Ford Escort/Lynx 1981-1990 Repair Guide

Glow Plugs


The diesel start/glow plug control circuit applies power to the glow plugs, which heat the combustion chambers, so that a cold diesel engine can be started.


Power is applied through heavy gauge wires to the starter relay located on the left-hand front fender apron, then to the starter solenoid. When the wait-to-start indicator light goes out, the ignition switch can be turned to the START position. Power is applied to the starter relay. The relay applies power to the solenoid coil, which in turn closes the contacts to apply battery power to the starter motor.

Even after the wait-to-start indicator goes out, the glow plugs must be kept hot because the combustion chambers may not be hot enough to keep the engine running smoothly. To compensate for this, the after-glow relay continues to provide power to the glow plugs until one of the following conditions occurs:

The vehicle moves
The coolant temperature rises above 86ºF (30ºC)
The glow plug voltage goes above 5.7 volts

Vehicle movement is defined as a clutch switch closed (not depressed) and the neutral switch closed (transaxle in any position except Neutral). This means pin 10 of the diesel control module is grounded.

The coolant temperature is measured by the thermoswitch. Whenever the coolant temperature is above 86ºF (30ºC), the thermoswitch is opened and there is no voltage on pin No. 8. This will prevent the entire glow plug circuit from operating, because the engine is hot enough to start and run without the glow plugs working.

If the voltage on the glow plugs is over 5.7 volts, they may overheat and burn out. Therefore, if more than 5.7 volts is detected at pin 11 (with the pre-glow relay off), then the after-glow relay is shut off.

When the ignition is in the START position, 12 volts is put on pin No. 7 of the module. This causes the pre-glow relay to cycle on and off to keep the glow plugs hot. The pre-glow relay will cycle only during cranking.

Power from fusible link B (which is connected to the starter relay) is applied to the fuel solenoid with the ignition switch in the START or RUN position. The fuel solenoid opens the fuel line to permit the engine to run. When the ignition switch is in the OFF position, the solenoid cuts off fuel flow and stops the engine.


The solid state diesel control module is mounted under the left-hand side of the instrument panel. It controls glow plug pre-glow time, after-glow time and the operation of the wait-to-start indicator light.

When the ignition switch is placed in the RUN position, the wait indicator lamp lights and the pre-glow No. 1 relay and the afterglow No. 2 relay go into operation. Voltage from the ignition switch is applied through pin 6 of the control module and then to the relays through pins 2 and 3. The contacts of the pre-glow relay close and the power is applied from fusible link S (located at the left-hand side of the engine above the starter) to operate the glow plugs. The plugs then start to heat up.

With power applied to the glow plugs, voltage is returned through circuit 472 yellow wire with a black tracer to the control module at pin No. 11. After three seconds, the wait-to-start indicator goes out and stays out. The glow plugs are now warm enough for the engine to be started. After three more seconds the pre-glow relay opens. Power is now applied through the after-glow relay and the dropping resistor (located in the air intake at the engine manifold) to keep the glow plugs operating at a reduced voltage.

The after-glow and the pre-glow relays are located at the top center area of the dash panel, which could be either underneath the instrument panel or near the center of the firewall.


See Figures 1, 2, 3, 4 and 5

Follow the steps described in the accompanying charts.

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Fig. Fig. 1: Glow plug system diagnosis

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Fig. Fig. 2: Glow plug system diagnosis (continued)

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Fig. Fig. 3: Glow plug system diagnosis (continued)

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Fig. Fig. 4: Glow plug system diagnosis (continued)

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Fig. Fig. 5: Glow plug system diagnosis (continued)


  1. Disconnect the battery ground cable from the battery, located in the luggage compartment.
  3. Disconnect the glow plug harness from the glow plugs.
  5. Using a 12mm deep-well socket, remove the glow plugs.

To install:
  1. Install the glow plugs, using a 12mm deep well socket. Tighten the glow plugs to 11-15 ft. lbs. (15-20 Nm).
  3. Connect the glow plug harness to the glow plugs. Tighten the nuts to 5-7 ft. lbs. (7-9 Nm).
  5. Connect the battery ground cable to the battery in the luggage compartment.
  7. Check to ensure proper glow plug system operation.