Powertrain Control Module

I am having a little problem on my 99 classic with my engine just shutting off like the ignition is cutting out. It happens when I hit a sharp bump and if I'm going fast enough it will stall the engine for a second and be fine after that. If I'm going slow it stalls and I have to turn the key to start it again. Its always fine after I re-start until I hit another bump. When I'm going fast enough for it to stay running I notice the gauges going slack and then picking right back up.

I figured that if it only happens when I hit a good bump that if I bang on and move around all my plugs and wiring while it was running I would find the problem when it stalled. The only time it stalls is if I hit the power-train control module. When I bang on it (and not very hard) it cuts right out like it does when I'm driving.

Now, I have read in my searches that a crank position censor could cause the same type of problem. But that doesn't seem to be an intermittent problem.

Do you guys think its worth it to go to a bone yard and get a PCM? Car-Parts.com says there are a few yards not far from me that have them for around $100.

99 out of a 100 PCM that are replaced end up not being the problem. Don't jump to conclusions, most likely its not the PCM, its a solid state electronic item, no moving parts. The PCM can burn out or certain circuits can burn out, they either work all the time OR the do NOT work every again. Its extremely extremely rare that a PCM has an intermittent problem.

More likely you've got a short in the wire harness, bumps tug the harness and short the wires. Tug the harness again, and the shorted wires pull apart and everything is fine again.

I'd look over the wire harness and look for signs of shorting or damage to the harness.

I've had shorted wire harness's (not in an XJ) and it acted a lot like your problem. I found the shorts, (the wire harness was chaffed) and fixed the wires and never had the problem again.

I'd start by checking the wiring at the battery terminals and all the grounds. This could be a loose ground connection somewhere.

Or, as Rick said, a short in the harness. I agree, it's generally very rare to see a PCM fail, that's usually a misdiagnosis, although it's certainly not impossible.

sounds like a harness. i had my wiring harness and PCM replaced under warranty. from what the dealer said, its a rather-common problem.
on 1999s, the gauges have been known to quit, then come back. its a problem with the plug that comes out of the dash and goes into the gauge cluster.

The first thing I suspected was a short. That's why I started moving the wire harness and plugs around. I also started to bang around on things. The only thing I found is if I hit the front part of the PCM it cuts right out. I don't even have to hit it hard, just drop my hand down on it. I agree that it can be the harness or plug right there but I don't think it is any other plug or part of the harness. I think if it was a plug under the dash it would cut out whei I banged on the dash or moved the wires there.

Could it be a over sensitive crash sensor/tilt switch?

I know this an old post, but if anyone has a PCM problem check out this post at CNET.

On my 96 ZJ 4.0L Auto 4x4, my PCM was cutting the signal to the ignition causing it to stall occasionally ($170 dealer diagnostics) Looking at another $500 to get a new PCM with the VIN flashed and installed, I discovered this miracle fix.

Two torx screws on the face of the PCM were removed and then cut in half (from 1/2" to about 1/4") and reinstalled. No more problems. Its just unbelievable. Check the link below for all the backgound on this fix.

http://forums.cnet.com/5208-7811_102-0.html?forumID=78&threadID=248386&messageID=2571760#