Heater System: Other GMC Pickup C2500

TEMPERATURE CONTROL LEVER

The lower lever on control panel controls discharge air temperature in all mode positions. Full right position provides maximum heat. Full left position provides cool (outside) air.

MODE CONTROL LEVER

When lever is in VENT position (if equipped), all discharge air is directed to upper outlets. When lever is in DEF (defrost) position, most air is directed to windshield (defroster) outlets with some air directed to floor (heater) outlets. When lever is in HEATER position, most air is delivered through floor outlets with some air directed to windshield outlets.

BLOWER CONTROLS

Blower control can be knob or lever type.

AUXILIARY HEATER BLOWER SWITCH

A 3-speed blower switch (if equipped) is located on instrument panel, to right of steering column. When switch is in OFF position, auxiliary system will not operate.

CONTROL CABLES

Cables are not adjustable.

BLOWER MOTOR INOPERATIVE

1. Disconnect Orange wire at blower motor. Connect test light from Orange wire at blower motor connector to ground. If test light glows, go to step 2). If test light does not glow, repair open in Orange wire between blower motor connector and blower resistor connector or between blower resistor connector and blower switch connector. If open is not found, go to step 3).
2. Connect test light from Orange wire to Black wire at blower motor connector. If test light glows, replace blower motor. If test light does not glow, repair open in Black wire circuit between blower motor connector and instrument panel ground connector.
3. Connect test light from Brown wire at blower switch connector to ground. If test light glows, go to step 4). If test light does not glow, repair open in Brown wire circuit between blower switch connector and fuse block. See WIRING DIAGRAMS.
4. Set blower switch to LO, MED or HI position. Connect test light between ground and blower switch connector wire that applies to position of switch. LO position is Yellow wire, MED position is Light Blue wire, and HI position is Orange wire. If test light does not glow, replace blower switch. If test light glows, repair open in wires between blower switch connector and blower resistor connector.

AUXILIARY BLOWER MOTOR INOPERATIVE

1. Turn ignition switch to RUN position. Set blower switch to HI position. Connect test light from White wire at blower motor connector (located on lower left side of instrument panel) to ground. If test light does not glow, go to next step. If test light glows, go to step 3).
2. Repair open in White wire between blower motor connector and blower resistor connector (located under instrument panel on heater housing) or between blower resistor connector and HI blower relay connector (located on auxiliary heater). If wire is okay, go to step 4).
3. Connect test light from White wire to Black wire at blower motor connector. If test light glows, replace blower motor. If test light does not glow, repair open in Black wire circuit between blower motor connector and ground connector. See WIRING DIAGRAMS.
4. Connect test light from Brown wire at HI blower relay connector to ground. If test light glows, go to step 5). If test light does not glow, repair open in Brown wire circuit between HI blower relay connector and fuse block.
5. Connect test light from Orange wire to Brown wire at HI blower relay connector. If test light glows, go to step 6). If test light does not glow, repair open in Orange wire between HI blower relay connector and blower switch connector. If wire is okay, go to step 7).
6. Connect an ohmmeter to Black wire from blower switch connector to ground. If continuity exists, replace blower switch. If continuity does not exist, repair open in Black wire circuit between blower switch connector and instrument panel ground connector.
7. Connect test light from Red wire at blower relay connector to ground. If test light glows, replace blower relay. If test light does not glow, repair open in Red wire circuit between HI blower relay connector and battery junction block (located on right rear side of engine compartment).

BLOWER MOTOR OPERATES IN LO AND/OR MED ONLY

1. Turn ignition switch to RUN position. Set blower switch to HI position. Connect test light from Orange wire at blower switch connector to ground. If test light glows, go to next step. If test light does not glow, replace blower switch.
2. Connect test light from Orange wire at blower resistor connector to ground. If test light glows, repair open in Orange wire connection at blower resistor connector. If test light does not light, repair open in Orange wire between blower resistor connector and blower switch connector.

