Shift Light: Testing and Inspection

Chart C-8A:

Chart C-8A Wiring Diagram:

CIRCUIT DESCRIPTION:

The shift light indicates the best point for maximum fuel economy. The ECM uses information from the following inputs to control the shift light:

^ Coolant temperature
^ TPS
^ VSS
^ RPM

The ECM uses the measured rpm and the vehicle speed to calculate what gear the vehicle is in. It's this calculation that determines when the shift light should be turned on.


TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. This should not turn "ON" the shift light. If the light is "ON", there is a short to ground in CKT 456 wiring, or a fault in the ECM.

2. When the diagnostic terminal is grounded, the ECM should ground CKT 456 and the shift light should come "ON."

3. This checks the shift light circuit up to the ECM connector. If the shift light illuminates, then the ECM connector is faulty or the ECM does not have the ability to ground the circuit.