Possible Cause & Correction
To track down possible causes of an intermittent SERVICE ENGINE SOON light, check the following items:
- Check for poor mating of one connector to another. Terminals may not be fully seated. Check for improperly formed or damaged terminals. Check wire to terminal connections.
- Check for poor connection from ignition coil to ground or arcing at spark plug wires or plugs.
- Check wire from SERVICE ENGINE SOON light to ECM for short to ground.
- Check wire from test terminal "B" of ALDL for intermittent short to ground.See Figure.
- Check for poor connections in ECM ground terminals.
- Check for loss of trouble code memory. To check code memory on fuel injected models, disconnect TP sensor and run engine at idle until SERVICE ENGINE SOON light comes on. Code 22 (or appropriate TP sensor code) should be stored and retained in memory when ignition is turned off. If code is not stored, ECM is faulty.
- Check for electrical system interference caused by a defective relay, or an ECM-driven solenoid or switch which may cause sharp electrical surge. This type of problem will normally occur when faulty component is operated.
- Check for aftermarket parts which may not have been produced to manufacturer's specifications. Solenoids without original- equipment diodes for circuit protection, and HEI-EST module or voltage regulator using transistors instead of silicon- chip circuitry may possibly cause voltage surges (up to 300 volts) in ECM wiring, causing temporary ECM shutdown. ECM shutdown is a normal response to system overvoltage (greater than 16-17 volts on most models). ECM will repower when condition no longer exists. This could cause a flickering SERVICE ENGINE SOON light and stumble, with no codes set in memory.
- Check for any open diodes in A/C or engine wiring.
- Check for improper installation of electrical accessories such as auxiliary lights or 2-way radios.
- Ensure EST wires are kept away from spark plug wires, distributor wires, distributor housing, ignition coil and generator. Ensure ground wire from ECM to distributor or ignition module is connected to a good ground.