Is your diesel-powered vehicle difficult to start when cold? If so, there may be a problem with the glow plugs.
Of course, because many different issues can cause hard starting, you’ll need to do some troubleshooting to determine whether the glow plugs are to blame. The good news is, you can perform a simple glow plug resistance test at home with a digital multimeter (DMM).
Gas-powered engines use spark plugs to ignite the air-fuel mixture inside the combustion chambers. On the other hand, Diesel engines rely on the heat from the compression of air to ignite fuel. Because of this, diesel engines can be difficult to start when cold.
Many diesel-powered vehicles use glow plugs to address this issue. The glow plugs act as heating elements to preheat the air in the combustion chambers, making the vehicle easier to start. Diesel engines typically have one glow plug per cylinder.
Each glow plug has a terminal, a threaded body, and a tip the protrudes into the engine’s combustion chamber. Applying voltage to the terminal end of the plug allows electrical current to pass through a heating element in the plug’s tip. The plug’s body, which is threaded into the cylinder head, acts as an electrical ground for the heating element.
Modern vehicles have a control module that manages the glow plugs. The module looks at inputs from various sensors, such as the coolant temperature and oil temperature sensors, when determining when to turn on the glow plugs. On many vehicles, the control module activates the glow plugs via a relay.
Over time, glow plugs wear out, and their internal resistance changes. When this happens, the plugs may cause hard starting and other issues. Fortunately, testing the glow plugs to see if they require replacement is a fairly straightforward affair.
Note: The following are general guidelines for educational and entertainment purposes only. Consult your vehicle’s factory information for specific repair instructions and recommended safety procedures.
If you’re diagnosing the glow plug system on a modern vehicle and the check engine light is on, the first thing you’ll want to do is check for diagnostic trouble codes (DTCs). A control module monitors the glow plug system. When the device detects a problem with one or more of the glow plug circuits, it will turn on the check engine light and store a diagnostic trouble code in its memory.
Note: If you have a scanner with OEM enhanced data, there is also a “key on engine running (KOER)” glow plug test you can perform on select Ford vehicles.
Next, you’ll want to perform a visual inspection of the glow plugs. Look for issues, such as damaged wires and poor connections. Make sure that the electrical connectors going to the glow plugs are clean and tight.
Repair any issues found during the visual inspection, clear the DTCs, and see whether the problem returns.
From there, you can move on to testing the glow plugs as necessary.
Before you whip out your digital multimeter, you might want to use an infrared thermometer to check for inoperative glow plugs. To do this, first, make sure the engine is cold. Then, cycle the ignition a few times to heat up the glow plugs, and point the thermometer at each plug.
If you find a plug that has a much lower temperature than the others, that plug isn’t working properly, either due to an internal failure or a damaged electrical connector.
Note: This test is only used to locate plugs that aren’t working at all. A glow plug may pass this test and still have carbon buildup or be out of specification resistance-wise.
As was mentioned, the internal resistance of the glow plugs changes as the plugs begin to degrade. You can test the resistance of the plugs with a digital multimeter (DMM).
The process for testing glow plugs may vary depending on the type of vehicle you have. For example, if the plugs are difficult to access, you may want to check the resistance at the wiring harness rather than at the plugs themselves. It’s a good idea to consult a repair manual or repair database for the proper test procedure.
In general, however, you can test the resistance of a glow plug by doing the following:
Keep in mind that a glow plug can be in good shape electrically and still have excessive carbon buildup. You may want to remove any suspect plugs to check for such a condition.
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Like any other car parts, glow plugs are susceptible to wear and tear, usually due to carbon buildup, excessive heat, and general damage. If your glow plugs start to fail, don’t think twice about replacing them.
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Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.