Ford Ranger/Bronco II 1983-1990 Repair Guide

Glow Plug System


The quick start; afterglow system is used to enable the engine to start more quickly when the engine is cold. It consists of the four glow plugs, the control module, two relays, a glow plug resistor assembly, coolant temperature switch, clutch and neutral switches and connecting wiring. Relay power and feedback circuits are protected by fuse links in the wiring harness. The control module is protected by a separate 10A fuse in the fuse panel.

When the ignition switch is turned to the ON position, a Wait-to-Start signal appears near the cold-start knob on the panel. When the signal appears, relay No. 1 also closes and full system voltage is applied to the glow plugs. If engine coolant temperature is below 30°C (86°F), relay No. 2 also closes at this time. After three seconds, the control module turns off the Wait-to-Start light indicating that the engine is ready for starting. If the ignition switch is left in the ON position about three seconds more without cranking, the control opens relay No. 1 and current to the plugs stops to prevent overheating. However, if coolant temperature is below 30°C (86°F) when relay No. 1 opens, relay no. 2 remains closed to apply reduced voltage to the plugs through the glow plug resistor until the ignition switch is turned off.

When the engine is cranked, the control module cycles relay No. 1 intermittently. Thus, glow plug voltage will alternate between 12 and four volts, during cranking, with relay No. 2 closed, or between 12 and zero volts with relay No. 2 open. After the engine starts, alternator output signals the control module to stop the No. 1 relay cycling and the afterglow function takes over.

If the engine coolant temperature is below 30°C (86°F), the No. 2 relay remains closed. This applies reduced (4.2 to 5.3) voltage to the glow plugs through the glow plug resistor. When the vehicle is under way (clutch and neutral switches closed), or coolant temperature is above 30°C (86°F), the control module opens relay No. 2, cutting off all current to the glow plugs.


  1. Disconnect the leads from each glow plug. Connect one lead of the ohmmeter to the glow plug terminal and the other lead to a good ground. Set the ohmmeter on the X1 scale. Test each glow plug in the like manner.
  3. If the ohmmeter indicates less than 1 ohm, the problem is not with the glow plug.
  5. If the ohmmeter indicates 1 or more ohms, replace the glow plug and retest.


See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Glow plug harness and buss bar, 2.3L

  1. Disconnect both battery ground cables.
  3. Disconnect the glow plug harness from the glow plugs. On the 2.3L diesel engine remove the nuts attaching the bus bar to the glow plugs.
  5. Using a 12mm deepwell socket, remove the glow plugs.
  7. Install the glow plugs, using a 12mm deepwell socket, and tighten to 11-15 ft. lbs.
  9. Install the glow plug harness on the glow plugs. On the 2.3L diesel engine install the nuts attaching the bus bar to the glow plugs.
  11. Connect both battery ground cables.