How to Replace a Car Glow Plug Timer

glow plug timer

The glow plugs in diesel engines need to know when to shut off, and for this there are glow plug timers (also called a relay or module depending on manufacturer). When certain criteria are met (temperature, run time duration, engine start) these timers or relays turn off and allow the glow plugs to cool down. There is no need for the plugs once the engine warms up enough to combust properly; automatically turning them off with a timer dramatically extends the plugs’ life. Symptoms of a failing timer or relay most commonly include failing glow plugs themselves. If they are overheated for an extended period of time because of a failing timer, the plugs can become brittle and even break.

Part 1 of 1: Replacing the glow plug timer

Materials Needed

picture displaying battery from the top

Step 1: Disconnect the battery. Always disconnect the negative battery cable on the car battery to cut power when working on any electrical system.

picture of the timer mounted to the vehicle

Step 2: Locate the glow plug timer. The glow plug timer is located in the engine compartment. It is usually mounted in an out of the way location, most likely on a firewall or sidewall.

If your car is equipped with a relay, it will be located in the main fuse box or close to the engine where it is less likely to get hot.

Step 3: Disconnect the timer. Several styles of timers or controllers require disconnection from the wiring harness. You will need to disconnect the terminal(s) on the unit.

Some simply pull out, which can be assisted with a pair of pliers, while others require removal of a retaining bolt with a small socket.

Newer models may use a relay which does not need to be disconnected.

Step 4: Remove the timer. Once the timer is disconnected, you can remove the bolts or screws holding it to the vehicle. You may want to clean any exposed contacts at this time.

  • Note: Bad communication between the sensors and the timer can cause failure-like symptoms. Make sure to clean the contacts to ensure a proper connection.

picture of module being held while screwed or bolted in

Step 5: Install the new timer. Compare your old timer to the new unit. You will want to confirm the number of prongs (if equipped) and the shape, size, and contacts are the same. Install the new timer and attach with the existing bolts or screws from the old timer.

picture of different style of timer with all of its terminals connected

Step 5: Reattach the terminals. Make sure the terminals are clean. Reattach the wiring terminals to the timer and hand tighten.

If the timer or relay plugs in, confirm that it is fully engaged and making a solid connection.

Step 6: Test the timer. Start the vehicle and check for proper glow plug function. They should turn off after a matter of moments depending on the ambient air temperature outside.

For specific times, check with the manufacturer of the replacement timer.

Glow plugs work hard and have to endure drastic changes in temperature with every use. It is common to have to replace them or the other parts associated with them, like glow plug timers. If you do not want to replace your glow plug timer on your own, schedule a convenient appointment with a certified mechanic from YourMechanic for service at your home or office.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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