Automotive AC – Replacing Blower Motor

Diagnosing and repairing the blower motor can be done by using good logic and strategy. Blower motor problems tend to fall into two categories: excessive noise and not working at all.

A blower motor that makes an audible unpleasant noise could be caused by an object or piece of paper getting stuck on the impeller wheel. This can be caused by the duct box system pulling the item into the case and it getting stuck on the impeller. The foreign material can typically be cleaned out to eliminate the noise. A faulty blower motor bearing can also cause a noise that has a metallic sound, such as a bad bearing. This can be repaired by replacing the blower motor assembly.

A blower motor that does not work at all needs to be diagnosed. The motor needs to have power and ground in order to operate. If the blower motor does not operate on any speed, then a DMM can be connected to the electrical connector of the blower motor by back- probing the terminals. After connecting the DMM, the ignition switch should be turned on and the blower switch should be set to high.

If the blower motor has system voltage dropped across it during this test, then the motor is faulty and needs to be replaced. The steps below list the typical method for replacing a blower motor that is located inside the cab under the dash area.

Replacing a Typical Blower Motor

1. Remove the lower trim panel on the passenger’s side.
2. Remove the glove box assembly to gain access to the area behind it.
3. Disconnect the blower motor electrical connector.
4. Remove the blower motor fasteners.
5. Remove the blower motor assembly.
6. Inspect the new blower motor to make sure it is compatible with the vehicle.
7. Install the new blower motor, being careful to not damage the impeller wheel.
8. Install the blower motor fasteners.
9. Connect the blower motor electrical connector.
10. Install the glove box and lower trim panel.
11. Test the blower for proper operation.

Other things that could cause problems with the blower motor include a faulty blower resistor or a faulty blower motor switch. A faulty blower resistor will typically cause the low speeds to not work, but the high speed will usually still operate. The blower switch is not a serviceable part on most vehicles built since about 1995, which would require the complete control head to be replaced if the blower switch is found to be faulty. However, the blower switch was replaceable on many older vehicles, which reduces that cost of repair.

Blower motors on late-model cars that use the brushless DC motors need to be diagnosed with an oscilloscope. These blower motors are not be diagnosed the same way as the older-style motors. The signal that causes these motors to change speeds is a variable pulse width signal that can be viewed using some type of oscilloscope. To test this signal, connect the oscilloscope leads to the signal wire and watch the signal change as the blower speed is changed on the control head. If the signal changes with the switch, then the motor needs to be replaced. If the signal does not change, then the control head is likely defective and will need to be replaced.

The voltage drop at the blower motor with the switch turned to medium should be about 7 volts. The voltage drop at the blower motor will be lower each time the blower motor switch is adjusted to a lower

The voltage drop at the blower motor with the switch turned to medium-high should be about 10

The voltage supplied to the blower motor can be tested with a voltmeter connected to the blower motor connector by back-probing the terminals at the blower motor. The voltage drop at the blower motor with the switch turned to high should be near 12

The blower motor resistor is mounted on the duct box near the blower motor. These devices need to be in the duct box so they can be cooled by the passing airflow. If one of the resistors in this device goes bad, then the lower blower speeds will not function

Old-style HVAC control heads may have serviceable parts, such as the blower motor switch. This blower switch can be unbolted from the control head and

The signal on the oscilloscope is about a 50 percent command, which means the blower motor should operate at a medium

The signal on the oscilloscope is about a 90 percent command, which means the blower motor should operate at a high

Pulse width modulated blower motors receive a varying square wave signal from the control module to change speeds. An oscilloscope allows this varying signal to be tested accurately. The signal on the oscilloscope is about a 15 percent command, which means the blower motor should operate at a low

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