Glow Plug Heating System (Diesel Engines) — General Information [Toyota RAV4 XA20]

To facilitate cold starting, diesel engines are equipped with a glow plug preheating system, which contains four spark plugs (by number of cylinders), the power relay unit, the warning light on the instrument panel, the ECM engine control module and the wiring connecting these components.

A spark plug is a miniature electric heating element enclosed in a metal housing with a tip on one end and an electrical terminal on the other. One spark plug is screwed into each combustion chamber. The spark plug tip is located in the path of the fuel spray from the injector. When power is supplied to the spark plug, its tip quickly heats up, heating the injected fuel, passing through the hot tip of the spark plug, and ignites, which allows the optimum combustion temperature to be reached faster.

Since all diesel models under consideration use an electronic control system, the control of the spark plug power relay unit is performed by commands from the ECM engine control module, which determines the required time for power supply to the spark plugs, based on signals received from the coolant temperature sensor. The system monitors the temperature of the air entering the engine, changing the glow plug warm-up time accordingly (supplying power to them).

An indicator located on the instrument panel warns the driver that the glow plug heater is switched on. Once the warm-up is complete, the indicator goes out, indicating that the engine can be started with the starter, although voltage may continue to flow to the spark plugs for some time until the engine has warmed up sufficiently (post-start warm-up). If there is no attempt to start the engine, the power supply to the spark plugs is cut off to prevent the battery from discharging and to prevent the spark plugs from burning out prematurely.

After the engine starts, the warm-up system continues to operate for a certain period of time, improving the combustion process during engine warm-up, resulting in smooth fuel combustion, less engine noise and reduced exhaust emissions.

If the pre-heating system malfunctions while the vehicle is being driven, the Check Engine warning light will come on and a diagnostic trouble code will be stored in the ECM. More detailed information about the on-board diagnostic system can be found in chapter 6B.

(The original is published on the website: ToyotaMAN)