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Jeep's Faulty Instrument Cluster Takes Some Tracing to Find CausePosted 3/11/2008By Tom Vadnais
I recently answered two hotline diagnostic calls concerning dead instrument clusters (IC) with none of the gauges working. One call was on a 1996 Jeep Grand Cherokee Laredo; the other, on a 1997 Jeep Grand Cherokee Limited. In both cases, the tech I was working with had already replaced the IC and the powertrain control module (PCM) with no success; I followed the normal diagnostic routine for a gauge problem. I had him run the cluster sweep test through the actuator test mode (ATM) with the scan tool. All of the gauges cycled through their normal range. I asked the technician to check for communication and fault codes with all of the other modules on the Chrysler collision detection (CCD) bus with the scan tool; all of the modules were responding. The modules involved are the body control module (BCM), PCM, electronic vehicle information center (EVIC), controller anti-lock brake (CAB), radio, driver door module (DDM), passenger door module (PDM), automatic temperature control module (ATC), airbag control module (ABCM) and the IC. When I suggested disconnecting all of the modules except the PCM and the IC, there was still no gauge operation. When the tech checked the CCD bus bias voltage, it was normal at 2.46 volts on the CCD bus plus and 2.49 volts on the CCD bus minus. At this point, all of the diagnostics pointed to a PCM not transmitting the information to the IC on the CCD bus. The tech tried yet another PCM, entered the vehicle identification number (VIN) into the new PCM with the scan tool and still no gauge operation. I then decided to check the power and ground circuits to the BCM, even though according to all of our information, the BCM on this vehicle should have nothing to do with the gauge operation. The power circuits consist of: 1) An orange wire, fused ignition switch output (start/run) on pin No. 18 of the C1 connecter, 2) A violet wire fused (B+) on pin No. 8 of the C2 connecter, and 3) A brown with pink trace wire fused ignition switch output (ACC/Run) on pin No. 15 of the C2 connecter. The grounds consist of: 1) A black/orange wire on pin No. 16 of the C1 connecter, 2) A black wire on pin No. 24 of the C2 connecter, and 3) A black wire on pin No. 8 of the C3 connecter. When checking the power and ground circuits into the BCM, we found that the circuit to pin No. 18 on the orange wire - which should have had fused (B+) from the power distribution center (PDC) - fuse No. 18, a 15-amp fuse, was actually at 0 volts. When we traced the wire back through the circuit from the PDC under the hood to the S101 splice located by the battery temp sensor take-off in the wiring harness at the right front fender to the C236 connecter, we discovered that the wire had corroded at the C236 connecter pin H8. This is a white 10-pin connecter located in the connecter bracket to the left of the steering column under the left side of the dash. Once it was repaired, the IC came back to life with all of the gauges working. A test drive confirmed that the Jeep was fixed.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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