How to Test Glow Plugs

What is a Glow Plug?

Glow plugs are basically large electrical resistors which heat the air or air-fuel mixture in an engine, to help with combustion. They are useful for diesels because diesel fuel cannot ignite when it is cold. The flash point (i.e. minimum temperature for ignition) of diesel fuel is usually between 52 and 96 °C (126 and 205 °F). Biodiesel’s flash point is even higher, usually at least 130 °C (266 °F). For comparison, gasoline’s flash point is about −43 °C (−45 °F), which is why gasoline engines start more easily in the winter.

What are the Symptoms of a Failing Glow Plug?

Some common symptoms of a failing glow plug are:

• Trouble starting engine when cold.
• Uncombusted (raw) fuel in the exhaust soon after startup.
• Rough-running after startup (especially at idle).
• Large amounts of smoke at startup.
• If glow plugs are causing the engine’s problems, the engine should run normally after it warms to operating temperature.

How to Test Glow Plugs With a Clamp-Meter

Clamp-meters are probably the best tool for testing glow plugs, because they are easy to use, and can test the complete system at once, without disconnecting anything. With a DC clamp meter, you can quickly check the following parameters (in a single test!) without removing anything:

• The glow plug power circuit.
• Glow plug heater element (resistance).
• The glow plug’s electrical connections.

Follow these steps to test the glow plug:

1. Make sure the engine is cold, and the glow plugs are turned off. Some glow plugs will not turn on if the engine is warm.
2. Turn on the clamp-meter, and select ‘DC current’ mode.
3. Put the clamp around the glow plug power wire, or around the top of the glow plug. Make sure that the clamp closes completely, and that the clamp meter is safe from damage.
4. Turn on the glow plugs!
5. Read the glow plug current off the clamp-meter display. Glow plugs usually consume between 2A and 6A each, when they are cold. Some systems will pulse the power to the glow plug to save energy, and maintain constant temperature.
6. Turn off the glow plugs.
7. Repeat this procedure for each glow plug on the engine.

If you find a problem, narrow down the potential causes by using some of the regular multimeter tests below.

How to Test Glow Plugs With a Digital Multimeter

You can test a glow plug with a digital multimeter (DMM), but it is a bit more work, and less thorough than testing with a clamp-meter. The most common DMM tests for glow plugs are:

• Checking the glow plug’s electrical resistance.
• Checking the glow plug power supply (electrical voltage).

To check a glow plug’s electrical resistance, first check your multimeter’s specifications, to see if it can measure low enough to give an accurate reading. Glow plugs usually have a resistance of 1Ω to 6Ω. If your multimeter cannot measure resistances that low, you can still use the ‘continuity’ function to check whether the glow plug is completely broken.

Testing glow plug resistance:

Make sure engine is cold.

1. Disconnect the power cable from the glow plug. If you don’t do this, you will be measuring the resistance of all the glow plugs at once, instead of just the one you want to test.
2. Turn on the multimeter and select ‘resistance’ or ‘Ω’ mode.
3. Connect one multimeter test lead to a ground on the engine (preferably near the glow plug).
4. Connect the other lead to the (power supply) terminal on the top of the glow plug.
5. Read the result off the multimeter display. Note that a resistance of less than 6Ω usually means the glow plug is ‘good’.
6. Turn off the glow plugs
7. Disconnect the test lead connected to glow plug power.
8. Reconnect glow plug power cable.
9. Repeat steps 5-9 for all the glow plugs on an engine.

Testing glow plug power supply:

Find out what voltage your glow plug uses. The easiest way to do this is to get the part number off the plug, and look it up on the manufacturer’s website. Most glow plugs require 12V, though some use other voltages.

1. Make sure engine is cold.
2. Turn on the multimeter and select ‘DC voltage’ mode.
3. Connect one multimeter test lead to a ground on the engine (preferably near the glow plug).
4. Connect the other lead to the (power supply) terminal on the top of the glow plug.
5. Read the result off the multimeter display.
6. Turn off the glow plugs.
7. Disconnect the test lead connected to glow plug power.
8. Disconnect the test lead connected to the engine ground.

How to Test Glow Plugs With a Glow Plug Tester

Glow plug testers sound like a good idea, but are usually very time consuming to use, and not very useful (because they only let you detect problems in the glow plug). Most glow plug testers are basically just an ammeter attached to four wires, and a switch. Caution: Some glow plug manufacturers (including NGK) recommend against applying battery power to the glow plug. This is because the plug can overheat and sometimes melt. Always follow directions from the glow plug manufacturer and tester manufacturer. Not all testers are compatible with all plugs.

Using a glow plug tester:

Decide whether you will be testing the glow plug on or off the engine. Some tester manufacturers recommend one or the other, and we recommend that you follow their recommendations. Find out what voltage your glow plug uses, and make sure the tester and power supply (which may be the vehicle battery) are compatible with each other and the plugs. The easiest way to do this is to get the part number off the plug, and look it up on the manufacturer’s website. Most glow plugs require 12V, though some use other voltages.

1. Make sure the engine is cold, and the glow plugs are turned off.
2. Disconnect the power cable from the glow plugs. If you don’t do this, your tester will be powering all the glow plugs instead of just one. Doing this will give you bad results, and may damage the tester.
3. Connect tester’s ground supply lead to your power supply ground.
4. Connect tester’s power supply lead to your power supply’s ‘high’ output.
5. Connect the tester’s plug ground to the glow plug’s ground. The glow plug ground is usually the threaded portion of the glow plug’s main body.
6. Connect the tester’s plug power lead to the glow plug’s power terminal. The glow plug power terminal is usually a threaded rod at the top of the plug.
7. Turn on the glow plug tester (if necessary).
8. Enable glow plug tester output.
9. Observe the result on the glow plug tester display or gauge. Most testers will have a ‘good’ range, and some will display the current. If a value is displayed, you should record it, for comparison with the other plugs.
10. Disconnect the tester’s plug power lead from the glow plug’s power terminal.
11. Disconnect the tester’s plug ground from the glow plug’s ground.
12. Repeat steps 6-12 for all the glow plugs on an engine.

That’s All!

If you’re looking for an accurate, easy-to-use clamp-meter, check out our CM100 1mA to 100A AC/DC Clamp-Meter. If you have any feedback for us, or ideas for topics we should cover in future blog posts, please send us an e-mail.