Glow Plug Control System; Description - GMC G Service Manual

6E2-58 DIESEL EMISSIONS

GLOW PLUG CONTROL SYSTEM

DESCRIPTION

GLOW PLUG CIRCUIT
In the diesel engine, air alone is compressed in the cylin­
der; then, after the air has been compressed, a charge of
fuel is sprayed into the cylinder and ignition occurs, due to
the heat of compression. Eight glow plugs are used to pre­
heat the chamber as an aid to starting (Figure 33).
The diesel glow plug system consists of an integral-elec-
tronic control/glow plug relay assembly, 6-volt glow plugs,
Cold Advance relay, ECM, and a glow plug lamp.
GLOW PLUGS (Figure 33)
These are 6-volt heaters (operated at 12 volts) that turn
"ON", when the engine control switch is turned to the run
position, prior to starting the engine. They remain pulsing a
short time after starting, then automatically turn "OFF".
INSTRUMENTATION
Vehicles with the diesel engine have special indicators
to permit the operator to properly follow the starting proce­
dure. A glow plug "wait" lamp, on the instrument panel,
provides this information on engine starting conditions.
Vehicles equipped with diesel engines have a water in
fuel lamp and low engine coolant lamp. Refer to FUEL
SYSTEM (SEC. 6C) and ENGINE COOLING (SEC. 6B1)
for information on these systems.
ELECTRONIC CONTROLLER/GLOW PLUG RELAY
ASSEMBLY (Figure 34)
The assembly contains the circuitry, which monitors and
controls glow plug relay operation. Information is received
at pins "B" and "C", which is used by the controller to deter­
mine glow plug operating requirements. Pin "B" senses
voltage at the starting motor solenoid. Pin "C" senses glow
plug voltage. Pin "D" supplies 12 volts, through the Cold
Advance relay, to operate the controller, only when engine
coolant temperature is below approximately 27 degrees C
(80 degrees F). Pin "E" is the controller ground. The control­
ler is mounted at the rear of the left cylinder head on two
studs.
CIRCUIT OPERATION
A normal functioning system operates as follows:
A. Key "ON", engine not running and at room
temperature.
1. Glow plugs "ON" for 4 to 6 seconds, then "OFF"
for about 4.5 seconds.
2. Then cycle; "ON" for about 1.5 seconds, "OFF
for about 4.5 seconds, and continue to cycle 1.5
"ON", 4.5 "OFF", for a total duration (including
the initial 4 to 6 seconds) of about 20 seconds.