Glow plugs are specialized heating devices used to aid in the starting of diesel engines. They are similar to spark plugs in design; however, they differ in their primary function. Instead of producing a timed spark to ignite the fuel mixture, like spark plugs do, glow plugs simply serve to produce additional heat that aids the diesel engine combustion process during cold starts.
Diesel engines rely entirely on the heat generated by cylinder compression to ignite the fuel mixture. When glow plugs begin to fail, this additional heat that aids the combustion process is gone, and the engine may become more difficult to start, especially in cold weather.
Another symptom of faulty glow plugs is black smoke being observed during start up, indicating the presence of unburned fuel due to an incomplete combustion process. In this guide, we will cover how to test the resistance of glow plugs to determine if they are functioning properly.
Part 1 of 1: Testing your glow plugs
- Basic set of hand tools
- Digital multimeter
- Paper and pen
- Service manual
Step 1: Determine the multimeter’s resistance value. Before testing the terminals, you must determine the resistance value of your digital multimeter. To do this, turn the multimeter on, and set it to read in Ohms.
Tip: Ohms are indicated by the omega symbol, or a symbol that looks like an upside down horseshoe (Ω).
Once the multimeter is set to read in Ohms, touch the two leads of the multimeter together and examine the resistance reading that is displayed.
If the multimeter reads zero, try changing the setting of the multimeter to a higher sensitivity until a reading is obtained.
Record this value on a piece of paper, as this will be an important value in calculating the resistance of your glow plugs later on.
Step 2: Locate the glow plugs in your engine. Most glow plugs are installed in the cylinder heads and will have a heavy gauge wire, similar to a regular spark plug wire, attached to them.
Remove any covers that may be obstructing access to the glow plugs, and use the flashlight for additional illumination if necessary.
Step 3: Disconnect the glow plug wires. Once all of the glow plugs have been located, disconnect any wires or caps attached to them.
Step 4: Touch the negative terminal. Take your multimeter and touch the negative leads to the negative terminal on your vehicle’s battery.
If possible, secure the lead to the terminal by tucking it inside or underneath the clamping mechanism of the post.
Step 5: Touch the positive terminal. Take the positive lead of your multimeter and touch it to the terminal on your glow plug.
Step 6: Record the glow plug’s resistance. With both leads touching the terminals, record the resistance reading that is indicated on the multimeter.
Again, the reading that you get should be measured in Ohms (Ω).
If no reading is obtained when you touch the glow plug, make sure that the negative lead is still in contact with the negative battery terminal.
Step 7: Calculate the resistance value. Calculate the true resistance value of the glow plug by subtracting.
The true resistance value of the glow plug can be determined by taking the resistance value of your multimeter (recorded in Step 2) and subtracting it from your glow plug resistance value (recorded in Step 6).
Step 8: Evaluate the resistance value. Compare your calculated true resistance value of your glow plug with the factory specification.
If the glow plug resistance exceeds the specification or falls out of the acceptable range, then the glow plug must be replaced.
- Tip: For most glow plugs, the acceptable range for the true resistance value is between 0.1 and 6 Ohms.
Step 9: Repeat for other glow plugs. Repeat the procedure for the remaining glow plugs, until they have all been tested.
If any of the glow plugs fail the test, it is recommended that the entire set be replaced.
Replacing just one, or a few, of the glow plugs may cause engine problems similar to a faulty glow plug if the resistance readings are too far apart.
For most vehicles testing the resistance of a glow plug is a pretty simple procedure, granted the glow plugs are in an accessible location. However if they are not, or this task is not something that you feel comfortable taking up on your own, this is a service that any professional technician, such as one from YourMechanic, should be able to perform quickly and easily. If necessary, they can also replace your glow plugs so that you can start your car like normal.