Shift Light: Testing and Inspection

Chart C-8C:

Chart C-8C Wiring Diagram:

CIRCUIT DESCRIPTION:
The shift light indicates the best transmission shift point for maximum fuel economy. The light is controlled by the ECM and is turned "ON" by grounding CKT 456. The ECM uses information from the following inputs to control the shift light.

^ Coolant temperature must be above 16°C (61°F).
^ TPS above 4%.
^ VSS.
^ RPM above about 1900.
^ Air Flow - The ECM uses rpm, MAP and VSS to calculate what gear the vehicle is in.

It's this calculation that determines when the shift light should be turned "ON." The shift light will only stay "ON" 5 seconds after the conditions were met to turn it "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, or has incorrect or faulty PROM.

DIAGNOSTIC AIDS:
A loss of vehicle speed input to the ECM should set a Code 24.