Automotive AC – Blower Motor

The blower motor is the device that forces the air through the duct box, and it is controlled by the HVAC control head. The speed of the blower motor is controlled by the amount of current that is delivered to the motor. The blower motor current is controlled by using a blower resistor device that routes the electrical flow through more resistors or fewer resistors, depending on the speed that is desired.

When a very low blower speed is needed, the blower switch directs the flow of electricity through two or three blower resistors. Each time the blower switch is moved to a higher setting, one of the blower resistors is bypassed, increasing the current in the circuit, which causes the blower motor to speed up. Ultimately, the blower motor receives more voltage each time a resistor is removed from the circuit. When the high blower speed is selected, the blower resistors are all bypassed and the blower motor gets full system voltage.

The wiring diagram in more advanced systems is helpful in troubleshooting these more modern systems. The main components in the circuit above include the AC compressor clutch coil, the AC compressor solenoid valve, the AC compressor clutch relay, the HVAC control module, and the AC refrigerant pressure sensor. These more advanced systems often require the use of scan tools and oscilloscopes to correctly diagnose and

Blower motor systems in late-model cars and trucks use a pulse width modulated–style system. These blower motors are controlled by varying the length of the pulse width signal that is sent to the motor. These systems use a blower control module instead of a blower resistor to accomplish the task of controlling blower speed. Different testing techniques are required to diagnose and repair these and will be covered later in this chapter.

The blower motor operates on 12 volts DC and is mounted in the duct box. The impeller sometimes needs to be transferred to the new motor when replacing the blower motor. The impeller sometimes gets debris, such as tissues or paper from the glove box, caught in it, which causes it to make an audible noise that is

A pulse width modulated blower motor will have more than two terminals and wires connected to the assembly. These types of blower motors require different testing techniques and

The blower resistor is used on 12-volt DC blower motor systems. This device is typically mounted in the duct box near the blower motor to allow for the airflow to keep it

The blower resistor is used on 12-volt DC blower motor systems. This device uses wire wound resistors and is typically mounted in the duct box near the blower motor to allow for the airflow to keep it

The climate control head allows the occupants of the vehicle to control the airflow in the duct box by controlling the distribution point, the temperature, and the velocity of the air being moved into the cab. The different modes the manual climate control systems include defrost, floor, and vent

The blower motor wiring diagram is a valuable tool to use when troubleshooting a blower motor problem. The key parts of a blower motor system include the blower switch, the blower resistor, the blower relay, and the blower motor. The switch and resistor are in the power side of this system. Using a blower wiring diagram along with a digital multimeter provides a method to find the problem in the

The blower motor in this wiring diagram is controlled with the switch and the blower resistor located on the ground side of the motor. The diagnostic procedure for this type of blower motor is different due to voltage being supplied to the hot side of the motor any time the heater control is turned

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