How to Troubleshoot Glow Plug Problems

by Tammy Bronson
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Glow plugs in a diesel engine preheat the injection chamber, making it easier to start the engine. The glow plug relay controls the glow plug system by the pre-heat cycle based on the reading from the temperature sensor of the cylinder head. Troubleshoot for glow plug problems if your car is running rough and has no pick-up, or if black smoke is pouring out of the rear of your car.

Step 1

Evaluate which parts of the glow plug system are easily accessible. Start with the easiest and work your way to the hardest. This process will help determine the logical order for testing the various parts of the glow plug system.

Step 2

Find the wire from the glow plug temperature relay. If the wire is disconnected, reconnect it.

Step 3

Observe if the glow plugs are getting the power they require by connecting a test light to the plug. If the test light lights up, the plug is receiving power.

Step 4

Evaluate whether or not the relay is able to supply the power required to start the diesel engine by testing the ohms of the circuit with an ohmmeter. A working relay will produce between 50 and 120 ohms. If the circuit is producing less than 50 ohms, the relay is suspect.

Step 5

Use a relay socket to test each individual pin, which must have voltage and/or power: 30 must have battery voltage, 50 must have power when the engine is running, 85 must show battery voltage, 86 must have power when the key is turned to the pre-glow slot, 87 must have power to the plugs directly, L must show that power is sent to the LED in the dashboard and T must show that the temperature sender is receiving input from the cylinder head. If any of these pins are not receiving or sending voltage and/or power, the relay is not working properly.

Step 6

Check the fusible link for loose wires or nuts, and if the contacts are dirty.

Run a wire from the glow plug to the LED on the dashboard. If you hear a clicking and see the light go on and off, yet there is no power reaching the glow plug, you know that the relay is faulty.

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