Shift Indicator - A/T: Testing and Inspection

Chart C-8D, 1-4 Shift Light Electrical Diagnosis:

1-4 Shift Light Circuit Diagram:

CIRCUIT DESCRIPTION:
The One to Four light display is in the instrument panel. The purpose of the One to Four light is to provide a display to the driver, which indicates the transmission should be shifted from first to fourth gear, for maximum fuel economy. The light is controlled by the ECM and is turned "ON" by grounding circuit 776. The One to Four light is turned "ON" if all the conditions listed below exist.

^ Vehicle speed is greater than 12 mph but less than 19 mph.
^ Engine speed less than 1200 rpm.
^ Throttle position is less than 35%.
^ Engine coolant temperature greater than 50°C (122°F).

The ECM uses the measured rpm and the vehicle speed to calculate what gear the vehicle is in. It is this calculation that determines when the One to Four light should be turned "ON".

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. This should NOT turn "ON" the One to Four light. If the light is "ON," there is a short to ground to circuit 776 wiring or a fault in the ECM.
2. When the diagnostic terminal is grounded, the ECM should ground circuit 776 and the One to Four light should come "ON".
3. This checks the One to Four light up to the ECM connector. If the one to four light illuminates, then the ECM connector is faulty or the ECM is unable to ground the circuit.
4. This would indicate a possible problem in the instrument cluster or wiring.