How to Test Glow Plugs From Start to Finish (With a Multimeter) - The Complete Guide

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My diesel Won't start? There's white smoke coming from the exhaust... It starts OK when the engine is warm?

For more information. See: https://do-it-up.com/vehicles/engine-transmission/diesel-wont-start-when-cold-how-to-fix/

– How to Fix?

Petrol engines start with the assistance of a spark from a spark plug to ignite the fuel. Whereas diesels start using the compression of the fuel mixture. However, diesels don’t like starting when cold. Hence diesel engines are fitted with glow plugs that heat the air fuel mix to aid starting when cold.

If a diesel engine is running well and starts easily when the engine is warm, but won’t start when cold, (or takes ages to get it started when cold). It could be an issue with the glow plugs, the glow plug system (glow plug module, relay, or voltage supply).

Diesel Cold Starting – Fault Symptoms

The engine turns over at a reasonable speed (it’s not slow cranking), but the engine won’t start?
The engine starts, but only after some time (and multiple attempts)?
White smoke is seen from the exhaust (unburnt fuel).
When the engine is warm, it starts easily.
You may see white smoke from the exhaust whilst trying to start the engine and for a short while when the engine starts (unburnt fuel).

Diesel Cold Starting – The Causes
If you see white smoke coming from the exhaust (unburnt fuel), after several starting attempts…

Some, or all of the glow plugs may have failed?
There maybe no voltage supplied to the glow plugs?
The glow plugs may switch off too soon. The timer (or control module) could be defective?

Diesel Cold Starting – Things to Check

The glow plug relay:
The glow plug timer (or control module):
The glow plugs:

Note:
The glow plug relay is sometimes a separate item, but it can also be incorporated inside the glow plug module?

With a Multimeter, we can measure:

The Supply Voltage:
The Timer (Control Module):
The Resistance (and Continuity):

What Should I Check First (three easy steps)?
For the initial testing, you will need a voltmeter, or a simple method of checking for a voltage. However, it’s not essential. If you don’t have a voltmeter, just go straight to the section on Testing Glow Plugs Without a Voltmeter (Multimeter)…

Is there a voltage at the glow plugs?
How long is the voltage applied to the glow plugs?
Check the glow plugs?

1. Is there a voltage at the glow plugs?
If there is a voltage supply to the glow plugs, then we know the glow plug relay and glow plug control module are probably working (also see 2. below).

2. How long is the voltage applied to the glow plugs?
There is a timer that applies a voltage to the glow plugs for a set amount of time. Therefore you need to check how long the supply to the glow plugs is switched on.

3. Check the glow plugs?
If the correct voltage is applied at the glow plugs (and for the correct amount of time), then the glow plugs will need to be checked.

Please refer to the vehicle manual, as each vehicle may be slightly different.

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