Glow Plug Circuit

Circuit Description:
With the key "ON," and coolant temperature below approximately 65°C (150°F), the ECM turns "ON" the cold advance relay to supply 12 volts to the glow plug controller and the cold advance solenoid. The controller, in turn, will cycle the glow plugs "ON" and "OFF" for a predetermined amount of time. After the engine is cranked, the glow plugs will again cycle "ON" and "OFF," for a short period of time.

Test Description:
Numbers below refer to circled numbers on the diagnostic chart.
1. Disconnecting the coolant sensor sends a low temperature signal to the ECM. The ECM turns "ON" the cold advance relay which sends 12 volts to Pin "D" of the glow plug controller. if the test light comes "ON" during this test, the cold advance circuit and ECM are "OK."
2. If the relay contacts are shorted together, there will be 12 volts supplied to the glow plugs at all times. Since these are 6 volt glow plugs, they will be damaged by the constant high voltage.
3. This step checks for high resistance in the feedback circuit to the controller. High resistance in this circuit will signal the controller to turn "OFF."
4. This step determines if the controller is cycling its internal relay. The test light should cycle "ON" and "OFF" if the controller is operating.
5. This step checks for a complete circuit between the controller and each glow plug. If the test light fails to light, that glow plug feed circuit is open. It may be necessary to turn the key "OFF" for 10 seconds and "ON" again to keep the controller cycling.