What is a Glow Plug and How Does it Work in Diesel Engines?

Glow Plugs on diesel engines glow red hot for a few seconds prior to starting the engine. This pre-warming of the air ensures reliable cold starting.

At cold start When the engine is cold. A lot of the compression heat is lost to the metal surfaces so ignition of the fuel isn't always possible, to overcome this an electric glow plug is fitted to each swirl chamber.

They are operated by a control unit and relays. and glow red hot for a few seconds prior to starting the engine. This pre-warming of the air ensures reliable cold starting. The time for which the glow plug and the warning lamp are lit is determined by the coolant temperature.

The engine is ready for starting. once the lamp has gone out. On some models to reduce this waiting time, the system is activated immediately after the ignition key is first inserted into the ignition switch. The glow plug continues to be energized during cranking and to reduce exhaust emissions, engine noise and improve the idling they often remain lit for a short time after starting.

Depending upon the model this can sometimes be at a reduced voltage. So although it can vary slightly according to the model. The glow plug system basically provides three functions pre-glow that's prior to starting the engine, cranking glow, and afterglow.

Inspection Procedure for Glow Plug on a Diesel Engines

The trouble diagnosis of the glow control system should be performed with the following procedures. Refer to the corresponding Service Manual for the vehicle you are working with.

1. Check the installation of the glow plug. Check that the glow plug connecting plate, nuts, and harness is securely installed.

2. Check the glow plug power supply circuit. Confirm that the engine coolant temperature is below approximately 25°C (77°F). If not, cool the engine down. Turn the ignition switch to OFF. Set the multi-meter probe between the glow plug and the engine body ground. Turn the ignition switch to ON. Check the voltage at the glow plug terminal.

If power is not supplied to the glow plug terminal, turn the ignition switch to OFF and remove the glow plug connecting plate. Then perform the continuity test between each glow plug and the cylinder head.

3. Check fuse, relay and fusible link.

4. Check the glow control unit power supply circuit. Disconnect the harness connector from the glow control unit. Check the power supply at the harness connector terminal when the ignition switch is turned ON. Connect the harness connector to the glow control unit.

5. Check the signal circuit of the glow relay. Check the continuity of the control signal circuit between the glow control unit and the ground through the glow relay.

6. Check the power supply circuit of the glow plug. Check the continuity of the power supply circuit between the battery and the glow plug through the glow relay. Check the dropping resistor and its circuit.

7. Check the engine coolant temperature sensor circuit.

8. Check the vehicle speed signal circuit

9. Check the glow indicator circuit. Check the alternator circuit for some models.

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