GMC Diesel Glow Plug Diagnosis
Douglas W. Usherwood
23 December 2012
The electronic glow plug control system uses an electronic module and a control sensor to control glow plug temperature, the glow plug relay and the wait light. An engine temperature switch controls the fast idle solenoid and housing pressure cold advance solenoid. The electronic module is located to the right of the steering column below the instrument cluster. It monitors the glow plug voltage and engine temperature to control the low plugs and wait light. The module also contains circuits to monitor the system for failures and hold the wait light on to indicate a problem in the system.
The control sensor is located in the right front intake manifold coolant passage; it senses engine coolant temperature to control glow plug feedback voltage to the module. The glow plug relay is located on the right inner fender filler panel and is pulsed on and off by the electronic module to control current to the glow plugs. This pulsing maintains glow plug temperatures without overheating. Do not manually energize or by-pass the glow plug relay as glow plugs will be damaged instantly.
The engine temperature switch is located on the left rear head bolt. At engine temperature below 95 degrees this switch will close to energize the fast idle relay which then energizes the fast idle solenoid. When the engine temperature switch closes, it also energizes the housing pressure cold advance solenoid which is in the injection pump. When the engine temperature reaches about 125 degrees the engine temperature switch opens, de-energizing both solenoids. The fast idle relay is located on the cowl and when energized provides voltage to energize the fast idle solenoid. When not energized, voltage may also pass through contact points in the fast idle relay to the fast idle solenoid anytime the A/C compressor clutch is engaged.
For normal operation turn ignition to run. The wait light and the glow plugs will come on. The wait light will remain on until glow plugs reach temperature the first time (which will take about seven seconds at zero degrees) and then the light will go out indication the engine is ready to start. There is no start light used with this system. The glow plugs will continue to pulse on and off after the engine is started for about twenty five seconds to provide afterglow. If the ignition is turned to run and the engine is not started (when it is freezing out) the glow plugs will continue to pulse on and off until the batteries urn down (usually around four hours). The wait light will stay on, if the system has a problem, to warn the driver to have the system checked. In most cases the engine can still be started but the glow plug control system should be repaired. The glow plug for the diesel engine is an important part and should never be overlooked and/or abused. Take good care of this system and it will take care of you on that cold day in December when you want your engine to start!
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