Gauge Cluster Inoperative and Data Bus Diagnosis
A 2021 truck has a complaint that the speedometer and tachometer are inoperative, and several warning lamps are illuminated. The technician scans for diagnostic trouble codes (DTCs) and finds U0100 (Lost Communication with ECM/PCM) and U0155 (Lost Communication with Instrument Panel Cluster). The battery voltage is 12.8V at rest and 14.3V running. The data bus is a CAN system with terminating resistors. The technician measures resistance between CAN High and CAN Low at the diagnostic link connector (DLC) with the key off and finds 120 ohms. The cluster powers up but shows dashes for speed and RPM. The ECM is communicating with the scan tool.
1. What is the likely cause of the communication issues, and how should the technician proceed?
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The DTCs indicate lost communication with the ECM and the instrument panel cluster (IPC). The CAN bus resistance of 120 ohms is within the normal range (typically 60 ohms with two 120-ohm terminating resistors in parallel, but some systems may show 120 ohms if one resistor is internal to the ECM and the other is in the IPC; however, 120 ohms suggests one terminating resistor is missing or the bus is open). The ECM is communicating with the scan tool, so the ECM is likely functional. The IPC is powered but not communicating, suggesting a fault in the CAN bus wiring to the IPC or a faulty IPC. Proceed by checking for continuity and shorts on the CAN High and CAN Low circuits between the ECM and IPC. Also, inspect connectors for corrosion or loose pins. If wiring is good, substitute a known-good IPC to test.
2. If the CAN bus resistance is 60 ohms, how would that change your diagnosis?
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If the resistance is 60 ohms, the bus termination is normal, indicating both terminating resistors are present and the bus is not open. In that case, the communication issue is likely due to a faulty IPC or a problem with the IPC's power or ground, or a fault in the CAN wiring specific to the IPC (e.g., a short to ground or power on one of the CAN lines). The technician should then check for voltage on the CAN lines (should be around 2.5V on both with key on) and look for shorts. Also, verify the IPC's power and ground circuits. If all are good, the IPC may be defective.