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Repair Question PCM

F

fwk442

NAXJA Forum User
Location
Florida
Hi all. So I've never had to post before as I've always been able to find answers to questions just by searching. However, I've got a specific issue that I am hoping to find some info on. My 96 XJ totally cut off while driving on the highway earlier in the week, no warning. Pulled off safely, tried to start it again and it turned over but would not start. No check engine light, visual inspection looked fine.
Had to have it towed to a random shop nearby as I was a ways from home. They first believed it to be the Crank Position Sensor. I had replaced this with one from the dealership about 4 years ago but told them to give it a try. Nope. Come to find the rotor in the distributor was fried. They now believe it to be the PCM. Saying there is no '5 volt crank signal' being sent from computer. They are currently trying to source a new ECM. I am not so sure about all this. Has anyone heard of such a thing? Any info would be greatly appreciated before I run up a bill. Tempted to have it towed home to work on myself.

Most recent work performed - In March 2021 - upgraded alternator to 130amp - upgraded all grounds and battery cables to 2 guage - full tune up including ign. coil - changed injectors to 4 port. Jeep had been running wonderfully until that single instance.
 
Were they able to get any codes out of the computer with a scanner?
 
OBD engine sensor failure or faults/failures of their wire harness or wires plugs is much more common than ECM failure.

Mechanics that have poor diagnostic skills, or have a boat payment due, typically are quick to blame the ECM for malfunctions they cannot locate/diagnose/resolve.

I would get it home and start testing the OBD engine sensors and inspecting/testing their wire plugs and wires.
 
Last edited:
Pull the plug on the CPS and the cam position sensor (called something else on the OBD motors the name escapes me now). See if your check engine light comes back on with the key in the run position. If either the cam position sensor or CPS has an internal short (no resistance) the computer will shut down the CPS, cam position sensor, speed sensor feed. Easy test maybe it will help you narrow it down.
Check for shaft side play in the distributor, too much play can explode the rotor. And it sounds feasible the flying debris damaged the cam position sensor. Just an idea.
 
The distributor phasing sensor/cam shaft sensor is the flat one under the distributor cap. If the shop is connecting to the obd2 port and the reader will not connect at all to the pcm with key on engine off then the pcm does not have power. Check all fuses in the PDC. Then the C1 (black) connector at the ecm should be reseated.
 
I'd be interested to hear what the solution is as I have a '96 XJ and if it isnt the CPS which would have been my first port of call .....
I've contacted Flagship One (just in case I ever need a replacement ECM) but they don't know if the export models (RHD; UK) had a different version - don't see why they should but maybe different fuel settings for emissions? I need to do more research.


Good luck and hope you get a fix
 
Everything I've come across on my 95 rhd has been the same besides the handful of front end stuff and the gauges in the cluster. I hated that mine were in metric so I got a standard full gauge set but the printed board or something was different so it wasn't plug and play. I dismantled both, bolted the lhd gauges into the rhd cluster and it all read correctly. Mine is obd 1 so I can't use a reader but you should be able to put a reader on your 96 and it would probably help determine if the programming is differently. Sensors are all the same for what that's worth.
 
Unclewolverine said:
Everything I've come across on my 95 rhd has been the same besides the handful of front end stuff and the gauges in the cluster. I hated that mine were in metric so I got a standard full gauge set but the printed board or something was different so it wasn't plug and play. I dismantled both, bolted the lhd gauges into the rhd cluster and it all read correctly. Mine is obd 1 so I can't use a reader but you should be able to put a reader on your 96 and it would probably help determine if the programming is differently. Sensors are all the same for what that's worth.
Click to expand...


As you say Unclewolverine, seems main changes on RHD versions are steering related ('natch!) at front end and reconfigured washer bottle, brake servo, and alternator placement. Aussie Jeep forum is helpful in this regard as they drive on the correct side of the road too :jester:


Fortunately our shared colonial history means we use imperial measurements like you in the US so none of this metric gibberish on UK instrument clusters. Our gallons are bigger tho'.:). I like the gauge solution - I had to do something similar when replacing the fuel pump as on the '96 only it's a module - pump+sender+pressure regulator - that sits in the tank. The fuel level measuring 'strip' feeding the sender's signal is unique to '96 so replacement module was wildly off. Replacement part unavailable worldwide so I had to do a frankenstein job using old part and that worked perfectly.



I have tried to find a cheap OBD reader that works on the '96 but nothing so far and only a $1000 specialist reader managed to collect anything and since I'm keeping my XJ on a budget that's not a viable option. I rely on the 'three twists of ignition key' and read the flashing codes: "12 & 55" and I'm happy.


If there's anything RHD you need and which I might be able to help with feel free to PM me.
 
