Glow Plug Controller
An electronic control unit or relay module that regulates electrical current and timing to the diesel glow plugs.
The glow plug controller is the electronic module or relay that manages the operation of the glow plugs in a diesel engine. Because glow plugs draw significant electrical current to heat the cylinders, their timing must be managed precisely to prevent battery drain or plug damage.
When the ignition key is turned to the 'on' position, the controller reads data from the engine coolant temperature sensor. If the engine is cold, the controller sends high-amperage current to the glow plugs for a calculated pre-heating period, lighting up the wait-to-start dashboard light.
After the engine starts, the controller typically keeps the glow plugs powered at a reduced rate for a short post-heating period. This helps stabilize combustion in the cold engine, reducing rough idling, noise, and white exhaust emissions during warmup.
If the glow plug controller fails, the glow plugs may not receive power (leading to hard starting or no starting in cold weather) or they may stay powered constantly (burning out the glow plugs). Diagnosing controller issues involves checking for voltage at the glow plug harness.