Glow Plugs Stuck On – Do this Immediately!

In diesel trucks it is normal for the glow plug indicator to come on when you insert the key, and go off indicating that the glow plugs are warm and the truck is ready to start. This system includes a few sensors, a controller, and a relay, and there are a few things that can go wrong causing the glow plug light to stay on. But there is one that is seen a lot more in the repair shop than the rest. Here is how to fix your glow plugs that are stuck on!

What do I do if the glow plug light won’t go off?

Shut the truck down right away and check the glow plug relay. If the glow plug light won’t go off, it could mean that the glow plugs are staying on, which will result in them burning out. Locate the glow plug relay, and tap it until it is knocked free and working again, or disconnect power to the glow plugs before driving the truck.

A Faulty Glow Plug Relay Causes your Glow Plugs to Stay On

After shutting down the truck, since it is already warm, you do not need the glow plugs for now. So if you are in a hurry and need to get somewhere, go ahead and disconnect the glow plug controller or the wire at the glow plug relay and get to where you need to go! This will prevent continuous power from burning out your glow plugs.

The glow plug relay is a very common reason for this to happen on diesels and is most likely the problem.

If you are familiar with starter motors, the same thing happens to the relay for glow plugs as it does the relay for the starter. The high current wears on the relay and can arc and cause the relay to be stuck in the closed position.

If you are lucky, a tapping on the relay with a screwdriver can knock it free. Otherwise you need to replace the relay.

The truck is still drivable and if it starts up, then the glow plugs are working fine. The problem is, if the glow plugs stay on continuously once the engine gets hot, if they do not go off the glow plugs will burn out. To prevent this from happening, if the glow plugs are stuck on, disconnect or replace the glow plug relay.

The glow plug controller is another likely cause of the issue, but much more likely it is the relay and should only be pursued as an option once the relay has been tested.

How to test glow plug relay:

The quickest easiest way is to have someone turn the key to the accessories position while you listen closely near the glow plug relay. You should hear a distinct click. If you do not then check it with a multimeter.

But the point of the check with your ears is that you may be able to very quickly determine that the relay is fine if you hear it click.

The relay acts as a switch when current is applied to it and that is what causes a click.

If this does not work for you, this video should be helpful to learn how to diagnose the relay properly.

Where is the Glow Plug Relay Located?

The glow plug relay is located in the engine bay generally mounted somewhere on top of the engine, the back of the firewall, or the side of the inner fender wells. The glow plug relay is identifiable by the cables that go directly to it from the glow plug controller and from the relay to the glow plugs.

Where is the Glow Plug Sensor Located?

The glow plug sensor is usually located on the passenger side valve cover near the front of the engine, or at the rear of the intake manifold, or near the firewall.

Other Causes of Glow Plugs that Stay On

There are two other primary causes of glow plugs that are stuck in the on position:

The glow plug controller or the temperature sensors.

A Bad Glow Plug Controller May Cause Glow Plugs to Stay On

The glow plug controller is the second most likely cause of the glow plug light and the glow plugs to stay stuck in the on position. Once you have confirmed that the glow plug relay is in working condition, remove and replace the glow plug controller. There is no easy DIY way to test this other than replacing it or sending the unit in to be tested.

Faulty Sensors may cause Glow Plugs to Stay On

Temperature sensors measure the temperature of the coolant and the ambient temperature for the glow plug controller to read. If one of these sensors have gone bad. The last thing you can do to fix your glow plugs that stay stuck in the on position is to replace both of these sensors.

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