Skylark L4-138 2.3L DOHC VIN D MFI (1988)
The shift light indicates the best transaxle shift point for maximum fuel economy. The light is controlled by the ECM and is turned "ON" by grounding
CKT 422.
The ECM uses inputs from various sensors to calculate when the shift light should be turned "ON" as follows:
Coolant temperature must be above -10~C (14~F).
ECM can determine the transaxle has been in a gear for at least 1.2 seconds, by comparison of vehicle speed (from VSS) with engine rpm.
TPS is between minimum and maximum calibrated values for each gear.
Rpm is above a calibrated value for each gear (maximum 6500 rpm).
The light will be turned "ON" after a calibrated delay time which is dependent on last gear change or downshift, and will remain on for a minimum of 10
seconds.
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 422 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.
Diagnostic Aids:
Check for Code 24 (no VSS). Faulty thermostat or incorrect heat range. Incorrect or faulty PROM.