AUXILIARY BLOWER MOTOR OPERATES IN LO &/OR MED ONLY

1. Turn ignition switch to RUN position. Set blower switch to HI position. Connect test light from Brown wire at HI blower relay connector (located on auxiliary heater) to ground. If test light glows, go to next step. If test light does not glow, repair open in Brown wire between HI blower relay connector and splice near relay leads.
2. Connect test light from Orange wire to Brown wire at HI blower relay connector. If test light glows, go to next step. If test light does not glow, repair open in Orange wire circuit between HI blower relay connector and blower switch connector. If wire is okay, replace blower switch.
3. Connect test light from Red wire at HI blower relay connector to ground. If test light does not glow, repair open in Red wire between HI blower relay connector and splice near relay leads. If test light glows, go to next step.
4. Connect test light from White wire at HI blower relay connector to ground. See WIRING DIAGRAMS. If test light does not glow, replace HI blower relay. If test light glows, repair open in White wire between HI blower relay connector and blower resistor connector.

BLOWER MOTOR OPERATES IN HI ONLY

1. Turn ignition switch to RUN position. Set blower switch to position which does not operate. Connect test light from wire of inoperable blower switch position at blower switch connector to ground. LO position is Yellow wire; MED position is Light Blue wire.
2. If test light does not glow, replace blower switch. If test light glows, connect test light from either Yellow wire or Light Blue wire (depending on switch position) at blower resistor connector to ground.
3. If test light does not glow, repair open in Yellow wire or Light Blue wire between blower resistor connector and blower switch connector. See WIRING DIAGRAMS. On "G" series, if test light glows, replace blower switch. On all other models, if test light glows, go to next step.
4. Connect test light from Orange wire at blower resistor connector to ground. If test light does not glow, replace blower resistor. If test light glows, circuit is okay.

AUXILIARY BLOWER MOTOR OPERATES IN HI ONLY

1. Turn ignition switch to RUN position. Set blower switch to position which does not operate. Connect test light from wire of inoperable blower switch position to ground. LO position is Yellow wire and MED position is Light Blue wire. If test light glows, go to next step. If test light does not glow, replace blower switch.
2. Connect test light from Brown wire at LO or MED blower relay connector to ground. If test light glows, go to next step. If test light does not glow, repair open in Brown wire between LO or MED blower relay connector and splice near relay leads. See WIRING DIAGRAMS.
3. Ensure blower switch is in inoperable position. Connect test light from Brown wire to Yellow wire or Light Blue wire at blower relay connector. If test light glows, go to next step. If test light does not glow, repair open in Yellow wire or Light Blue wire between blower relay connectors and blower switch connector.
4. Connect test light from Red wire at LO or MED blower relay connector to ground. If test light glows, go to next step. If test light does not glow, repair open in Red wire between blower relay connector and splice near relay leads.
5. Ensure blower switch is in inoperable position. Connect test light from Yellow wire or Light Blue wire at blower relay connector to ground. If test light glows, go to next step. If test light does not glow, replace blower relay.
6. Connect test light from Yellow wire or Light Blue wire at blower resistor connector to ground. If test light glows, replace blower resistor. If test light does not glow, repair open in Yellow wire or Light Blue wire between resistor block connector and LO or MED blower relay connector.

REAR TEMPERATURE DOOR INOPERATIVE

1. Turn ignition switch to RUN position. Set blower switch to LO, MED or HI position. Connect test light from Brown wire at temperature door relay connector (located on heater) to ground. If test light glows, go to step 2). If test light does not glow, repair open in Brown wire circuit between temperature door relay connector and fuse block.
2. Connect an ohmmeter from Black wire at blower switch connector to ground. If continuity exists, go to step 3). If continuity does not exist, repair open in Black wire circuit between blower switch connector and instrument panel ground connector. See WIRING DIAGRAMS.
3. Connect test light from Brown wire to Dark Green wire at temperature door relay connector. If test light glows, go to step 4). If test light does not glow, repair open in Dark Green wire between temperature door relay connector and blower switch connector. If open is not found, replace blower switch.
4. Connect test light from Brown wire to Black wire at temperature door relay connector. If test light does not glow, repair open in Black wire circuit between temperature door relay connector and ground connector. If test light glows, connect test light from Brown wire at temperature door relay connector to ground. If test light glows, go to step 5). If test light does not glow, replace door relay.
5. Connect test light from Brown wire at temperature door motor connector to ground. If test light glows, go to step 6). If test light does not glow, repair open in Brown wire between temperature door motor connector and temperature door motor relay connector.
6. Connect test light from Brown wire to Yellow wire at temperature door motor connector. If test light glows, replace temperature door motor. If test light does not glow, repair open in Yellow wire between temperature door motor connector and temperature door relay connector.

HEATER CORE