Mazda Trucks 1972-1986 Repair Guide

Glow Plugs



This engine uses a quick start glow plug system, enabling the operator to start the engine relatively quickly after the key-on sequence. One glow plug per cylinder is used, controlled by a control module, two relays, a resistor, a coolant temperature switch, and clutch and neutral switches.

Relay, power, and feedback circuits are protected by fusible links in the harness. The control module is protected by a 10A fuse in the fuse panel.

When the ignition switch is turned ON, a Wait-to-Start signal appears near the cold start knob on the instrument panel. At this time, relay no.1 closes and full system voltage is applied to the glow plugs. If coolant temperature is below 86ºF (30ºC), relay no.2 also closes. After three seconds, the module turns off the Wait-to-Start light. If the operator does not start the engine and the key is left ON, the no.1 relay opens and cuts off voltage to the glow plugs within three seconds.

However, if coolant temperature is below 86ºF (30ºC) when the no.1 relay opens, the no.2 relay will remain closed, continuing reduced voltage to the glow plugs until the ignition switch is turned OFF.

When the engine is cranked, the control module cycles relay no.1 intermittently, providing the glow plugs with between 4 and 12v, depending on which replay is closed.

Once the engine has started, the alternator output signals the control module to open the no.1 relay and the afterglow function takes over, supplying between 4 and 5v to the glow plugs through the no.2 relay as long as the coolant temperature remains below 86ºF (30ºC).

Once the truck is in motion, the clutch and neutral switches close, opening the no.2 relay if the temperature switch hasn't already done so.


See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Using an ohmmeter to test glow plugsif the reading is greater than 1 ohm, replace the glow plug

  1. Disconnect the leads from each glow plug. Connect one lead of an ohmmeter to the glow plug terminal and the other lead to a good ground. Set the ohmmeter on the X1 scale.
  3. If the ohmmeter indicates less than 1 omega , the problem is not with the glow plug. If the ohmmeter indicates greater than 1 omega , replace the glow plug and retest.


  1. Disconnect the battery ground cables.
  3. Disconnect the glow plug harness.
  5. Using a 12mm deep well socket, unscrew the glow plugs.
  7. Installation is the reverse of removal. Torque the glow plugs to 11-15 ft. lbs.