Repeated PCM Failure

Jeff Diagnostician Cathedral City, California Posted   Latest   Edited  
Question
Driveability
2006 Jeep Grand Cherokee Limited 4.7L (N) 5-spd (545RFE)—1J4HS58N26C288082
P0882 - TCM Power Input Signal Low
P0846 - Transmission Fluid Pressure Sensor/Switch "B" Circuit Range/Performance
P0871 - Transmission Fluid Pressure Sensor/Switch "C" Circuit Range/Performance
P0988 - Transmission Fluid Pressure Sensor/Switch "E" Circuit Range/Performance
P0876 - Transmission Fluid Pressure Sensor/Switch "D" Circuit Range/Performance

This Jeep is a first time customer referred by our trans shop , Also including the listed codes it has original vin mismatch codes, loss of comm with ecm tcm codes spread out through all the modules. Initially had no scan tool communication with the tcm and Ecm and no voltage on the SCI lines at the aldl , after unplugging the ecm for testing and plugging it back in I got scan tool comms back. I chased the listed trouble codes down to the TCM/ECM wasnt powering up the TCM relay. Looks like I have a bad ECM , customer informs us that this is his 2nd ECM and they only last about 3 years each. I havent gotten to load test the power and ground yet BUT I did capture some interesting waveforms from ground , while it is running, there appears to be a voltage spike that coincides with the ignition events , I have channels hooked up to the alt to look at the diodes , the #1 coil primary , the ground and a low amp wave form of the Ignition coils. Im gonna look into this further on Wed when I get back from Vacation if they approve more time. Anyone seen this before ? or have any thoughts on the scope wave forms ?

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

The voltage spike on the ground is interesting. Where do you have your ground lead hooked? I think a trace of the voltage supply to the ignition coils would be in order. Over the years, the only thing I have ever seen that would cause voltage spikes like is some very high resistance in the ignition secondary. From memory, there should be a capacitor on each cylinder bank that is hooked to the…

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Jeff Diagnostician
Cathedral City, California
Jeff Default
   

That ground is at the battery , but it's the same every where I hook it to , capacitors came to mind as well but I didnt see any on the wiring diagram (doesnt mean there not there ) you mentioned high resistance in the secondary which makes me think I read somewhere about Chrysler spark plugs causing weird electrical issues. I have channel D set to AC and on the Alternator post and you can see…

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Although ignition spikes are always visible on the ground, (and a good trick to ID a misfire), spiking to 13 volts is crazy bad for an ECM I think. Around that time-frame there are numerous cases of mystery stalls with no DTC set, all from ignition spikes. IIRC there was even a TSB basically stating to make darn sure all the spark plugs are 100% correct OE for the application. Pretty sure they…

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Robert Diagnostician
Fair Oaks, California
Robert Default
 

Had a Durango that had trans codes and blown fuse for the transmission. After replacing the fuse and trying to trace the short, Lost communication with the ECM/TCM. Turned out the wiring harness down by the trans had the insulation on the wires disintegrate inside the tape. Wires shorted together and fried the ECM. Fixed the harness and replaced the ECM and all was good. Good luck!

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Robert Diagnostician
Fair Oaks, California
Robert Default
 

I see igniton spikes on sensor waveforms all the time with the scope. Ignition creates alot of electrical noise that can be transferred to other circuits with induction. Could be just the way your leads are run in the engine bay. I use them for diagnostics sometimes to see if there is spark while using the channel for other signals.

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Dave Mobile Technician
Ironton, Ohio
Dave Default
 

I have seen the same problem, but the short would take out a pcm at the first key cycle. His problem is way more intermittent.. I have seen a lot of Chrysler ecm failures that were due to cracks in the circuit board. Flexing the connector can sometimes make the problem appear and disappear.

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Robert Diagnostician
Fair Oaks, California
Robert Default
 

This vehicle had been having transmission issues and blew the fuse a few times before coming into the shop. I hooked up my short detection tool that pulses to find the short. Even though the computer was not turned on by the time I found the short the computer was fried and the truck would not start and no communication with the ECM. With all the trans codes I would take a good look at the…

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