In a diesel engine, the glow plugs help to raise the temperature of the cylinders to assist with combustion on cold days. Diesel engines rely on the temperature of combustion to heat the engine and ignite fuel and air by compression. The glow plug relay or glow plug time relay is activated by the engine control module and turns on the diesel glow plugs for a set amount of time before indicating that the vehicle can be started to ensure optimal combustion and prevent engine damage.