Bad or damaged GMPP PCM??
09-08-2023, 06:56 PM
#1
Bad or damaged GMPP PCM?? GMPP# 19354333
This is the PCM for an LS376/525.
Here is my situation. The engine starts and idles at around 950-1050 rpm. You can turn on the AC and the AC clutch engages, air blows cold.
Once the coolant temp reaches approx 205F, the SPAL fans turn on. The temperature starts to slowly drop and once it reaches about 192F, there is a bump up in the idle RPMs and immediately following this bump the DBW throttle pedal goes offline/dead, the AC clutch disengages (ambient air blowing), and the engine starts misfiring badly.
Obviously this is hard to troubleshoot with only this information, but it gives you an idea of what I am up against and this post will be very long if I go into complete detail over how everything is set up. It is a very outside the box setup, but likely nothing too crazy for many of you here. This is a first for me so I'm looking for some feedback from guys who have run into similar issues.
This is the PCM for an LS376/525.
Here is my situation. The engine starts and idles at around 950-1050 rpm. You can turn on the AC and the AC clutch engages, air blows cold.
Once the coolant temp reaches approx 205F, the SPAL fans turn on. The temperature starts to slowly drop and once it reaches about 192F, there is a bump up in the idle RPMs and immediately following this bump the DBW throttle pedal goes offline/dead, the AC clutch disengages (ambient air blowing), and the engine starts misfiring badly.
Obviously this is hard to troubleshoot with only this information, but it gives you an idea of what I am up against and this post will be very long if I go into complete detail over how everything is set up. It is a very outside the box setup, but likely nothing too crazy for many of you here. This is a first for me so I'm looking for some feedback from guys who have run into similar issues.
09-08-2023, 07:40 PM
#2
ModSquad
iTrader: (6)
Moving this one to diagnostics because you’ll get more views over here…
09-10-2023, 07:51 AM
#4
TECH Resident
A data log and the tune file posted might be a help.
"I am up against and this post will be very long if I go into complete detail over how everything is set up. It is a very outside the box setup".
Not sure anyone will spend the time to play "20 questions" to make a number of SWAGS, without these details.
"I am up against and this post will be very long if I go into complete detail over how everything is set up. It is a very outside the box setup".
Not sure anyone will spend the time to play "20 questions" to make a number of SWAGS, without these details.
The following users liked this post:
G Atsma (09-10-2023)
The following users liked this post:
Old Buzzard (09-10-2023)
09-10-2023, 06:39 PM
#6
Left the battery disconnected for 36 hrs.
Reconnected the battery.....I always turn on ACC power prior to starting to permit the fuel pump/system a moment to prime up (eliminate any variables, this is a test/development car)....turn on power, and right off the bat, even though I am not cranking the car and the clutch switch is not engaged, I can hear the starter clicking away for a second....it stops, I crank the car and it starts without hesitation but straight into dead-throttle-pedal-and-misfire mode. AC is commanded OFF.
I turn the car off basically immediately. Give it about 5 or 6 seconds....Restart, fires right up and is running "normally". No misfire, throttle pedal is functioning, AC is commanded OFF.
Just as before, things seem normal for a cold start from the driver seat, I watch the coolant temps slowly rise to 203F, SPAL fans kick online, coolant temps slowly drop back down to approx 192F, car is steadily idling at 950-1000 rpms, then there is a ~200rpm bump in the idle and the misfire is back, throttle pedal is dead, and I check the AC, turn it on, it is blowing ambient air. SPAL fans do not cut off at 192F they just keep going I have made no changes in the GM calibration file apart from final gear ratio and tire size.
Maybe related?
Attached VCM scanner log file. Issue happens about 30-60 seconds before the end of the log, at which point you can see I shut it off.
Reconnected the battery.....I always turn on ACC power prior to starting to permit the fuel pump/system a moment to prime up (eliminate any variables, this is a test/development car)....turn on power, and right off the bat, even though I am not cranking the car and the clutch switch is not engaged, I can hear the starter clicking away for a second....it stops, I crank the car and it starts without hesitation but straight into dead-throttle-pedal-and-misfire mode. AC is commanded OFF.
I turn the car off basically immediately. Give it about 5 or 6 seconds....Restart, fires right up and is running "normally". No misfire, throttle pedal is functioning, AC is commanded OFF.
