HOW TO: Diagnose Heater A/C Blower System


old_master
12-16-2007, 08:44 PM
Manual HVAC System Blower Motor Diagnosis

There are two fuses used in the HVAC blower circuits: A 30amp fuse in the Underhood Bussed Electrical Center, (UBEC) and a 20amp fuse in the Instrument Panel, (IP) fuse block, always check them first. The HVAC blower motor is mounted in the HVAC housing, under the hood on the passenger side, near the hood hinge. There are 2 wires connected to the blower motor. The purple wire comes from the blower relay. The black wire must have less than 5 ohms resistance to ground. Using a fused jumper wire, apply battery voltage to the purple wire. The blower motor should operate at high speed. If not, the blower motor is faulty.

The blower motor resistor is usually mounted near the blower motor in the HVAC housing with only the connector visible. The three lowest speeds go through the resistor. The electrical connector has 4 wires. To check the circuits from the blower switch, unplug the connector from the resistor. Turn the ignition switch to the RUN position, and the HVAC mode switch to the HEAT position. Use a volt/ohm meter or test light to check each circuit.
The brown wire should have battery voltage when the blower switch is in the low position.
The tan wire should have battery voltage when the blower switch is in the medium 1 position.
The light blue wire should have battery voltage when the blower switch is in the medium 2 position.
The dark blue wire goes to the blower relay and should have less than 5 ohms resistance to the terminal in the blower relay connector. Reconnect the blower resistor connector. If any of these tests fail, either the blower switch or the associated wiring is faulty.

The blower motor relay is usually located near the blower motor resistor, either attached to the HVAC housing or to the firewall. It is about 1 inch cubed in size. All blower speed circuits go through the blower relay. The 3 lower speeds use one circuit, and high speed uses a separate circuit within the relay. The electrical connector has 5 wires. To check the circuits, unplug the connector from the relay. Turn the ignition switch to the RUN position, and the HVAC mode switch to the HEAT position. Use a volt/ohm meter to check each circuit. The larger orange wire, (from the UBEC) should have battery voltage at all times.
The smaller orange wire, (from the blower switch fed by the IP fuse) should have battery voltage when the blower switch is in the HIGH position only.
The purple wire goes to the blower motor and must be less than 5 ohms resistance to the terminal at the blower motor connector.
The black wire must have less than 5 ohms resistance to ground.
If all of these tests pass, the relay is faulty.
The dark blue wire comes from the blower resistor and voltage will vary depending on the blower speed setting. Low speed should read the lowest voltage, and increase with medium 1, and medium 2. If voltage is not present on one or more blower speed settings, the resistor is faulty. Reconnect the blower relay connector.

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