Blower Motor Malfunction

Blower Motor Malfunction

Diagnostic Instructions
^ Perform the [Diagnostic System Check - Vehicle Diagnostic Information] prior to using this diagnostic procedure. Diagnostic System Check - Vehicle Diagnostic Information
^ Review [Strategy Based Diagnosis] for an overview of the diagnostic approach. Strategy Based Diagnosis
^ [Diagnostic Procedure Instructions - Vehicle Diagnostic Information] provides an overview of each diagnostic category. Diagnostic Procedure Instructions - Vehicle Diagnostic Information

Diagnostic Fault Information






Circuit/System Description

Blower Motor Control Processor
The lower motor control processor an interface between the HVAC control module and the lower motor. The lower 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 lower motor speed control processor in order to command the lower motor speed. The processor supplies 12 volts to the lower motor through the lower motor voltage supply circuit. The control processor uses the lower motor ground or low reference as a low side control to adjust the lower motor speed.

Air Speed
The lower motor forces air to circulate within the vehicles interior. The vehicle operator determines the lower motors speed by placing the lower motor switch in a desired speed position. The lower motor will only operate if the lower motor switch in any position other than OFF, and the ignition switch in the RUN position. Once a lower speed selected, the lower speed remains constant until a new speed selected.
As the requested lower speed increases, the following conditions occur:
^ The HVAC control module increases the amount of time that the lower motor speed control circuit modulated to ground.
^ The voltage and duty cycle, measured between the lower motor speed control circuit and ground, decrease.

As the requested lower speed decreases, the following conditions occur:
^ The HVAC control module decreases the amount of time that the lower motor speed control circuit modulated to ground.
^ The voltage and duty cycle, measured between the lower motor speed control circuit and ground, increase.

Diagnostic Aids
Be sure the lower motor housing not shorted to a grounded surface causing the lower motor to always be on in high speed.

Circuit/System Verification
Ignition ON, command the lower motor ON and OFF with a scan tool. Monitor/feel the operation of the lower motor.

Circuit/System Testing
1. Ignition OFF, disconnect the harness connector at the lower motor control processor.
2. Test for less than 1 ohm of resistance between the ground circuit terminal 5 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 lower motor speed control processor supply voltage terminal 6 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/BLWR 40-amp) fuse in the instrument panel (I/P) bussed electrical center (BEC) open, test all components connected to the B+ circuit and replace as necessary.

4. Connect a test lamp between the lower motor speed control circuit terminal 3 and the lower motor supply voltage circuit terminal 6 of the lower motor control processor.
5. Turn the lower 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 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 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 lower motor control processor.
7. Disconnect the harness connector at the lower motor.
8. Ignition ON, verify that a test lamp illuminates between the lower motor supply voltage circuit terminal B of the lower motor control processor and B+.
^ If the test lamp does not illuminate, test the lower motor supply voltage circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the lower motor control processor.

9. Connect a test lamp between the lower motor supply voltage circuit and the lower motor control circuit of the lower motor control processor.
10. Turn the lower motor ON high speed then to low speed. The test lamp intensity should be right when ON high speed and very dim or out when the lower on low speed.
^ If the test lamp intensity not correct, test the lower motor ground circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the lower motor control processor

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

Repair Instructions
Perform the [Diagnostic Repair Verification - Vehicle Diagnostic Information] after completing the diagnostic procedure. Diagnostic Repair Verification - Vehicle Diagnostic Information
^ [Blower Motor Control Module Replacement]
^ [Blower Motor Replacement]