·
Update

Alright so I got the alternator hooked up and battery charged. That was the hardest plug to reach in my life, had to unbolt it and rock it forward and plug it in from underneath. The car is idling fine. 14V of power while running, back to the issue. Since the car isn’t shutting off on me i decided to run some diagnosis on the codes.

First I decided to check throttle body response. I noticed the TPS had a value other than 0% meaning it was being read. I shut the vehicle off and turned accessories on to see if the value was reading a resting number differently, it was. I had my girlfriend push the throttle plate in while I monitored the scanner, it showed response by changing its value.

Secondly I scrolled down to read pedal A and B. Both pedals were reading 0%. So I pressed the pedal through out the press the value remained 0%. Okay hmm… I click to view live graph data on it and press again, nothing flat line 0%. So now I know this pedal isn’t reading anything. I fiddled with the wires on the back of the connector to see if any were out any, no change.

Currently I’m thinking this pedal may be dead, unless it has a fuse that goes to it that may be blown. Any other opinions? I’m calling it a night at 12:22 AM.
 

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I do not see you saying why you swapped the main wiring harness ?
What else did you change, such as any of the controllers ?

APP will not work if it does not get 5 volts, check from fuse panel for the 5 volts reference fuse is good and getting voltage
Check for 5 volts then to the APP off the gas pedal

You could with scanner with just key on, engine off and step on gas pedal and see if APP1 and 2, along with TPS if they show functional

Example of scanner PIDs to monitor (your APP may be different and have not 2 put 3 sensors, So look at wiring drawing for your vehicle)

Rectangle Font Parallel Slope Number


If ringing out APP, other than getting 5 volts, ground and one APP reports 0 to 5 volts when moving gas pedal and the other APP sensor then goes 5 volts to 0 volts and TPS goes from 0 % when off pedal to 100 % at WOT.
 

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Discussion Starter · #15 ·
Hello Team-ZR-1,
I purchased (for $1000) this vehicle from a family who ran it through a ditch and ripped the wiring harness out. When I went to look at the vehicle I could see they had tried re-wiring the original harness, and also tried putting another in. They said they just wanted to be done with the car and couldn’t figure it out. I towed the vehicle home, and began looking over what it needed. (They had removed the fuse box, and several other under the hood things such as MAF and placed them in the trunk. So i had no idea of the routing of the engine harness either. Anyways I purchased an engine harness from a pull-a-parts yard because I couldn’t find one new for 200$ and installed it. After several trouble shooting events I have the vehicle now idling fine, no misfires nothing. Just no pedal response.
 

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You need to go through the debug of what I posted to even see if the fuse panel to the gas pedal are even getting ground and the 5 volts.
Test the fuse for the 5 volts reference, which is the fuse panel sends 12 volts to the PCM/TCM and the PCM then sends out 12 volts that other sensors like TPS use
So test both sides of the fuse(s) to make sure the fuse panel buss is giving one side of the fuse 12 volts, then check fuses are not blown and then have to see if APP is getting 5 volts from PCM/TCM
Also need to check the wiring pins that connect to the DBW motor on throttlebody

And then if the APP sensors are reporting to the PCM as to the angle of butterfly of the throttlebody

The TCM (controller for the drive by wire) is not going to allow the gas pedal to work if it does not meet the safety
checks for DBW

This would be easier if you had GM's wiring drawing for DBW, voltage, ground
as for gas pedal to work requires the TPS, TB, TCM, PCM with proper voltages, grounds, and return signals from them

Again using a scanner do simple tests to see if APP sensors and TPS agree on the throttle angle to what APP and TPS report to PCM
 

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Discussion Starter · #17 ·
This is my first Cadillac so bear with me on this.
So from what I’m understanding by your post, is that I should test both 5v reference 1 & 2 to ensure they are both reading 5v (I can’t recall if my scanner reads this), check the pins on my throttle body sensors, In the event that one of the 5v ref isn’t reading correctly I need to look at the 5v wiring diagram ensure everything is plugged in correctly, and start evaluating wires to find a short to ground? Also I’m not sure if my Accelerator pedal position A and B is the APP 1 and 2 you speak of when I’m using my scanner. My scanner reads 0% on those two sensors. Even when TPS is reading (I believe 21-22? When idling) (30 when off) (I had my girlfriend push the throttle plate in and it read up to 80% on my scanner when testing it.
 