Just as before, things seem normal for a cold start from the driver seat, I watch the coolant temps slowly rise to 203F, SPAL fans kick online, coolant temps slowly drop back down to approx 192F, car is steadily idling at 950-1000 rpms, then there is a ~200rpm bump in the idle and the misfire is back, throttle pedal is dead, and I check the AC, turn it on, it is blowing ambient air. SPAL fans do not cut off at 192F they just keep going I have made no changes in the GM calibration file apart from final gear ratio and tire size.
Maybe related?
Attached VCM scanner log file. Issue happens about 30-60 seconds before the end of the log, at which point you can see I shut it off.
Trending Topics
09-10-2023, 06:53 PM
#8
Tune file is almost completely untouched from the GM calibration, the only changes were made in the following table:
AIRFLOW > GENERAL > MAF Calibration
Those changes are according to what worked on past LS376/525 cars.
The "plan" was to dyno tune this car in the next week or two, with these values as the starting point just to get it running and the new motor broken-in.
The other changes made to the GM file are Speedo calibration stuff....VSS pulses per rev, final drive ratio, and VSS Tire Revs per
AIRFLOW > GENERAL > MAF Calibration
Those changes are according to what worked on past LS376/525 cars.
The "plan" was to dyno tune this car in the next week or two, with these values as the starting point just to get it running and the new motor broken-in.
The other changes made to the GM file are Speedo calibration stuff....VSS pulses per rev, final drive ratio, and VSS Tire Revs per
09-11-2023, 12:54 PM
#10
Left battery disconnected over night. Cold started, started right up, idling as expected 950-1000rpm, commanded AC ON while coolant temps were rising and both fans commanded OFF.....blows ambient air, AC clutch back to not working, secondary fan not ON as it should be when AC is commanded ON.
Pulled up the DTC window.....
"0x7E8: P2135 - Throttle/Pedal Position Sensor/Switch A/B Voltage Correlation (Permanent)"
Cleared DTC's and waited for temps to rise, at 203F both SPAL fans commanded ON (this is not correct fan function, but ok maybe it is related), coolant temp slowly drops to 192F......no RPM bump like yesterday immediately before the misfire, no misfiring, fans remain ON, coolant temps hold around 196F
Cliff's Notes: Misfire gone, throttle pedal (for) now does not go dead, but AC clutch is inop.
Some setup info re: fans and AC....
Dual SPAL fans
Primary fan powered by GM harness wire, grounded to chassis
Secondary fan powered by native chassis harness fan wire, grounded via native chassis harness
The blue fan relay trigger wire from the ganged pair (blue + green) in the GM harness is rerouted to the native chassis fan relay.
AC clutch is powered by native chassis AC clutch relay
Fans should be independently operated, Primary commanded by temp only, Secondary commanded by AC controls and/or temp....that is not what is happening, 203F is commanding both fans ON regardless of AC command. However the Secondary fan WAS working independently of the primary when AC was commanded ON and temp had not reached 203F....and now it is not working at all....what any of this has to do with the DBW throttle pedal is a mystery to me so far.
Wiring is according to what has worked on near identical cars. I have double checked.
Engine is brand new crate motor, with less than 40 miles and possibly 3 to 4 hours max of runtime.
Pulled up the DTC window.....
"0x7E8: P2135 - Throttle/Pedal Position Sensor/Switch A/B Voltage Correlation (Permanent)"
Cleared DTC's and waited for temps to rise, at 203F both SPAL fans commanded ON (this is not correct fan function, but ok maybe it is related), coolant temp slowly drops to 192F......no RPM bump like yesterday immediately before the misfire, no misfiring, fans remain ON, coolant temps hold around 196F
Cliff's Notes: Misfire gone, throttle pedal (for) now does not go dead, but AC clutch is inop.
Some setup info re: fans and AC....
Dual SPAL fans
Primary fan powered by GM harness wire, grounded to chassis
Secondary fan powered by native chassis harness fan wire, grounded via native chassis harness
The blue fan relay trigger wire from the ganged pair (blue + green) in the GM harness is rerouted to the native chassis fan relay.
