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Know your auto parts: Glow plug control modules

Glow plug control modules work hard to ensure your diesel car starts, but what else do you know about them? For example, do you know how they work? How to choose the right glow plug control module for your vehicle? Or how and when to replace them? Continue reading to find out more.

What is a glow plug control module?

As you already know, a glow plug is a heating device in each cylinder of a diesel engine that helps the engine to start. But how is the overall operation of your glow plugs monitored and controlled? This is done by a glow plug control module.

Located on the engine and with a direct connection to the engine control unit, the control module uses a microprocessor to analyse input from the engine control unit to determine information related to the working of the glow plugs. This includes when to switch the current for the glow plugs on and off, and how much current is required as this will differ depending on the starting temperature of the engine. The current to the glow plugs is turned on during the pre-heating to warm up the engine, stand-by time when the engine should be started, and post-heating to meet increasingly strict exhaust gas standards and optimisation of combustion processes.


When you start up a diesel-powered vehicle, the following actions happen:

  1. The glow plug control module determines the timing and electric current required by the glow plugs based on input from the Engine Control Unit;
  2. The heating element in the glow plug is electrified so it heats up and emits visible light (hence the name);
  3. Intake air is compressed;
  4. The fuel injector spray directs fuel onto the hot tip of the glow plug;
  5. The injected fuel mixes with the compressed air, evaporates and starts combustion almost simultaneously, even when the engine is cold;
  6. The glow plug remains on during the pre-heating phase to ensure correct combustion of the fuel and reduce the environmental impact of your car.

This process is helped by the increase in temperature of the intake air during compression and the relatively low ignition temperature of diesel. However, a glow plug control module is still essential, as without it controlling your glow plugs, the compression temperature might not be high enough to ensure proper ignition of the injected fuel in a cold diesel engine. This would result in increased exhaust emissions.

What are the symptoms of a defective glow plug control module?

A diesel engine has one glow plug control module which controls the glow plugs. This means that you might not notice when a single glow plug goes bad as this only affects one cylinder in your engine. However, if two of three go bad or the control module itself malfunctions, you’ll quickly find it difficult to start the engine. In addition to watching out for the glow plug light on your dashboard, here are some common symptoms of a defective glow plug or glow plug control module:

  • White exhaust smoke – a defective glow plug will cause diesel to leak into the exhaust where it burns.
  • Hard to start – this could mean the engine is slow to start in warm weather or fails to start in cold weather. However, this can also be a symptom of a problem in the fuel system or battery.
  • Poor engine power – after a difficult start, bad glow plugs will make it difficult for your car to run properly, due to improper combustion which reduces power and efficiency.

How can you test your glow plug control module?

First things first, is it a problem with your glow plugs or your glow plug control module?

The easiest is to check your glow plugs before testing your glow plug control module.

To test your glow plugs, simply connect a 12-volt test light to the positive battery terminal. Then disconnect the wires from each of your glow plugs and touch the probe of the test light to the terminal of the glow plug itself (not the wiring harness). If the light doesn’t light up, the glow plug is bad and should be replaced.
Easier is to use a professional glow plug tester device.

There is some discussion about whether you should only replace the glow plugs that have gone bad, or if it is best to replace them all. As one glow plug going bad could be a sign that the others will shortly follow, some experts recommend replacing them all at the same time while others suggest keeping a close eye on the remaining glow plugs.

If your glow plugs are all working correctly, move onto your glow plug control module. Start by checking the battery voltage to the glow plug control module, a voltage drop of over a half volt (or more) means your control module should be replaced.

How to choose the right glow plug control module?

As glow plugs and glow plugs control modules don’t come in a one-size-fits-all format, it is important to select the correct type for your car and the type of fuel that you use. Additionally, using premium components is recommended, as a low-quality part can cause expensive consequences for your engine. For example, a swollen glow plug constructed from sub-par materials would either be impossible to remove later or the tip would break off in the cylinder, causing damage to the cylinder, valve and head.

When should you replace your glow plug control module?

Replacing your glow plugs or glow plug control module is a straightforward project that is recommended every 60,000 miles (95,000 km). This will help ensure that you don’t find out they’ve gone bad on a really cold day.

Before you start, ensure that you have read all relevant instructions in your owners’ manual and have all relevant tools at hand.


Champion glow plug control modules

Champion offers a range of quality glow plugs and glow plug control modules suitable for all types of diesel engines. Discover the full range for yourself.

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The content contained in this article is for entertainment and informational purposes only and should not be used in lieu of seeking professional advice from a certified technician or mechanic. We encourage you to consult with a certified technician or mechanic if you have specific questions or concerns relating to any of the topics covered herein. Under no circumstances will we be liable for any loss or damage caused by your reliance on any content.

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