Bad PCM?

For all who read my previous post on the intermittent Air conditioner and check guages light, I sent the jeep to the dealer for diagnosis. I had told them that I had already replaced the relay and the fuse with no success. They bypassed the PCM and the damn thing worked. So they say it's a bad PCM. $565!!!! Looks like I'm either off to the junk yard or joining the 90mph club. Not only that, when they were poking around they said they noticed that the exhaust manifold was cracked. They quoted me $900 installed! F**K THAT. I might as well go for a friggin header and free flow exhaust at that rate. Anyone have any experience with changing out the PCM? Is it a "plug'n'play" operation? Plus we're talking a 1997 here. How many of those you think I'll be able to find in a bone yard?!!! On my way out of the dealer lot I had to stop and look at the two Rubicon's they had. SOOOOOO TEMPTING! If I didn't just buy a new minivan a month ago........


Rock on

Deyman

New PCM on a '97.... Is not really a normally replaced part. I'd get another opinion to make sure. Is not out of the question that it could be bad, but another opinion is in order i think. What happened to cause it's problems?? Something short out the wires somewhere?? If is determined to be bad, i'd think since is something that is fairly dependible, a junkyard replacement would be okay. Make sure what could have caused your's to go bad is made good first!!

As far as the exh man, i've got a good used one at home.... with headers out there in use as is my case there have to be used ones available cheap. The stock replacements are really expensive because they now include stainless flex sections to help with cracking. I think a header of some type would be a great alternative. A few out there even bolt to the stock pipe.

Before you jump on one of those Rubicons, ck the reviews first and see if you'd be willing to deal with the crude road manners....

Having had a bad pcm diagnosis that turned out to be a bad wiring harness, I would suggest that you invest in a good ohmmeter, and start looking for wiring problems first. You can get a pretty cheap but effective digital meter for under 20 bucks, and even if you end up needing the PCM the meter will be handy for years to come.

In my case it was a slightly resistive internal splice, which got worse as it heated up, and just bad enough to cause one injector to misfire. After weeks of struggling, and hundreds of dollars worth of new parts, sensors, etc. etc. I fixed it in 10 minutes with a foot of wire and a soldering iron.

As far as replacing the PCM, it is a plug and play operation. I did this with a boneyard PCM on the above vehicle, to no good effect. Too bad yours isn't a 93, or I could loan you my spare for testing!

Sounds like a good idea, but I wouldn't even know where to start. I don't know the circuitry of the jeep or how to read or use the ohm meter. My luck, I'd cross the wrong wires or something and really screw things up. I'd like to take the time to mess around with it, but with a 1 1/2 yr old, that kind of limits free time. What do you have to do? Just disconnect one end of the wire at the PCM and test for the proper current? Or do you have to disconnect at all junctions and test? Seems like that would take forever. I gotta go back to work. Later


Deyman

Deyman said:

Sounds like a good idea, but I wouldn't even know where to start. I don't know the circuitry of the jeep or how to read or use the ohm meter. My luck, I'd cross the wrong wires or something and really screw things up. I'd like to take the time to mess around with it, but with a 1 1/2 yr old, that kind of limits free time. What do you have to do? Just disconnect one end of the wire at the PCM and test for the proper current? Or do you have to disconnect at all junctions and test? Seems like that would take forever. I gotta go back to work. Later


Deyman

Click to expand...

BAsically, you should find a wiring diagram that shows where the various connectors are for the thing you're trying to diagnose, with pin numbers and possibly wire colors identified. The factory service manual is, of course, ideal. Then check various spans of the wiring from one connector to the other, looking for resistance. Almost every circuit of the car wiring will involve a stretch of wire in the harness, with a plug at each end, into which you can insert the ohmmeter probes. The exact locations of these plugs may or may not be intuitive, because wires will break out or into a harness at various points. The color coding helps a lot, though.

A good wire will show about the same resistance you get when you touch the two test leads together (often a tiny bit, owing to the fuse in the meter). The other thing the ohmmeter can do is check for grounding. With one lead connected to ground, you can probe wires that are not supposed to be grounded. Any reading other than "infinite" or out of range suggests a short to ground. Finally, since most such meters are "multi" meters, and include voltage, etc., you can check the voltage at various terminals to see if there is some fault. For instance if you find a terminal that should be seeing 12 volts, and it reads lower, you can suspect a problem.

This might sound stupid, but does the jeep have to be running to test the wiring? Would it be necessary to yank out the dash and instrument cluster? There probably is a short somewhere. The airbag light comes on every now and then, and the guages all go dead. If I bang on the dash, the light goes out and the gauges come back to life. Gremlins? Sounds like the damn thing needs an exorcism. Thanks for all the info.

Deyman said:

This might sound stupid, but does the jeep have to be running to test the wiring? Would it be necessary to yank out the dash and instrument cluster? There probably is a short somewhere. The airbag light comes on every now and then, and the guages all go dead. If I bang on the dash, the light goes out and the gauges come back to life. Gremlins? Sounds like the damn thing needs an exorcism. Thanks for all the info.

Click to expand...

For testing with an ohmmeter, you have everything turned off, and preferably disconnected. The ohmmeter sends its own very small current through a wire, and the wire shouldn't be live. For voltage testing, you would likely have the ignition and whatever accessory you are testing turned on, but whether the engine is running would depend on what you're testing. For most of the things you would test regarding your problem and the PCM you would not have to get behind the dash.

If you have a problem that responds to whacking the dash, I'd bet on a bad connector somewhere. I think some of the later XJ's had a common problem with one of the main plugs on the instrument cluster. Sometimes it's enough to unplug everything you can find and just plug it back in.

Deyman....

From the sounds of it i think the problem you are having is better left to the professionals. Wire and/or sensor and computer problems are among the most difficult to diagnose. There can be so many possibilities and good diagnostic skills are so very important. I would recommend again a second opion from a reputable shop in this case. Good luck man.

I'm going away on vacation on Sat. I'll probably mess around with it when I get back. Thanks for the advice. Need to get an ohmmeter though. As for the exhaust manifold, I'll have to think about that one. Is welding it a possibility? Are there any products out there that will seal a crack? later

Deyman