AC clutch is powered by native chassis AC clutch relay
Fans should be independently operated, Primary commanded by temp only, Secondary commanded by AC controls and/or temp....that is not what is happening, 203F is commanding both fans ON regardless of AC command. However the Secondary fan WAS working independently of the primary when AC was commanded ON and temp had not reached 203F....and now it is not working at all....what any of this has to do with the DBW throttle pedal is a mystery to me so far.
Wiring is according to what has worked on near identical cars. I have double checked.
Engine is brand new crate motor, with less than 40 miles and possibly 3 to 4 hours max of runtime.
09-11-2023, 01:38 PM
#12
Now with all that said above, why would I be concerned about a damaged PCM?
Before the misfire and dead throttle pedal issues...the car was technically running fine for the initial 30 mile break-in oil interval, BUT initially the AC clutch was not working, the fan function was (and still is) incorrect, AND there was a significant parasitic drain....
To make a long story short, during the process of trying to resolve that parasitic drain, I had the classic brain fart and reversed polarity on the jumper battery for a moment. It was not the battery in the car, that battery is relocated into the trunk, but the trunk latch is electronically controlled, so I needed an external battery connected to the jump point in the engine bay (hood latch is not electronically controlled) in order to open the trunk and connect the drained battery to the charger, so I did NOT crank the car with reverse polarity nor did I even switch on 12v ACC, it was simply connected to ground and the jump point located between the installed battery and the starter for a few seconds before I realized what I had done and pulled it back off, but long enough for the jumper cables to begin heating up quickly.
This has me wondering if that was enough to damage the PCM....but again, apart from the dead throttle pedal and misfire, these fan and AC issues preceded my brain fart and persist today...but I have to wonder if these new issues (throttle pedal & misfire) are a result of that mishap.
Shortly after my dumbassery, I resolved the parasitic drain and at the same time the AC clutch magically started working and the AC blew cold air when commanded ON.
It was a couple of test drives before this new issue with the misfire, throttle pedal, and AC clutch being kicked offline surfaced...
Now it could be that whatever happens at ~192F that causes the RPM to blip up and trigger these new issues was always there but maybe the coolant temp never got back down to ~192F to trigger these new issues, so it was not brought to my attention until I started the car in the shop and let it sit idling (I was troubleshooting some other non-critical stuff that I was seeing during those test drives)
Of course the easy solution is get a new PCM and see what happens, but they are $700 PLUS another pair of HPTuner credits, so I need to rule everything else out before I spent another $1k all said and done.
Before the misfire and dead throttle pedal issues...the car was technically running fine for the initial 30 mile break-in oil interval, BUT initially the AC clutch was not working, the fan function was (and still is) incorrect, AND there was a significant parasitic drain....
To make a long story short, during the process of trying to resolve that parasitic drain, I had the classic brain fart and reversed polarity on the jumper battery for a moment. It was not the battery in the car, that battery is relocated into the trunk, but the trunk latch is electronically controlled, so I needed an external battery connected to the jump point in the engine bay (hood latch is not electronically controlled) in order to open the trunk and connect the drained battery to the charger, so I did NOT crank the car with reverse polarity nor did I even switch on 12v ACC, it was simply connected to ground and the jump point located between the installed battery and the starter for a few seconds before I realized what I had done and pulled it back off, but long enough for the jumper cables to begin heating up quickly.
This has me wondering if that was enough to damage the PCM....but again, apart from the dead throttle pedal and misfire, these fan and AC issues preceded my brain fart and persist today...but I have to wonder if these new issues (throttle pedal & misfire) are a result of that mishap.
Shortly after my dumbassery, I resolved the parasitic drain and at the same time the AC clutch magically started working and the AC blew cold air when commanded ON.
It was a couple of test drives before this new issue with the misfire, throttle pedal, and AC clutch being kicked offline surfaced...
Now it could be that whatever happens at ~192F that causes the RPM to blip up and trigger these new issues was always there but maybe the coolant temp never got back down to ~192F to trigger these new issues, so it was not brought to my attention until I started the car in the shop and let it sit idling (I was troubleshooting some other non-critical stuff that I was seeing during those test drives)
Of course the easy solution is get a new PCM and see what happens, but they are $700 PLUS another pair of HPTuner credits, so I need to rule everything else out before I spent another $1k all said and done.