REDUCED HEATER PERFORMANCE REPLACE HEATER CORE. GMC Truck GMC C Pickup

Model and Year: 1988 C/K EQUIPPED WITH DELUXE/HEAVY DUTY HEATER (C42) OR AIR CONDITIONING (C60)

Some 1988 C/K trucks may experience reduced heater performance due to a missing separator in the heater core. If a vehicle exhibits this condition and the VIN of the vehicle is within the breakpoints listed below, the following diagnostic procedure should be used:

1. Run the vehicle until the thermostat opens. The vehicle must be at normal operating temperature.

NOTE: This may be accomplished by (a) physically driving the vehicle to achieve operating temperature or (b) letting the vehicle idle in the service area and visually monitor the coolant flow or record the coolant temperature using a thermometer via the radiator cap opening. If method (b) is used, it may be necessary to block off the radiator to restrict air flow to aid in achieving operating temperature.

2. The remaining procedure must be conducted at 60-80 degrees Fahrenheit ambient temperature in order to properly diagnose the heater core performance.

NOTE: Conducting this test at 60-80 degrees Fahrenheit will insure proper performance of the heater core in cooler ambience.

3. Set control to HEATER mode.

4. Set fan speed on HIGH.

5. Engine speed at IDLE.

6. Check the vehicle heater outlet temperature for several minutes with a thermometer.

NOTE: The outlet temperature should be a minimum of 50 degrees Fahrenheit higher than the ambient temperature.

If outlet temperature is less than 50 degrees Fahrenheit above ambient, the heater core should be replaced with P/N 3091316. Vehicles built between the following VIN breakpoints may be affected by this condition.

PLANT VIN BREAKPOINTS ----- --------------- Fort Wayne JZ559713 To JZ560949 Oshawa J1583737 To J1584197 Pontiac East JE536922 To JE537614

Parts are currently available from GMSPO.

For vehicles repaired under warranty use:

Labor Op: T7765 Check heater performance (includes measuring air temperature at heater outlet). .5 Hr.

Add: If necessary to replace heater core: Gas vehicles with A/C .7 Hr. Diesel vehicles with A/C (incl. charge A/C system) .9 Hr. Gas/Diesel vehicles without A/C 1.0 Hr.

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.