The glow plug system has the following parts (refer to Figure 8-12):
• Eight glow plugs
• An electronic controller/relay assembly
• A GLOW PLUGS indicator lamp
• Related wiring
The glow plug system is used to assist in providing the heat required to begin combustion during engine starting at cold ambient temperatures. Electrical heating occurs before and during cranking, as well as during initial engine running.
Each glow plug threads into a cylinder head hole so that its tip is near the center of the pre-combustion chamber (see Figure 8-13, view A). The glow plug has an electrical heater coil element (rated at 6 volts) that is designed to provide very rapid heating when a 12-volt power circuit is completed for a few seconds (see 8-13, view B).
Each glow plug connects to power from a terminal stud of the electronic controller/relay assembly, using a harness wired in parallel (refer to Figure 8-15). The other terminal stud of the electronic controller/relay connects to battery power, using fusible link protection at the vehicle junction block. Each glow plug is grounded through its cylinder head mounting.
The glow plug wiring for cylinders #4 and #6 is protected from exhaust manifold heat by shields (see Figure 8-14, view A). At each of these locations, an extension wire connects the glow plug terminal with its wiring harness terminal (see Figure 8-14, view B). During service, a special tool is used for removal and installation of the extension wires.
The electronic circuit inside the controller/relay receives power at terminal D that is controlled by the ignition switch and protected by the GAGES fuse (see Figure 8-16). The ground side of the electronic circuit inside the controller/relay connects to a wire at terminal E. The wire has a terminal that attaches to the vehicle sheet metal at a point near the junction block.
The electronic controller receives a feedback signal at terminal C from a wire in the glow harness (refer to Figure 8-17). The cranking motor solenoid circuit also provides an electrical input to the electronic controller/relay at terminal B.
The glow plug harness includes parallel wiring for the GLOW PLUGS indicator lamp mounted in the instrument cluster (refer to Figure 8-17). The lamp will illuminate whenever the electronic controller/relay provides power to the glow plugs.
The glow plug controller electronic circuit operates the relay with cycling action that varies in length, based on the underhood air temperature and engine temperature sensed at its mounting bracket.
At room temperature, the glow plug system operates as follows:
• When the vehicle driver rotates the ignition switch to the RUN position, the following things occur (refer to Figure 8-18):
– The controller electronic circuit completes the relay coil circuit, causing glow plug and indicator lamp operation for 4 to 6 seconds.
– Based on the temperature and feedback inputs, the controller electronic circuit opens the relay coil circuit for 4 to 5 seconds.
Note: At this time, the vehicle driver would normally rotate the ignition switch to the CRANK position to start the engine.
• If the vehicle driver keeps the ignition switch in the RUN position, the following things occur:
– The controller electronic circuit completes the relay coil circuit again, causing glow plug and indicator lamp operation for 1 to 2 seconds.
– Based on the temperature and feedback inputs, the controller electronic circuit opens the relay coil circuit for 4 to 5 seconds.
– The ON/OFF cycling action will continue until a total cycling time of approximately 20 seconds has elapsed.
• If the vehicle driver rotates the ignition switch to the CRANK position during or after the previous cycling sequence, the following things occur (refer to Figure 8-19):
– The controller electronic circuit completes the relay coil circuit again, causing glow plug and indicator lamp operation for 1 to 2 seconds.
– Based on the temperature and feedback inputs, the controller electronic circuit opens the relay coil circuit for 4 to 5 seconds.
– The ON/OFF cycling action will continue until the total cycling time after the ignition switch has returned to the RUN position is approximately 20 seconds.
Note: The maximum length of glow plug cycling does not depend on whether or not the engine runs after cranking.
Glow plug system cycling times are approximate, because temperature and feedback voltage inputs vary. As a rule, colder ambient starting temperatures result in longer initial ON times and total duration of cycling.
A controller/relay that applies power to the glow plugs for longer than five seconds may cause damage to the glow plugs. If all eight glow plugs are replaced because of open circuit faults, the controller/relay should also be replaced.