Diagnostic Aids

The PCM/TCM relies on the Power Switch Module (PSM) to indicate what gear range has been selected. PSM must be functioning properly or Code 85 may be set. Check all connections, especially those at the transmission pass-through connector. Compare Scan tester gear ratio reading to specifications in GEAR RATIO table.

Fig 1: Code 85, Undefined Ratio Troubleshooting Chart (Lt. Truck)

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