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Engine guides and mods

Testing Glow Plugs

How to test the glow plugs to see which one is faulty.

Modification Details

If your glow plug light has come on, the immediate reaction is that the glow plugs need changing.
This isn't always the case, it could be a faulty glow plug controller. On the flip side, you might be
hard up for cash and only want to change the glow plug that is faulty instead of swapping all 3.

So, how do you know which of your glow plugs is faulty? Luckily enough they are basic
and can be checked with nothing more than a multimeter. You do own a mutimeter right?

The glow plugs are behind the indent cut outs on the back of the plastic name plate (circled).

Each one has a push on connection that can be hooked underneath and removed.

With the connector removed you will see the upper most metal pin, this is the positive connection.
Below that you'll see the 10mm hexagon. This is the negative terminal. They earth to the engine.

With your multimeter set on OHMs (Ω), place the red probe on the end and the black probe on the hexagon.
This is obviously a lot easier with the glow plugs removed but it isn't totally necessary.

You are looking for a reading around 1 Ohm, this is the reading I got from a new glow plug.

Any reading approaching 2 or more means that there is a fault with the glow plug.

Bear in mind that testing the glow plug whilst they are still in the engine is an inefficient way to test them
so you may get slightly higher readings than if you test them out of the engine. You must test them directly,
not through their wiring or through the engine earthing point. This will lead to a very high Ohm reading.

If the resistance is too high, the glow plug has internal damage to the heating element.

If you get 0 Ohms (no resistance) then the glow glug is internally short circuited.

If you get no reading at all, the glow plug internals are damaged and are not making contact.

Don't Have A Multimeter

Why the hell not! A basic one is really cheap, usually less than £8.

Without a meter, your only option is to remove the glow plugs and run some power through them.
Attach your jump leads to your car battery, on the other end, clamp the red lead to the positive connection
and then connect the black lead to the hexagon. Don't leave it connected for more than a few seconds.

When you do this, keep your hands clear of the other end as there's a good chance it's going to get very hot.

A correctly functioning glow plug will glow from the very end and quickly work back.
It should glow white hot at the tip. If it doesn't glow from the tip, the element inside is damaged.

Any faulty glow plugs should be replaced with new ones.

Thanks to Tolsen whose posts I read before creating this page.

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