"Service Engine Soon" Light Inoperative

Diagnostic Circuit:

Circuit Description:
When the engine is started, the ECM grounds terminal "A10" to turn "OFF" the "Service Engine Soon" light. When ALDL terminal "B" is grounded it alternately grounds and opens "A10" to flash a code.

Service Engine Soon Light Diagnostic Chart:

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This checks for open ECM fuse or open in "Service Engine Soon" light circuit, including I/P connector, printed circuit, and "Service Engine Soon" lamp. Normal response is lamp "ON."

2. This checks for a shorted ECM. Grounded ECM terminal "A10" will turn the "Service Engine Soon" light "OFF." If disconnecting ECM turns light "ON," ECM is shorted. Normal response is lamp "ON."

3. This checks for grounded CKT 487 from terminal "C", of lamp driver, to ECM terminal "A1O", an open CKT 439 to terminal "B" of lamp driver, a bad ground, or faulty lamp driver. A normal reading is about 9 to 11 volts, because of the drop through the upper resistor in the lamp driver. Over 11 volts indicates there is no drop in the lamp driver. This indicates a bad ground or faulty lamp driver.

4. This step checks for an open in the wire to terminal "B". Normal voltage is approximately battery voltage.

5. This checks for a grounded CKT 487 from driver terminal "C" to ECM terminal "A10". Normal response is light "ON."