Been There Done That

Hey everyone, thanks a lot.
Sorry for the dead air but I had the Jeep towed home and I recently have been working on the XJ (luckily not primary vehicle), and this is what I've found:

Crank but no spark. Fuel pump cycles on.
No codes. Check engine light cycles when key on and then turns off.
Inspection found rotor was toast. Like burnt up.
Replaced rotor, cap and ignition coil.
Changed the CKPS and CPS (cam and crank sensors)
Checked all fuses and swapped relays to confirm working.
My scanner communicates with PCM.
Tested for 5v signal to MAPP with key on and I get 5v. Need to check 5v reference on other sensors.

Most recently I back probed the harness to the ignition coil.
2 wires, 1 signal wire and the other should be 12v constant, as I understand. Green signal wire will light the tester when cranking but I get no 12v constant on the other wire. I believe power to coil is routed to the PDC (power distribution center) and tied to the ASD relay (which is good).

All grounds visually look good and all were replaced back in March, along with 130 amp alternator and 2 gauge battery cable upgrade. Will be going through and physically cleaning all connections next and thoroughly inspecting all harness wires.

Not sure where to go from here. Is there a way to definitively test the PCM? Even say removed from vehicle and run through a diagnostic?
Should I take a wiring diagram and start back probing the PCM?

Any help in how to work through this would be greatly appreciated.
Thanks.
 
Re: Been There Done That

fwk442 said:
Hey everyone, thanks a lot.
Sorry for the dead air but I had the Jeep towed home and I recently have been working on the XJ (luckily not primary vehicle), and this is what I've found:

Crank but no spark. Fuel pump cycles on.
No codes. Check engine light cycles when key on and then turns off.
Inspection found rotor was toast. Like burnt up.
Replaced rotor, cap and ignition coil.
Changed the CKPS and CPS (cam and crank sensors)
Checked all fuses and swapped relays to confirm working.
My scanner communicates with PCM.
Tested for 5v signal to MAPP with key on and I get 5v. Need to check 5v reference on other sensors.

Most recently I back probed the harness to the ignition coil.
2 wires, 1 signal wire and the other should be 12v constant, as I understand. Green signal wire will light the tester when cranking but I get no 12v constant on the other wire. I believe power to coil is routed to the PDC (power distribution center) and tied to the ASD relay (which is good).

All grounds visually look good and all were replaced back in March, along with 130 amp alternator and 2 gauge battery cable upgrade. Will be going through and physically cleaning all connections next and thoroughly inspecting all harness wires.

Not sure where to go from here. Is there a way to definitively test the PCM? Even say removed from vehicle and run through a diagnostic?
Should I take a wiring diagram and start back probing the PCM?

Any help in how to work through this would be greatly appreciated.
Thanks.
Click to expand...
Just for a little better understanding of how the coil works. The dark green/orange wire supplies power to the coil, the injectors, the O2 sensor heaters and the alternator when the ASD relay is activated.

The black/grey wire goes from the coil to the PCM and is the trigger from the coil to ground. When the powered (from the ASD) the coil electromagnetic field collapses (to ground) through the PCM trigger, the coil generates a high voltage.
Ohm test the coil the dark green pole to the black grey pole. You should get continuity with some resistance. Many auto parts stores have a coil tester.
 
Re: Been There Done That

I would back probe the crank and camshaft sensors while cranking and verify that you see a pulsing signal. O-Scope would be best but a voltmeter on a/c should show something. Same with the coil trigger. That would narrow down whether the sensors are putting something out and whether the pcm is doing it's job. Are injectors firing? If they're firing then the sensors should be good.

Low probability but worth checking if all else fails - Check the bellhousing bolts, and that the flexplate isn't broken.
 
My PCM on my 01 XJ went bad last Monday. Actually, I think it had been intermittently acting up since I got it last November. The check engine light has never functioned, not even at start up (bulb was good).

A couple of times here lately it had acted like it was experiencing heat soak, but would start and drive fine once it cooled off. This past Monday, I was driving home from work and it died on the freeway. Managed to get off the road. It would start, but only stay running if you feathered the pedal and then it just died. The only code was P1296, no voltage to MAP sensor. I was tired of messing with it, so had Jeep towed to a mechanic I trust. He verified MAP sensor was functioning, etc. and deduced it was PCM.

He had to order it and it should be in today. I'll update how it went. But this was the first PCM that I or any of my friends had to replace.
 
I wanted to follow up as the Jeep is running again!
Took some time and ended up enlisting the help of a retired mechanic that lives in my neighborhood.
In the end the problem was that the oil pump seized and locked up the distributor causing the gear to shear. (see link to pics) This caused the engine to immediately cut out while driving down the highway and the no-spark situation that followed.
That one sucked. But I am now intimately acquainted with the wiring, sensors and relays under the hood. Just happy she's humming along again.
Thank you everyone.

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