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).
List of checks that can be carried out:
With a Multimeter:
Without a Multimeter:
This page contains all the detail on glow plugs fault finding, their fault symptoms, testing and repair.. If you just want to view the videos, they can be found on this page ‘Diesel Won’t Start When Cold – The Video’s‘. – Opens in a new tab
This article is for guidance only…
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). The video below shows an example of what happens:
If you see white smoke coming from the exhaust (unburnt fuel), after several starting attempts…
Diesel Cold Starting – Things to Check
The glow plug relay is sometimes a separate item, but it can also be incorporated inside the glow plug module?
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)…
* If you are unsure about working on vehicle electrics, seek the advice of an electrical engineer *
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).
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.
If the correct voltage is applied at the glow plugs (and for the correct amount of time), then the actual glow plugs will need to be checked.
It’s easy to check the glow plugs, the glow plug timer and relay with a voltmeter. If you don’t have a voltmeter, you can still check the glow plugs, the glow plug timer and relay with a car battery and a jump lead (see the two options below).
Short video on how to remove glow plugs:
There isn’t much you can check on the relay and glow plug control module, other than voltages. Therefore, all you need is a simple voltage meter, voltage tester (or you could use a vehicle lightbulb and some wire). If you have replacements, you can always swap the units over to check their operation?
Please refer to the vehicle manual, as each vehicle may be slightly different.
Remove (unclip) each of the glow plugs connectors and check the voltage to each of the glow plugs when the ignition is switched on. See the video at the end of 2. ‘Testing the Supply Voltage & Glow Plug Timer‘.
Check how long the glow plug timer (control module) voltage is supplied to each of the glow plugs. It should be for around 10 to 20 seconds dependant on the vehicle.
Other things to check:
Short video on testing the supply voltage and glow plug timer:
With a voltmeter (multimeter), we can check the resistance of the glow plugs. This will tell us if they are:
The glow plugs can be tested either when fitted, or removed. However, a less accurate reading may be obtained if testing whilst still fitted (additional resistance in the circuit).
At the end of section 3. is a table of ‘Typical Glow Plug Resistance Values‘, and whether they pass or fail.
We can measure the glow plug resistance whilst they’re still installed. This may only give an indication of their condition, as there may be additional resistance in the circuit. However, it can prove that there is a circuit and that corrosion is not a factor in the glow plug failing to work.
Short video on how to measure the glow plug resistance (when fitted):
What can we check..?
Carry out a visual check for general condition, such as corrosion, or any obvious damage etc.
The resistance check is a more detailed check on the condition of the heater element. It can indicate if the glow plug is worn, or on the way out (see the table of Typical Glow Plug Resistance Values below).
Short video on how to measure the glow plug resistance (when removed):
A continuity check will check if the glow plug is open circuit (broken internally). This can be just a simple ‘buzzer’ test. If you don’t get a ‘buzz’, (open circuit) then it’s dead!
Short video on how to test the continuity of glow plugs:
How to Test Glow Plugs From Start to Finish (With a Multimeter) – The Complete Guide
The table below shows typical glow plug resistance values and whether they would pass or fail. Note the difference in values between measurements taken when installed and removed. This could vary greatly if there is any corrosion present, or there is a ‘bad contact’ when the glow plugs are inserted.
Typical Glow Plug Resistance Values
|Glow Plug||Condition||Resistance (Fitted) 1||Resistance (Removed) 1||Pass or Fail|
|1||Old||3.031 MΩ||2.181 MΩ||✖|
|2||Old||16.24 KΩ||15.24 KΩ||✖|
|3||Old||47.26 KΩ||45.77 KΩ||✖|
|4||Old||211.2 Ω||208.9 Ω||✖|
|5||New||1-5 Ω 2||1 Ω||✔|
|6||New||1-5 Ω 2||1 Ω||✔|
|Note:||1 The difference in resistance for the same glow plug (when fitted and removed)…|
|2 The resistance (fitted) may vary and is approximate…|
There are some easy checks you can carry out, using a:
You can test the glow plugs when fitted or removed, as indicated below:
Do not short the battery connections, (or yourself across the battery, it could give you a shock!) i.e. Between the live (+ve) and the earth, or bodywork (-ve). 12 volts is not usually enough to kill you, but batteries larger than 12v may do? Hydrogen gas given off by the battery can ignite, causing an explosion. Sending bits of battery and acid everywhere…
The glow plugs can be tested for a working circuit (when fitted), by using a wire from a live 12v (or 24v depending on the system) supply and touching the end of the exposed glow plug (with the connector removed). If a spark is present when the end of the glow plug is touched, we know current is passing through the glow plug back to the battery. This indicates the glow plug is not open circuit.
When a glow plug is removed, we can carry out a visual check whilst testing to see if the glow plug works.
For safety reasons:
In this case, use a jump lead for testing, not a thin piece of wire…
To save time, two test’s can be carried out the same time. Goto 3. Does the Relay and Timer Work? below for more detail.
This test is similar to ‘Connect to the actual glow plug supply‘ above, but has an additional test. You can time how long the glow plug works for, (before the timer switches the supply off).
Short video on how to test glow plugs without a test meter:
How to Test Glow Plugs From Start to Finish (With Jump Leads & Wire) – The Complete Guide
Is the same as for the removal, except…
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