Glow plug warning lamp (Engine management system - Td5) (page 2 of 2)

The purpose of the glow plugs is: l Assist cold engine start. l Reduce exhaust emissions at low engine load/speed.

The main part of the glow plug is a tubular heating element that protrudes into the combustion chamber of the engine. The heating element contains a spiral filament that is encased in magnesium oxide powder. At the tip of the tubular heating element is the heater coil. Behind the heater coil and connected in series is a control coil. The control coil regulates the heater coil to ensure that it does not overheat and cause a possible failure. The glow plug circuit has its own control relay located in the engine compartment fuse box.

Pre-heat is the length of time the glow plugs operate prior to engine cranking. The ECM controls the pre-heat time of the glow plugs based on battery voltage and coolant temperature information via the glow plug relay.

Post-heat is the length of time the glow plugs operate after the engine starts. The ECM controls the post-heat time based on ECT information. If the ECT fails the ECM will operate pre/post-heat time strategies with default values from its memory. The engine will be difficult to start.

Input/Output The glow plugs receive voltage from the glow plug relay that is controlled by the ECM. The ECM provides the earth path for the relay coil closing the relay contacts and supplying the glow plugs with battery voltage. The supply voltage heats the coils to approximately 1000 °C (1832 °F). The glow plug circuit is wired in parallel, the body of each glow plug is screwed directly into the engine block which provides each glow plug with an earth path.

The glow plugs can fail in the following ways: l Heater coil open circuit. l Control coil open circuit. l Poor earth quality. l Short circuit to vehicle supply. l Short circuit to vehicle earth. l Wiring loom fault. l Relay windings open circuit. l Incorrect relay fitted.

In the event of a glow plug failure any of the following symptoms may be observed: l Difficult starting. l Excessive smoke emissions after engine start.