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