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Blower Motor Malfunction



Blower Motor Malfunction

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Diagnostic Fault Information






Circuit/System Description

Blower Motor Control Processor

The blower motor control processor is an interface between the HVAC control module and the blower motor. The blower motor speed control, battery positive and ground circuits enable the control processor to operate. The HVAC control module provides a pulse width modulation (PWM) signal to the blower motor speed control processor in order to command the blower motor speed. The processor supplies 12 volts to the blower motor through the blower motor voltage supply circuit. The control processor uses the blower motor ground or low reference as a low side control to adjust the blower motor speed.

Air Speed

The blower motor forces air to circulate within the vehicles interior. The vehicle operator determines the blower motors speed by placing the blower motor switch in a desired speed position. The blower motor will only operate if the blower motor switch is in any position other than OFF, and the ignition switch is in the RUN position. Once a blower speed is selected, the blower speed remains constant until a new speed is selected.

As the requested blower speed increases, the following conditions occur:

* The HVAC control module increases the amount of time that the blower motor speed control circuit is modulated to ground.
* The voltage and duty cycle, measured between the blower motor speed control circuit and ground, decrease.

As the requested blower speed decreases, the following conditions occur:

* The HVAC control module decreases the amount of time that the blower motor speed control circuit is modulated to ground.
* The voltage and duty cycle, measured between the blower motor speed control circuit and ground, increase.

Diagnostic Aids

Be sure the blower motor housing is not shorted to a grounded surface causing the blower motor to always be on in high speed.

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the blower motor control processor connector.
2. Test for less than 1.0 ohm of resistance between the ground circuit connector terminal A and ground.

• If greater than the specified range, test the ground circuit for an open/high resistance.

3. Ignition ON, verify that a test lamp illuminates between the B+ circuit of the blower motor speed control processor supply voltage terminal connector terminal C and ground.

• If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse (HVAC 15-amp) fuse in the instrument panel (IP) BEC is open, test all components connected to the B+ circuit and replace as necessary.

4. Connect a test lamp between the blower motor speed control circuit connector terminal B and the blower motor supply voltage circuit connector terminal C of the blower motor control processor.
5. Turn the blower motor ON high speed and then OFF. The test lamp should dimly turn ON and OFF when changing between the commanded states.

• If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the HVAC control module.
• If the test lamp is always OFF, test the speed control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the HVAC control module.

6. Ignition OFF, connect the harness connector at the blower motor control processor.
7. Disconnect the harness connector at the blower motor terminal A.
8. Ignition ON, verify that a test lamp illuminates between the blower motor supply voltage circuit terminal C of the blower motor control processor and ground.

• If the test lamp does not illuminate, test the blower motor supply voltage circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the blower motor control processor.

9. Connect a test lamp between the blower motor supply voltage circuit terminal C and the blower motor ground circuit terminal A of the blower motor control processor.
10. Turn the blower motor ON high speed then to low speed. The test lamp intensity should be bright when ON high speed and very dim or out when the blower is on low speed.

• If the test lamp intensity is not correct, test the blower motor ground circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the blower motor control processor.

11. If all circuits test normal, replace the blower motor.


Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests

* Blower Motor Control Module Replacement
* Blower Motor Replacement
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