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With Drive By Wire GM has to put safety functions in to prevent it going wrong and causing crashes so

Depending on year/model/brand

1. Either the Throttle Control Module (TCM) is a separate controller or in newer vehicles it was rolled into the PCM)
2. Again as above the gas pedal sensors (APP) either there is 2 or 3 sensors

That then has a Throttle Position Sensor (TPS) and a throttle body DBW electric motor

GM has the controllers cross-checking for safety to know 100 % what the angle of the butterfly is (by TPS) and is doing electronic voltage compares to assure what that angle is

So as example if your vehicle's gas pedal has 2 APP sensors

As more steeping on gas pedal APP1 goes from 0 to 5 volts while at the same time APP2 is going from 5 to zero volts and the TCM is doing a compare from that and what TPS reports as to butterfly angle percent

So the system is double or triple safety checking
In that example then not on gas pedal then APP1 reports 0 volts and APP2 reports 5 volts
Now if at WOT then APP1 now reports 5 volts and APP2 report 0 volts,

So as pedal angle changes, those voltage levels ramp 0 to 5 and 5 to 0 volts

At fuse panel there are fuses for the PCM, and it maybe marked as 5 volt Reference, it is a 12 volt circuit
but when received by the PCM its hardware converts that 12 volts to 5 volts, and it goes to certain sensors and functions including for DBW
So that is a redundancy check and then add TPS angle as another cross-check

If using your OBD-II scanner APP1 and 2 not showing any changes as you move on and off the gas pedal then you have to solve that, starting with does APP module getting ground and 5 volts

This would also be seen if using a multi meter and disconnecting the wiring connector of the DBW motor on the side of TB and testing to see if the voltages and ground getting there from APP

Yes APP A and B is just another way to label them

I would check the wiring from APP at gas pedal as if not getting ground or voltage then DBW will be dead
and if those same circuits are not seen at the DBW motor then wiring issue from there to APP and PCM
 

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Discussion Starter · #19 ·
Update

Hello everyone, so today I began working on diagnosing again.

So I started off simple, and began with the DBW throttle body sensor, I hooked the sensor and began testing the 5v ref, it read 5v and I looked at the pins on both the plug and sensor, both looked good. I plug it back in knowing it was good.

Next I climb underneath the dash and remove the pedal plug, I tested the 5v’s on the pedal plug and notice nothing on each. I test all the way across the pedals and nothing.

Confused I start looking for unplugged sensors thinking there may be a correlation. I find nothing, I go over to my 6 page wide engine harness schematic diagram where I have everything highlighted for what’s needed to fix this. I studied it for a bit and start examining wires. My brother and I are looking over the car together and I keep mentioning the gauge cluster plug, over and over, how it’s what the pedals received its signal through and I said “What if this plug isn’t plugged in correctly?” I go to unplug it and realized I didn’t have it locked down. I remove the plug and examine the pins to ensure nothings bent. I re-insert the plug and lock It down firm. I joke with my brother about how the car will respond now because it’s always the dumbest of things.

Sure enough I start it up and push the pedal and she roars to life, responding immediately. I take a quick victory lap around the yard and re-park the car. Tomorrow calls for an Oil change, fresh gas, and to fill the radiator up.
 

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I learned long ago that when the whole world turns brown, go back and recheck what you last did when the trouble started. I have met the enemy and he is me. With my last project which was resealing the passenger side headlight that was full of water, when I got through the heater didn't work. I had pulled out the under hood fuse box for access and didn't get it plugged in correctly when I replaced it. About a half hour redoing my work and all is well.
 
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