Jeep electrical problems - A thing to try
This was posted on http://www.allpar.com/fix/gauges.html; it relates to the the same problems (dead gauges, etc) on a Plymouth minivan, but it sounds like his issue ultimately was with the BCM.
My wife's 92 Plymouth Voyager minivan instrument cluster went on the blink. Every gauge in the instrument cluster was dead with the "check gauges" light on. This means no speedometer, odometer, fuel gauge, etc., almost like driving with blinders on.
The factory service manual states the mechanical instrument cluster is electronic with analog display gauges. All messages to the instrument cluster gauges are sent over the "CCD" bus from the body computer. The CCD bus is a two wire differential communication bus which allows the micro-processor in the body computer to communicate with the instrument cluster microprocessor.
I called the local Plymouth/Chrysler dealer and they said they would fix it (for a small fee $$). After questioning them on what they do to fix it the service rep broke down and said it goes to an outside specialty repair service company that just fixes onboard auto electronics. He was nice enough to give me their name and phone number, and said I could go direct to them for the repair of my instrument cluster.
Ray Parker wrote: Hey, I just fixed my 89 year old mother's 1992 Dodge Caravan by following your instructions. I replaced only the Q110 transistor as all her gauges were dead, and put the body computer back in and voila, it worked again, thus saving her a bucket of money. Thanks.
I called this specialty repair company and told them my problem with the nonfunctional instrument cluster. They said they would be glad to fix it for $229 plus $58 removal/installation. I asked to speak to a tech to help me diagnose the problem over the phone to be sure this was the problem. A tech came on the phone and after telling him about my instrument cluster failure he said the problem is not in the cluster electronics but with the body computer! They have repaired over 1,000 of these body computers and it has fixed the instrument cluster problem only by replacing it with a later model body computer. The tech asked me to check the part number on the body computer to see if it was a 4686058 (it was). This part number was known to fail and the fix is to replace it with a new model 4741423 body computer.
Since I figure I was out $300 I decided to pulled the body computer module and check the PC board out myself. The body computer is under the steering column cover just to the right of the steering column in a sheet metal box with two large connectors. Once the cover is off it removes very easily with just two screws, but you should pull two connectors cabled to the body computer first. The body computer sheet metal cover has one screw (star type) attaching it. I removed this metal cover and popped out the pc board. It is a typical high volume low cost circuit board with a Motorola microprocessor and lots of discrete parts (capacitors, resistors, transistors). I took out my trusty digital volt/ohm meter and after a few minutes found two bad transistors, Q105 and Q110. The transistors were both found to have open base-emitter junctions. Without a schematic, I could trace one transistor to the chime transducer and the other to what appeared to be a bias for the "CCD" bus. Both were marked with factory numbers but I could deduce that the were "NPN" type transistors in a plastic TO-92 package. I called my brother, an EE who has an electronic lab in his home, and ask him to recommend a substitute part. He said he has a drawer full of 2N3904 (which you could pick up at a local Radio Shack for less than a buck) and to come over and get a couple. After carefully unsoldering the defective parts and soldering in the new transitors it would be a matter of minutes to check it out. I reinstalled the body computer and upon inserting the key into the ignition with the driver side door open I heard the familiar "key-in-ignition" chime again!! Good so far!
Now I started the engine and all the instrument cluster gauges came alive again!! Success!!! My brother figured the original transistors were from a bad batch and were doomed to a short life (about 4 years I guess).
I don't recommend this repair approach for everyone, but with some electronic knowledge you should be able to do this body computer repair yourself saving hundreds of dollars.
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