Monday 9th March 2015
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What are glow plugs?  You’ll find glow plugs in diesel engines.  When a diesel engine is cold, the compression process may not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electrically heated wire which almost acts like a toaster. It heats the combustion chambers and increases the air temperature when the engine is cold so that the engine can start.  According to Cley Brotherton, a Journeyman heavy equipment technician, glow plugs act little electric heaters helping to start a diesel engine through glowing and emitting heat in the combustion chamber and warming the engine block.  Essentially, they are pieces of metal with heating elements at the tip.

Diesel engines can be much harder to start in cold weather as the cylinder block and cylinder head absorb the heat of compression and stop ignition because of the higher surface to volume ration.  Inducted air temperature is low in cold engines as they barely get any heat from the engine cylinder walls. As the air is compressed and heats up a lot of the heat is surrendered to the cold cylinder walls, further reducing the temperature.  Glow plugs act as the solution to this problem as they briefly activate until the temperature of the combustion chamber reaches a level where it can combust itself. The glow plugs are located in the pre-chambers or in direct injected engines in the combustion chamber.  When electricity reaches the heating element at the tip it lights up because of its resistance to electricity and heat is created, hence the name glow plug.

You should check glow plugs every 45,00 to 60,000 miles so that you can be more assured of your car starting in low temperatures.  Here’s some advice from on how to test and check glow plugs.

Glow plugs

How to test glow plugs in the engine

Using a digital multimeter would be the best way to check that glow plugs are working. Negative (black) and positive (red) leads must be connected correctly. You will need to test the current and resistance, but you won't need to test each separately. Use the dial in the middle to adjust the settings and make sure it is set correctly. Identify the resistance value of the multimeter and cross the two leads over each and note the result.  On a digital multimeter the readings appear on the screen.  Subtract the amount from the glow plug reading.  Test the voltage of the battery by putting the multimeter in Volt reading mode. Add the negative lead to the negative terminal of the battery and the positive lead into the positive terminal.  The reading you get should be 12.5 Volts when the car is not running and closer to 13 Volts when it is running.  Find the glow plugs and remove their caps or connectors to attach them to the multimeter clamps.  While doing this check the connector and pins are clean and don’t have any rust or corrosion.  If they do, give them a clean.  Attach the multimeter’s negative lead to the earth point under the bonnet, usually a point on the body chassis. Connect the multimeter’s positive lead to the top of the glow plug and evaluate the reading.  The plug should be between .6 and 2 ohms of resistance.  Subtract the resistance value of the multimeter from the resistance value of the glow plug to get the real reading for the plug.  All of the glow plugs in the engine should have similar readings, if one has a higher resistance than the others then the engine can be affected even if the plug itself is good. If glow plugs don’t have the correct voltage running into them  they won’t work correctly.  Even if only one glow plug is faulty you will still need to replace them all.

How to test glow plugs removed from the engine

Use your vehicle manual to locate the glow plugs and for the best way to remove them.  Set the multimeter to a range between 200 -1000 ohms.  If the glow plug is faulty then it will measure too high for the multimeter’s setting.  Next, find the resistance value of the multimeter and cross its two leads over each other, record the result, then subtract this figure from the glow plug reading.  Put the negative lead of the multimeter on the glow plug’s nut but do not place the negative lead higher than the nut.  Remove the cap from that of  the glow plug and attach the positive lead onto the tip.  Again,  subtract the mutimeter’s resistance value from that of the glow plug and the real reading for the glow plug will be the difference.  If the glow plugs do not have similar readings then some are faulty and they should all be replaced.

How to test glow plugs with a battery charger

Remove the glow plugs from the engine and take a 10-12 amp battery charger   If the battery charger is not within this range it will not provide enough heat to properly test the glow plugs.  Put the negative lead from the charger onto the body of the plug and place the positive lead from the charger onto the plug tip.  If you find that the plug does not glow very quickly then it is faulty, although some faulty plugs may still glow and not heat the engine.

Modern diesel cars have innovative glow plugs with three phases of glow technology; pre-heating, heating during starting and post heating.  This protects the engine and is more environmentally friendly. Some glow plugs have regulator coils that protect them from over heating which makes them more reliable.  Do you think you would check and replace your glow plugs yourself?