Have a 96 Dodge Ram 2500 Cummins with a flakey (I think) PCM. The failure mode originally intermittent tach, followed by tach failure (0 RPM), then the Gen/Engine idiot lights cycle, then nothing. The converter drops lockup, the tranny downshifts to 3rd (no OD), the OD switch doesn't work, the alternator drops off line and a/c clutch drops out.

I had it "rebuilt" and reinstalled it. Initially the unit was completely non-responsive (as if it were out of circuit.) It then spontaneously worked, initially for a few miles, then failed as above. It has progressively performing better, but still fails randomly, and remains failed until it sits a few days, then works. The engine does not have EFI control, has (I'm pretty sure) mechanical injectors without electronics and will start, run and respond to throttle without the PCM installed, the tranny will engage and run gears 1-3, but the converter will not lock and 4th will not engage. When the tach is indicating normally, everything is normal.

Next steps?

 

Sounds like they shafted you on the rebuild. All Computer Resources built mine and it worked really well. Couple things you can do, have somebody with a code reader pull and reset your codes. Start it up and see if it throws the same codes. If you have several intermittent problems the PCU/PCM can definitely be the issue. Check all your bases first, ensure the ground is solid and isn't loose or breaking connection. If you do not have access to a code reader unplug your battery and turn your key to the start position for about a minute to drain the capacitors in the computer. Then plug her back in and see what you got. With as many different issues as you have, its related to the PCM, it could be faulty wiring that is screwing with you or it is the unit itself.

Now when you said everything is normal with the tach working, is that still with the PCM out of the loop or is it with it all connected?

 

If it is doing the same thing it was prior to the rebuild, likely the rebuild failed. Might try simply replacing it.

 

When everything is normal, with the tach working, the PCM is in the loop. Here is how it fails:
1. Startup is fine, everything is operating. key switch to on, initial lights operate, pre-heat (wait to start) goes out and engine starts and idles normally.
2. Engine responds as expected to throttle, tach indicates normal idle and appropriate rpms for speed/gearing (ie 1750-1800 at 55-60 mph).
Failure mode:
1. Tach begins to bounce around, becomes intermittent then drops to zero.
2. Once the tach drops to zero, the converter drops out of lock up, then the tran drops out of O/D, and the engine sounds like its turning closer to 2500, and requires much more throttle. Sometime around this time the alternator voltage drops to battery voltage on the volt meter on the panel.
3. Intermittently the Gen/wait-to-start/etc. lights on the panel will flash on, then off, and that's it until it sits.
When it is in failure mode, there is no generator output and system voltage is at 12 v.

When the PCM is unpluged from the system, the engine/tranny behave as in failure mode, as above, except no winky blinky lights.

Code reader: I do have a code reader for OBD2. I'm not sure the '96 used OBD2 or OBD1, but I'll check and see if I can pull any codes.
I am concerned about the rebuild quality myself.

Thanks for the help. Also it appears I may have put this in the wrong forum group. Sorry about that. When I was searching a pop up asked me to post my problem and this is where it put it.

 

If I could find one, I would. The local Dodge dealer told me he couldn't get one, but he wants to sell me a new truck, too.
I've thought about seeing if a performance shop that specializes in Diesels could get one, but haven't a clue where to look. It's doing similar but slightly different things.

My original thought was that it was a bad/dirty connector, but I've checked and cleaned all of them in every place I could find them. Next time it fails, I'll check the -12VDC feeds to see if that has dropped out. The problem with intermittent issues is I need to be watching that feed at the time it fails. I've thought about building a voltmeter for a raspberry pi and recording the line voltages during the failure point.

Thanks for your input. If you have a good source for a replacement PCM, I'd be interested.

 

Try car-part.com. They may be able to locate a used one for you. Being yours is a 96, its just a simple swap. No re-programming required.

 

Once I figured out that the engine was mechanical multi-port injection pumps and no electronics in the fuel injection/throttle system I figured that would be the case.

Near as I can tell the only reason I need it is to watch the engine speed, match the transmission gear, lock the converter when appropriate. If I needed to I could rewire and bypass it for the voltage regulator/alternator and the a/c compressor. Chrysler went supercheap when they threw all that junk in there. On the '96 diesels, all the injection circuits appear to be unused. Probably not the case with the gas version. Might be cheaper to just drop the auto trans and put a manual in. . .

That's why I thought and was searching for a diesel performance geek type shop that may have an aftermarket unit. Overall the truck and engine are in good shape and its mainly used for hauling on relatively shorter trips.