Glow plugs are specialized heating devices used to aid in the starting of diesel engines. They are like 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.
If your engine is having trouble starting or you see smoke coming from your exhaust, then one or more of your glow plugs might be failing. Take a few simple steps and test the glow plug yourself, so you can avoid a trip to the mechanic.
1. Determining the resistance value of the multimeter: 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. Now, 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. This will be an important value in calculating the resistance of your glow plugs later on.
2. Locating glow plugs in your engine: These are installed in the cylinder heads and will have a heavy gauge wire, similar to a regular spark plug wire, attached to them.
3. Disconnecting the glow plug wires: Once all the glow plugs are located, disconnect any wires or caps attached to them.
4. Touch the negative and positive 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. Repeat the same for positive terminal.
5. 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.
6. Calculate the resistance value: 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). 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.
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.
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, 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.