Petrol engines start with the assistance of a spark from a spark plug to ignite the fuel. Whereas diesel engines 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 (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 plug 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 fuel is not as volatile as petrol. It doesn’t ignite so easily, especially when the air temperature (engine starting temperature) is cold. Diesel engines ignite the air/fuel mixture injected into the engine cylinders, by using the compression created by the pistons. Unlike petrol engines, diesel engines don’t have spark plugs. Diesel engines also needs a bit of warmth to help the process…
As part of an explanation, a few years back, I took part in a demonstration to show if diesel fuel would ignite easily (when there was a diesel spill on water). This was in controlled conditions where diesel was added to a water filled tray. The diesel floated on top of the water. A torch flame was used to try and ignite the diesel. However hard we tried, the diesel would not ignite. However, when we heated the diesel in one corner of the tray, and it began to give off a vapour. We were able to ignite this diesel vapour easily. The heat from the flames in the corner of the tray, then started to heat the rest of the diesel floating on top of the water. This created more vapour, and we soon had a raging fire on our hands (with the whole tray alight). We then had fun putting out the test fire with foam fire extinguishers!
Hopefully this explanation shows how glow plugs can help igniting the diesel when the engine is cold to aid starting (more so in colder temperatures).
Diesel Cold Starting – Things to Check
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 Without a Voltmeter (Multimeter)…
* If you are unsure about working on vehicle electrics, seek the advice of an electrical engineer *
Three Easy Steps:
If there is a voltage supply (the correct voltage), then we know the relay and control module are probably working (also see 2. below).
There is a timer that applies a voltage for a set amount of time. Therefore you need to check how long the supply is switched on.
If the correct voltage is applied (and for the correct amount of time), then the actual glow plugs will need to be checked.
It’s easy to check the timer and relay with a voltmeter. If you don’t have a voltmeter, you can still check them 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 connectors and check the voltage to each of the glow plugs when the ignition is switched on. See the video below (at the end of section 2). ‘Testing the Supply Voltage & Timer‘.
There is a large number of different voltage specifications for vehicles, ranging from 4.4v all the way up to 23v. The majority appear to be rated at 11v. However, it should be noted that the 11v glow plug will work perfectly fine on 12v, which is the case in the videos shown here. I guess they are rated at 11v, in case the output from the vehicle’s electrical system is low. Plus they will have a + or – operating tolerance.
You could assume that:
You may find a manufacturer’s colour coding system, to indicate the operating voltage (i.e. yellow or orange for 5v). However, I would only use a colour indicator as a guide (past experience, where manufacturer’s don’t strictly adhere to helpful indicators).
Always check your part numbers on the manufacturers website to ensure the correct part is being fitted. I.e. If your vehicle system is 12 volts and you find 5 volts is supplied (and you don’t have a manual showing this type of detail). Check the specifications on the manufacturers website, to check the voltage is correct. The voltage may be regulated down (by the vehicles control unit) to a different voltage…
Check how long the 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. This will tell us if they are:
They 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 Resistance Values‘, and whether they pass or fail.
We can measure the 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 Resistance Values below).
Short video on how to measure the glow plug resistance (when removed):
A continuity check will test if there is an open circuit (it’s 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 From Start to Finish (With a Multimeter) – The Complete Guide
The table below shows typical 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 they’re 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||1-6 Ω 2||1 Ω||✔|
|6||New||1-6 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…|
Low resistance is good. It means the glow plug will heat up…
“The total energy dissipated by the circuit is proportional to current, so the resistance of the heating element has to be low enough to draw sufficient current to generate enough heat”.
“The resistance of the heating element has to be high enough so that most of the energy is dissipated by the heating element”.
In other words…
You want a low resistance, so the energy is dissipated in the glow plug and it heats up. But… Not a really, really low resistance (like a dead short). Otherwise, the energy will be dissipated in the glow plug, and in the rest of the circuit. Blowing the fuse and potentially melting the wires (dependant on the fuse rating and how quickly the fuse reacts!).
There are some easy checks you can carry out, using a:
You can test 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 removed, we can carry out a visual check whilst testing to see if it works.
For safety reasons:
In this case, use a jump lead for testing, not a thin piece of wire…
Testing the glow plugs by connecting directly to the battery is not recommended...
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 without a test meter:
How to Test From Start to Finish (With Jump Leads & Wire) – The Complete Guide
The following table gives examples of the test equipment, tools, etc you can buy to fix your vehicle. If you’re not sure what you need, sometime it’s worth having a look? The list below may give you an idea of what’s what’s available, what to look for and how much it costs…
Ordering Parts – Examples of What You Can Buy:
|Glow Plug Removal Tools||UK||https://ebay.us/TbyHfH|
Is the same as for the removal, except…
I have seen vehicles where the warning indicator on the dashboard only appears very briefly. So briefly, you can miss it. Whereas on other vehicles, the indicator stays on a lot longer? As a guess, it’s probably down to the circuit design for that particular vehicle (if the vehicle starts and runs fine, with no white smoke). The periodicity the symbol displays may vary from vehicle to vehicle… However, always assume the glow plugs are needed, and wait for them to heat the cylinders, to aid the engine starting process (see the notes below for more info…).
If you need to check they are actually working:
With a test meter:
Without a test meter:
– click or tap any image to view full size –