Service Engine Soon Light Inoperative
When engine is started, the ECM grounds terminal A10 to turn off the SERVICE ENGINE SOON light. When ALDL terminal "B" is grounded, ECM alternately grounds and opens terminal A10 to flash a code.
Test Number 1) This tests for an open ECM fuse or an open in the SERVICE ENGINE SOON light circuit, including instrument panel connector and printed circuit board. Light "on" is the normal response.
Test Number 2) This tests for a shorted ECM. A grounded ECM terminal A10 will cause the SERVICE ENGINE SOON light to go off. If light comes on after disconnecting ECM, ECM is shorted. Light "on" is the normal response.
Test Number 3) This tests for a grounded circuit No. 487 from terminal "C" of remote lamp driver to ECM terminal A10. It also checks for an open circuit No. 439 to terminal "B" of remote lamp driver module, a bad ground, or a faulty remote lamp driver. Because of the voltage drop through the remote lamp driver upper resistor, a normal voltage reading should be 6-11 volts. If voltage is greater than 11 volts, there is no voltage drop in the remote lamp driver module due to bad ground or faulty module.
Test Number 4) This tests for an open wire to remote lamp driver terminal "B". Normal reading should be close to battery voltage.
Test Number 5) This tests for a grounded circuit No. 487 from terminal "C" of remote lamp driver to ECM terminal A10. Light "on" is the normal response.