How do I Fix a 6.2 Diesel GMC Glow Plug?

by William Boyce

Glow plugs are essential components in starting a diesel motor. The GM 6.2 L diesel was installed in GM and Chevy trucks between 1982 and 1993 and it had two different electronic controllers during this production period. You must determine which type of controller your ignition system uses, then troubleshoot the ignition system. Glow plugs come on before, during and briefly after engine ignition. How long they stay on depends largely on the temperature of the engine coolant, and they stay on longer in cold weather than in warm weather. A burned-out plug usually indicates problems elsewhere in the ignition system.

Fixing Your GM 6.2 Diesel Ignition System

Make sure you have the right tools before you begin diesel ignition repairs.

To check operation of the glow plug system, put the ignition switch in the run position (but don't crank it) and see if the glow plug light on the dash lights up for eight to 10 seconds and goes out.

Start the engine. The glow plug light should come on briefly after ignition and possibly cycle on several times. If it does not come on or simply stays on, your ignition controller likely needs repair.

Be careful not to short out electrical connections.

Check all the glow plugs with the ammeter. If a glow plug heats up when electrified, it is working properly. If it does not heat up or does not stay heated, replace it.

Be sure all electrical connections are clean and tight.

Check that all your fuses and ground wires are intact, clean and properly tightened.

The ignition controller is often the root cause of glow plugs not working properly.

Get behind the wheel and watch the glow plug light on the dashboard to see if it is cycling properly. If the glow plug light only comes on once after ignition, the problem is likely the controller. Replace the controller.

Replace any glow plugs you determine to be faulty by disconnecting the glow plug wire, then lowering a socket on a socket extender down into the glow plug well. Use your fingers to turn the extender until the socket fits snugly over the glow plug. Attach the socket handle, then loosen and remove the glow plug. Insert the replacement plug in the socket and reverse the procedure, finishing by reconnecting the glow plug wire.


  • check Find a good repair manual if you are serious about learning to repair your GM 6.2 diesel engine's ignition system.
  • check The GM 6.2 L diesel has a gray controller with a silver label. If this does not describe the controller on your 6.2 diesel, it either has an after-market controller or the wrong controller. You should probably check with a diesel shop to verify whether the controller on your diesel engine's ignition system is the right one.
  • check If one or more of your glow plugs is defective, the problem is likely in the electrical system that's supplying energy to the glow plugs. Fixing electrical problems in the ignition system is an involved process requiring a sound knowledge of both electrical testing and diesel ignition. Possible problems include overly advanced injection timing, diode failure at the alternator, faulty glow plug controller, faulty glow plug temperature inhibitor switch, blown fuses, faulty glow plug relays, faulty ground wires, electrical shorts and burned-out or shorted glow plugs. Find a reliable diesel mechanic familiar with the GM 6.2 L diesel for troubleshooting and repairing electrical problems.


  • close The 12-volt currents in this electrical system can cause burns and fire, and short circuits can melt wiring and damage system components.

Items you will need


About the Author

William Boyce started writing professionally in 2009. His work has appeared on and his own blog. Boyce was a registered professional forester in central British Columbia for 24 years. He has a Bachelor of Science in forestry from the University of British Columbia.

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