Another Bad PCM on a Buick 3.8L!

[clinebarger]

2003 Chevy Impala, 3.8L/4T65E, Unknown mileage as the cluster display is dead, Car/Owner is from out of state (Northeast) so the car is RUSTY & I'm not used to dealing with that. Car is very ROUGH POS.
*Complaint......High Idle, Hard start, & Poor fuel economy.
NO trouble codes present.

Scan data revealed high TPS voltage at closed throttle (1.63vdc), Thought to myself--Bad TPS! 'til I unplugged it & the Voltage went up to around 2.55vdc!
*Jumping Low Reference to the TPS signal brought it back down to 1.63vdc.
*Jumping a Ground right off the battery to the TPS signal resulted in the same 1.63vdc.
*Jumping the 5vdc Reference to TPS signal resulted in 4.99vdc in scan data (Normal).

Decided to look through the rest of the scan data PID's before I "Disassemble Johnny Five" & backprobe all powers & grounds at the PCM.
*Everything looked normal (KOEO/Engine at ambient temp) EXCEPT the O2 sensor voltages were hanging out at around 80Mv where they should be right around a half volt (.447Mv) or so with a cold O2.
*Disconnected the front O2 as it was easy to access & nothing changed per the B1S1 PID.
*Using my incandescent test light hooked to 12vdc.....Voltage would jump 10.23vdc. This IS high voltage for this circuit....But the test light WILL NOT allow enough amperage pass to do harm!
*With my test light hooked to ground.....No change it voltage.

This is a situation where Operational Theory is your best friend!!!
*The TPS signal has a Grounded resistor inside the PCM to assist in providing a clean/noise free low voltage Sensor Input. Therefore the Voltage on the circuit should drop to 0vdc when the TPS is unplugged.
*The O2 signals (GM) have Resistors that are Powered with a power supply within the PCM @ .447Mv, Same theory as the TPS but uses a bias voltage that is exact CENTER of the voltage range for the O2.

At this point I already culled the PCM as faulty.....However I did my due diligence & installed the old 3.8L/4L60E that was in my Camaro before I V8 swapped it (They are "keyed" the same & the SAME PCM besides the calibration)
*TPS voltage fell to 0vdc
*O2 sensor voltages rose to .447Mv.

I concede that I lucked out by having a test PCM in my cabinet to prove out my diagnoses, However......The rest of the Input sensors worked flawlessly & The 5 Volt power supply was operational which tells me Powers & Ground to the PCM are sound.

First photo.....TPS grounded resistor. Arrow.
Second photo.....Bias line on the O2 signal circuits. Arrows. It doesn't denote what voltage but that's how I read them. Some diagrams will denote a 5vdc resisted bias!

 

[clinebarger]

Scan Data......

First photo.........Faulty PCM TPS voltage with the sensor unplugged.
Second photo....O2 volatges, My poor photography skills caused the missing PID descriptors, 74Mv is B1S1 & 82Mv is B1S2.
Third photo.......Remanfactured PCM TPS voltage at closed throttle Idling....Poor photography again, Top arrow is TPS voltage-Bottom arrow is Throttle percentage .

 

[clinebarger]

Looks as the resistors/voltage sensing circuits were burned-up somehow? Someone had tried to fix the high idle with a new IAC motor & a new pigtail that was hacked-in.....They even broke the lock on the new connector! The estimate to correct the hack job was declined.

 

[dishdude]

Nice detective work! Who would use crimp connectors in the engine bay?

 

[nthach]

I wonder if that hacked wiring introduced enough resistance to burn up the resistor on the PCM's PCB or maybe carelessness when the piggytail was being spliced in?

 

[Trav]

Originally Posted by clinebarger
Looks as the resistors/voltage sensing circuits were burned-up somehow? Someone had tried to fix the high idle with a new IAC motor & a new pigtail that was hacked-in.....They even broke the lock on the new connector! The estimate to correct the hack job was declined.

What a hack job, I see broken connectors on 90% of the VW cars I do and they are one of the easiest connectors to remove. I would almost bet they buggered it up and shorted something when they were performing this masterpiece.
Whats going on with the yellow wire next to the lower red connector? Is that broken?

 

[clinebarger]

Originally Posted by Trav

What a hack job, I see broken connectors on 90% of the VW cars I do and they are one of the easiest connectors to remove. I would almost bet they buggered it up and shorted something when they were performing this masterpiece.
Whats going on with the yellow wire next to the lower red connector? Is that broken?


Guess they didn't have wire strippers so they just split the insulation with a knife or something? And didn't bother to cut the excess insulation away. I bet the connector pigtail leads were pre-stripped.

I did wrap each wire with some electrical tape to prevent them shorting together.....Which I shouldn't have done since they declined the very reasonable repair estimate, But if something were to happen.....They would come back screaming warranty!!!!

 

[SatinSilver]

How much was the repair estimate?

 

[clinebarger]

Originally Posted by SatinSilver
How much was the repair estimate?


$20.......I had a used IAC connector/pigtail out of my own stash that I would have given to the job, I just needed to bill out my time to solder & heatshrink.

This was separate from the PCM, PCM R&R, PCM programming & the diagnostic charge. I don't charge diagnostic time on every job but do on these kind as I don't make hardly anything swapping a PCM & flashing it!

 

[SatinSilver]

Yes, very reasonable estimate and their loss for not getting it fixed by you.