Heater System Trouble Shooting GMC S15 Pickup

Slow Warming

1. Incorrect operation of controls.
2. Low coolant level.
3. Control cables not attached.
4. Blower motor not operating.
5. Air doors not fully opening.

Exhaust Fumes in Vehicle

1. Leaking seal between engine compartment and plenum.
2. Leaking seal between air inlet duct and dash.

Cold Drafts on Floor

1. Improper operation of vent cables.

Insufficient Heat to Rear Seat Area

1. Obstruction on floor.

Drop in Heater Air Temperature at Any Blower Motor Speed

1. Low engine coolant level.
2. Air in cooling system.

Failure of Cooling System to Warm Up

1. Incorrect or inoperative engine thermostat.
2. Incorrect coolant level.

No Coolant Flow Through Heater Core

1. Kinked heater hoses.
2. Plugged heater core.
3. Inoperative water valve (auxiliary heater only).

Inadequate Removal of Fog or Ice

1. Air door not opening.
2. Defroster door not opening.
3. Temperature door not opening.
4. Obstructions in defroster outlets at windshield.
5. Dinged defroster outlets.
6. Blower motor feed or ground disconnected.
7. Blower motor or blower motor fuse inoperative.
8. Inoperative blower motor switch.

Too Warm in Vehicle

1. Temperature door improperly adjusted.
2. Incorrect operation of controls.

Blower Motor Not Operating

1. Blown fuse.
2. Inoperative motor.
3. Open circuit.
4. Inoperative blower motor switch.
5. Shorted or open blower resistor.

Heater Wiring Diagram (Blazer, P/U and "G" Series Van Models). Scheme 7

Heater Wiring Diagram (All Others). Scheme 8