Fixing Instrument Panels
Ray Parker wrote:
My wife's 1992 Plymouth Voyager minivan instrument cluster went on the blink. Every gauge in the instrument cluster was dead with the "check gauges" light on.
The instrument cluster is electronic, with analog display gauges. All messages to the instrument cluster gauges are sent over the two-wire differential communication "CCD" bus from the body computer.
The local Plymouth/Chrysler dealer said they would fix it, but that it goes to an outside specialty repair service company; and gave me the company’s phone number. They said they would be glad to fix it for $229 plus $58 removal/installation; but their tech 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.
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 easily, with just two screws, but you should pull the 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 board. It is a typical high volume, low cost circuit board with a Motorola microprocessor and lots of discrete parts (capacitors, resistors, transistors). Using my trusty digital volt/ohm meter, I found two bad transistors, Q105 and Q110. The transistors both had 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. They 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 transistors, 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! The cluster gauges all came alive on startup. 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.
Another dead gauge cluster
Ken Humphrey’s 1992 Dodge Daytona had a dead gauge cluster; he tried swapping one in from another Daytona but it made no difference. “The tachometer was dead but now seems to have come back to life, but the speedometer and the fuel, oil, volt and temp gauges are consistently out.” The Traveler’s trip odometer was still working.
Bob Lincoln commented that, if the odometer is working, then the vehicle speed sensor is good, and it's sending the signal to the BCM (body computer). If both clusters do not work, it suggests that the problem may be in the harness to the cluster. There are two plugs as I recall, both very tight-fitting, so I would not suspect a loose connection at the plug.” (It was not the fuse — there is one for the instrument panel.)
Ken checked pin 14 to ground and got 90 ohms resistance, indicating a bad ground. Russ90Daytona wrote, “Sounds like the firewall connector would be the most likely problem. Use a can of electrical contact cleaner on the connections while you're in there.”
Ken came back and noted that “the wiring harness that crosses the firewall from the connector and then runs across the passenger side strut tower had come loose from the tower and was rubbing against the power steering pump pulley. It wore throught two wires: one red that has 12V all the time and a blue with orange trace wire (this one was held together by a single strand). I thought for sure I'd found the problem but after repairing the wires the cluster is still dead, except for the tachometer. I added a pigtail off of the cluster connection ground wire and ran it to a metal frame brace and verified that there was 0 ohms from pin 14 to the frame -- still no difference. According to the service manual, is the voltage and ground check out OK and the gauges don't work, then the PCB (computer) is bad.”
Bob Lincoln noted that the small daughter board on the right side the cluster is the tachometer drive board. The PCB is the entire board the gauges connect to.
The solution, from Ken: “I got the ground repaired at the connector but the gauges were still inoperative. I pulled the gauges out of the cluster and checked the voltage and resistance at the pins -- resistance to ground was high so I removed the PCB and re-soldered all the connections, sprayed contact cleaner in the plugs and tried it again. The gauges now work, and the speedometer came back to life after driving a few miles.”