Chart C-8B Shift Light Check

The shift light indicates optimum shift point, for manual transmission vehicles, to achieve best fuel economy. Light comes on when ECM provides ground to terminal A2.

The ECM uses information from coolant temperature sensor, TPS, VSS, and engine RPM to determine when to turn on shift light.

1. This step should not activate shift light. If shift light is activated, there is a short to ground in Brown/Black wire A2, or a fault in ECM.
2. When diagnostic terminal is grounded, ECM provides ground for terminal A2, and shift light should come on.
3. This step checks shift light circuit to ECM connector. If shift light comes on, ECM connector is faulty or ECM does not have ability to ground circuit.

Fig 1: Chart C-8B Schematic - Checking Shift Light

Courtesy of GENERAL MOTORS CORP.

Fig 2: Chart C-8B Flow Chart - Checking Shift Light

Courtesy of GENERAL MOTORS CORP.