Automotive Systems

Formerly Automotive Systems I

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Glow Plugs


The purpose of a glow plug is to beat up the air that is drawn into the precombustion chamber to assist starting, especially in cold weather. Glow plugs are common on precombustion chamber engines, but not on direct injection diesels because they use shaped piston crowns that produce a very effective turbulence to the air in the cylinder. Direct injection engines also have less immediate heat loss to the surrounding cylinder area than in a precombustion engine and generally have a higher injection spray-in pressure.

A glow plug is used for each cylinder located just below the injection nozzle and threaded into the cylinder head (fig. 5-43). The inner tip of the glow plug extends into the precombustion chamber. The glow plugs may be turned on using the ignition switch with the length of time being controlled from an electronic module. On some older vehicles and construction equipment, glow plugs are operated by manually depressing a switch or button for 15 to 30 seconds. During colder weather, the system may have to be cycled more than once to start the engine.

Glow plugs are not complicated and are easy to test. Disconnect the wire going to the glow plug and use a multimeter to read the ohms resistance of the glow plug.

Specifications for different glow plugs vary according to the manufacturer. Be sure and check the manufacturer’s service manual for the correct ohms resistance value.

Figure 5-43.—Typcial diesel glow plug.
Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

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