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How to test glow plugs?

How to test glow plugs?

On a diesel vehicle, the purpose of glow plugs is to... preheat the diesel-air mixture so that the vehicle starts faster. An engine that struggles to start or thick black smoke when starting can indicate worn glow plugs.

In the winter, it's important to know if your car's glow plugs are in good condition. Glow plugs wear out over time, and it's important to check them regularly to avoid difficult or impossible starts and excessive fuel consumption.

Fortunately, there are a few simple ways to tell if your glow plugs are working or not.

What are the symptoms of a bad glow plug?

If you notice any of the following symptoms, it's probably time to replace your glow plugs:

  • You're experiencing severe engine vibration;
  • Smoke, especially when the engine is cold;
  • Abnormally high oil consumption;
  • You have trouble starting;
  • Your vehicle uses a lot more fuel than usual.
  • A drop in engine performance
  • Difficulty getting up to speed

These symptoms are often associated with normal or premature glow plug wear. However, these signs of wear can also come from other mechanical parts, such as the injectors, battery or glow relay. That's why it's important to test your glow plugs to be sure.

How to test glow plugs with a multimeter?

Step 1: Remove the hood

Park the vehicle on a level surface and allow the engine to cool. Once the engine is cold, open the hood and remove the engine cover to gain access to the glow plugs.

Step 2: Locate the glow plugs

The location of the glow plugs varies by vehicle model. They are usually located near the fuel injectors.

Step 3: Set up your multimeter

You'll need a multimeter set to ohms. Place the two metal tips of the multimeter's leads in contact, read the multimeter's resistance value, and make a note of that number. You'll need it later to determine the actual resistance of the spark plugs.

Step 4: Test the battery voltage

To test a car battery, first set your multimeter to the DC voltage scale. Next, place the black multimeter lead on the negative terminal of the battery and the red lead on the positive terminal. You should see a reading of 12.5 volts when cold and 13 volts when the engine is running. If you get a reading far from these values, you have a battery or alternator problem. You don't need to replace your glow plugs.

Step 5: Disconnect the battery's negative terminal

For safety's sake, it's imperative to disconnect the battery's negative terminal when opening the hood to avoid the risk of a short circuit.

Step 6: Prepare Glow Plugs by Removing Glow Plug Cap

Once you've identified where the glow plugs are mounted, remove the connector caps to make them accessible. Each glow plug has a cap, and removing them is a fairly straightforward operation, as all you have to do is pull them up. If you notice traces of oxidation, such as rust, on the plugs, take this opportunity to clean them with a wire brush.

Step 7: Measure glow plug resistance

To check the resistance of your glow plugs, connect the black lead of your multimeter to one of the two engine grounds and the red lead of the multimeter to the top of the glow plug.

You should see a reading on your multimeter, from which you'll need to subtract the resistance reading you got from the multimeter (step 3). After the difference, the actual resistance value of each glow plug should be between 0.6 and 2 ohms.

If a glow plug has a resistance of less than 0.6 ohms, you can clean it and test it again to see if its resistance has increased. If not, replace all glow plugs.

Note: If one glow plug is not working properly, all glow plugs must be replaced.

Now you know how to test glow plugs without removing them! You will need to measure their resistance with a multimeter.

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