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.

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 10mm studs and held by two 10mm nuts.

GLOW PLUG INHIBIT SWITCH

The glow plug inhibit switch opens above approximately 125 DEG. 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 temper- ature. The initial ON time and cycling ON/OFF times vary also with system voltage and/or temperature. Longer duration of cycling is pro- duced by lower temperature only. The temperature switch opens at approximately 125 DEG. 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 Nm (7-9 lb ft) 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 Nm(3-4 lb ft). 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 DEG. 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

Connect an ammeter in series (induction type meter may also be used)* with DK red or orange wire leading from the Glow Plug Relay to LH bank of glow plugs. Operate the system and note the ammeter reading. Repeat the procedure for the red or orange wire leading from the Glow Plug Relay to the RH bank of glow plugs.

Operate the system and note the reading.

VEHICLE MODEL NORMAL AMP READINGS

G-Van C/K, R/V, P D-CUCV 50 amps. MIN. 55 amps MIN. 35 amps MIN. | | | |-------------------------------|----------------------------| |-------------------------|--------------------------| | | | LH BANK AMMETER READING AMMETER READING NORMAL RH BANK AMMETER READING LESS THAN NORMAL | LESS THAN NORMAL | | | | | | One or more glow plugs on Glow plug system One or more glow plugs on LH bank not operative. operating normally RH bank not operative. Check individual glow plug Check individual glow plug leads by connecting ammeter leads by connecting ammeter in series with GRN wire in series with GRN wire that feeds glow plug. Operate that feeds glow plug. the system and note the reading Operate the system, note on ammeter. Repeat procedure the reading on ammeter. for each glow plug. Each Repeat procedure for each individual wire should have a glow plug. Each individual reading of approximately: wire should have a reading | of approximately. | | | C/K, R/V, P = 14 amps | | G-Van = 13 amps | | D-CUCV = 9 amps | ------------------------------------------------------- | | | | READINGS NORMAL READINGS LESS THAN NORMAL | Glow Plugs and Harness OK. On those cylinders with less than normal readings, check for continuity through the harness by disconnecting the lead and connecting a 12 volt test light from the connector to ground. Operate the glow plug system. | |----------------------------| | | TEST LIGHT LIGHTS TEST LIGHT DOES NOT | LIGHT WHEN GLOW | PLUGS ARE OPERATING | | | | Harness OK. Repair or replace harness. Replace glow plug Retest glow plug for proper operation.

*If using an in line ammeter read both banks at once. Do not cut wire

(snap-on meter MT552, VAT-40, or equivalent)

1985-88 C/K, R/V, G, P 6.2L DIESEL ELECTRICAL SYSTEM DIAGNOSIS

Engine does not start cold - "GLOW PLUG" light may or may not come on.

1. Fuel system checked and is OK. 2. Battery voltage is 12.4 volts or more with IGN off. 3. Cranking speed OK (100 RPM or more). 4. Refer to the Electronic Glow Plug System Schematic (Figure No. 1) for wiring harness terminal identification.

Use voltmeter to measure voltaqe at battery stud (single red wire) on glow plug controller. |------------------------------------------| | | Battery Voltage No Voltage

With ignition key "off" measure voltage Locate and repair battery to glow at glow plug feed stud (twin lead) plug controller circuit. on glow plug controller. | | |------------------------------------------| No Voltage Battery Voltage

Disconnect harness from all glow plugs. Relay contacts shorted. Replace Use ohm meter to measure continuity glow plug controller and all between glow plug terminal and engine glow plugs. block (ground).Replace glow plug if measurement is greater than 2 ohms. Reconnect all glow plugs before continuing with diagnosis. | | Remove controller connector and measure voltage at harness connector terminal "D" with the ignition key in "run". (1988 with LH6, remove coolant sensor harness connector before measurement.)

|------------------------------------------| | | Ignition Voltage No Voltage

Measure continuity between terminal "E" Locate and repair ignition feed of connector and engine block (ground). circuit to controller. (1988 If measurement is greater than 1 ohm, with LH6, refer to cold advance locate and repair ground circuit to electrical system check.) controller. ----| | Measure continuity between terminal "C" and "E" of connector. | |------------------------------------------| Less than 2 ohms Greater than 2 ohms

Reconnect controller harness connector Remove connector from temperature and insure complete engagement inhibit switch (not used on 1988 (connector locking latch should "click" with LH6) and measure continuity over controller locking tab). Measure between the two terminals on the the voltage at glow plug feed stud (twin switch. (Open above 125 DEG.F). lead) on glow plug controller when turning | the ignition key from "off" to "run". | | |----------------------------| | Less than 1 ohm Greater than 1 ohm | | Locate and repair cir- Replace temperature | cuit between con- switch and return to | nector at Pin C and terminal "C" to "E" | glow plugs. continuity test. | |------------------------------------------| Battery Voltage No Voltage

Measure the voltage at any one glow Replace glow plug controller. plug harness connector (test both right and left banks) when turning the ignition key from "off" to "run". | |------------------------------------------| Battery Voltage No Voltage

If instrument panel "GLOW PLUG" Locate and repair glow plug feed lamp does not come on, locate circuit to glow plugs. and repair bulb or circuit.

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 retumed 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 DEG. F or less), turn ignition switch to the "RUN" position and allow glow plugs to cycle. Wait two minutes. Tum 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 switdh 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. .

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.