Glow Plug Circuit Check

Glow Plug Circuit Check:

Glow Plug Circuit - C/K:

CIRCUIT DESCRIPTION
With the ignition switch "ON," and engine coolant temperature below approximately 65°C (150°F), the ECM/PCM turns "ON" the cold advance relay to supply 12 volts to the glow plug control switch and the cold advance solenoid. The control switch, in turn, will cycle the glow plugs "ON" and "OFF" for a pre-determined 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
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Disconnecting the engine coolant sensor sends a low temperature signal to the ECM/PCM. The ECM/PCM turns "ON" the cold advance relay which sends 12 volts to Pin "D" of the glow plug control switch. If the test light comes "ON" during this test, the cold advance circuit and ECM/PCM 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 signal circuit to the glow plug control switch. High resistance in this circuit will signal the glow plug control switch to turn "OFF."

4. This step determines if the glow plug control switch is cycling its internal relay. The test light should cycle "ON" and "OFF" if the glow plug control switch is operating.

5. This step checks for a complete circuit between the glow plug control switch 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 glow plug control switch cycling.