A glow plug is a rod shaped heating element, with an internal heating coil, which can withstand extremes of high temperature. One end of the glow plug has a point which heats up, emitting a glowing light, when fed electricity. Glow plugs are used in a diesel engine instead of a spark plug. The primary function of a glow plug is to produce heat in order to help with the startup process in the engine. Unlike a petrol engine, the fuel that is injected into a diesel engine does not need a spark to ignite it. But when starting a diesel engine in cold conditions, some extra help is required, which is where the glow plugs and glow plug relay come into action. Glow plugs operate with a glow plug relay which functions to open or close the electrical circuit between the glow plugs and the battery.
The glow plug relay will be housed in the relay box, usually found underneath the dashboard. The relay connects to the car's battery, as well as connecting to the specific part that the relay is functioning, in this case the glow plugs. The glow plug relay is crucial to the glow plug system. It receives information from the glow plug sensor, which is positioned on the cylinder head, on the heater hose fitting and uses this to determine how long to heat the glow plugs for. The colder the engine, the longer the glow plugs will be supplied with the electrical current to heat them. The glow plug relay basically acts as an ON/OFF voltage control switch, allowing an electric current to flow to the glow plugs, by closing or opening the contacts that provide voltage to them. The cylinders in the engine are heated up by the glow plugs, when the ignition is turned on, but before the engine is actually started. In modern vehicles the glow plugs are usually automatically switched off by the glow plug relay when the desired heat for engine startup has been reached. If the engine is started prematurely this can cause damage to the glow plugs.
Signs that the glow plug relay may need changing are; an engine that struggles to start or will not start at all. It is always a good idea to consult with your mechanic when engine problems strike, to make sure the correct fault has been identified. When you need to change the glow plug relay, a good quality glow plug relay will guarantee a part that fits well, performs properly and has a long service life. Relays can fail because of oxidisation, overheating or due to a faulty connection in the relay socket. Faults can occur with the electrical contacts, relay coil and wiring. The connections can overheat and corrode. It is important when replacing the glow plug relay that you buy a part that will be compatible with the make and model of your vehicle. Buying the glow plug relay will guarantee you get the right component that is designed specifically for the make and model of your car.