Engine - Glow Plug System Diagnostics
87chevy18GMC NUMBER: 88-T-39
GROUP: 6 - Engine Diagnosis
DATE: September, 1987
CORPORATE
NUMBER: 866001
SUBJECT: 1985 - 88 6.2L DIESEL ENGINE GLOW PLUG SYSTEM DIAGNOSIS
MODELS: 1985 - 88 C/K, R/V, G, P - 1, 2, 3 WITH 6.2L DIESEL ENGINES
THIS BULLETIN CANCELS AND SUPERSEDES TRUCK SERVICE BULLETIN 85-B-55/ CHEVROLET BULLETIN 85-56, DATED MARCH, 1985, ALL COPIES OF WHICH SHOULD BE DESTROYED.
GENERAL DESCRIPTION
The 1985-88 6.2L diesel engine glow plug system consists of an integral-electronic controller glow plug relay assembly, 6-volt glow plugs, a glow plug light, and a glow plug inhibit temperature switch.
FIGURE 1:
ELECTRONIC CONTROLLER/GLOW PLUG RELAY
The Electronic Controller/Glow Plug Relay Assembly contains the circuitry which monitors and controls glow plug relay operation. Inputs at pins "B" and "C" of the controller provide information to determine glow plug operating requirements (see Figure No. 1). Pin "B" at the controller senses "engine cranking". Pin "C" senses glow plug voltage through the glow plug inhibit switch which is wired in series with the glow plug voltage sense lead to the glow plugs. The electronic controller is installed at the rear of the left hand cylinder head on two 10 mm studs and held by two 10 mm nuts.
GLOW PLUG INHIBIT SWITCH
The glow plug inhibit switch opens above approximately 125~F to prevent glow plug operation above this temperature.
CIRCUIT OPERATION
A normal functioning system (green eye in batteries) 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 approximately 4.5 seconds.
2. Then cycle; ON for approximately 1.5 seconds, OFF for approximately 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.
B. If the engine is cranked during or after the above sequence, the glow plugs will cycle ON/OFF for a total duration of 25 seconds after the ignition switch is returned from the crank position, whether the engine starts or not. The engine does not have to be running to terminate the glow plug cycling.
All the "times" shown above are approximate because they vary with initial engine temperature. The initial ON time and cycling ON/OFF times vary also with system voltage and/or temperature. Longer duration of cycling is produced by lower temperature only.
The temperature switch opens at approximately 125~F, and above this temperature the glow plugs are not energized.
If the system does not operate as described above, check all connectors to ensure they are fully seated. As in all 6.2L diesels, the engine harness ground connection to the engine is critical. Make sure the nut is tightened to 10-12 N-m (7-9 ft.lbs.) and the ground ring terminal is tight. The other connections to check in the engine compartment are:
1. Four-wire connector on controller. If this connector is not fully seated and latched, the glow plugs may not function.
2. Both controller copper stud upper nuts. Tighten to 4-5 N-m (3-4 ft.lbs.). Do not tighten lower nuts.
3. Temperature switch connector at top rear of RH head on 1985-1986 models and in the water crossover manifold on 1987-1988 models. If this connection is not made, the glow plugs will not energize. (Engine temperature must be below 110~ F for glow plugs to operate.)
If the glow plugs function normally, but the "Glow Plug" light does not, check all the connections and the bulb in the jumper harness in the IP area.
If all connections are intact, but the glow plug system does not operate as stated, proceed with normal diagnostic procedure. DO NOT MANUALLY BYPASS THE RELAY IN THE GLOW PLUG CONTROLLER. DO NOT JUMP START WITH MORE THAN A 12-VOLT SYSTEM.
PRELIMINARY DIAGNOSIS WITH AMMETER
The following procedure provides a fast way to determine whether the glow plug system is functioning correctly or if you have another condition to contend with. It is suggested that this procedure be performed whenever there is doubt about correct system operation. Then refer to the diagnosis charts that follow to pinpoint the condition.
1985-88 C/K, R/V, G, P 6.2L DIESEL
GLOW PLUG FUNCTIONAL CHECKING PROCEDURE
Another feature of the glow plug controller is that it provides glow plug operation after starting a cold engine. This after-start operation is initiated when the ignition switch is returned to "RUN" from the "CRANK" position. While loss of this function may not cause a cold start complaint, it may result in excessive white smoking and/or poor idle quality after start. To check for proper operation of this circuit, proceed as follows: With cold engine (80~ F or less), turn ignition switch to the "RUN" position and allow glow plugs to cycle. Wait two minutes. Turn ignition switch to "CRANK" for about one second (engine starting is immaterial) and return to "RUN". The glow plugs should cycle at least once. If the plugs do not turn on, disconnect the controller connector and check connector harness terminal "B" with a grounded 12-volt test light. The light should be off with the ignition switch in "RUN", and on with the ignition switch in "CRANK". If the light does not operate properly, repair a short or open in the engine harness purple wire. If the light operates properly, but the after-start glow plug feature does not, the controller should be replaced.