Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Relay-Glow Plug: Operation
When the ignition (key) switch is place in the ON position, a signal is sent to the ECM relating current engine coolant temperature. This signal is sent from the engine coolant temperature sensor.
After receiving this signal, the ECM will determine if, when and for how long of a period the glow plug relays should be activated. This is done before, during and after the engine is started. Whenever the glow plug relays are activated, it will control the 12 volt 100 amp circuit for the operation of the four glow plugs. Each relay control two glow plugs.
The Glow Plug lamp is tied to this circuit. Lamp operation is also controlled by the ECM.
With a cold engine, the glow plug relays and glow plugs may be activated for a maximum time of 200 seconds. Refer to the following Glow Plug Control chart for a temperature/time comparison of the glow plug relay operation.
In this chart, Pre-Heat and Post-Heat times are mentioned. Pre-Heat is the amount of time the glow plug relay control circuit is activated when the ignition (key) is switched ON, without the engine running. Post-Heat is the amount of time the glow plug relay control circuit is activated after the engine is operated. The Glow Plug lamp will not be activated during the post-heat cycle.
| Engine Coolant Temperature "Key ON" | Wait-To Start Lamp "ON" (Seconds) | Pre-Heat Cycle (Glow Plugs On Seconds) | Post-Heat Cycle (Seconds) |
| -30C | 20 SEC. | 35 SEC. | 200 SEC. |
| -10C | 8 SEC. | 23 SEC. | 180 SEC. |
| +10C | 6 SEC. | 21 SEC. | 160 SEC. |
| +30C | 5 SEC. | 20 SEC. | 140 SEC. |
| +40C | 4 SEC. | 19 SEC. | 70 SEC. |
| +70C | 1 SEC. | 16 SEC. | 20 SEC. |
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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