Auxiliary Blower Motor Malfunction

Auxiliary 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.

Circuit/System Description

The auxiliary blower motor is a variable speed fan. The HVAC control module controls the fan speed by sending a pulse width modulated signal to the auxiliary blower motor control module. The auxiliary blower motor control module varies the duty cycle across the auxiliary blower motor in relation to the pulse width modulated signal. The auxiliary blower motor speed can be adjusted from 10 percent to 90 percent duty cycle (PWM), 90 percent is high speed fan.

Circuit/System Verification

Ignition ON, command the auxiliary blower motor ON and OFF with a scan tool. Observe that the auxiliary blower motor turns ON and OFF with each command.

Circuit/System Testing

1. Ignition OFF, disconnect the X2 harness connector at the auxiliary blower motor control module.
2. Ignition OFF, 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 terminal 6 and ground.

• If the test lamp does not illuminate, test the B+ circuit for an open/high resistance.

4. Test for the following voltages between the signal circuit terminal 3 and ground with the auxiliary blower motor speed set at the specified levels:

* Off-Less than 1 volt
* Level 1-0.5-8.5 volts
* Level 2-8.5-9.5 volts
* Level 3-9.5-10.5 volts
* Level 4-10.5-11.5 volts

• If less than the specified range, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the HVAC control module.

• If greater than the specified range, test the control circuit for a short to voltage. If the circuit tests normal, replace the HVAC control module.

5. Reconnect the X2 harness connector at the auxiliary blower motor control module. Disconnect the harness connector at the auxiliary blower motor.
6. Verify that a test lamp illuminates between the B+ circuit terminal A 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, replace the Auxiliary Blower Motor Control Module.

7. Install a test lamp between the B+ circuit terminal A and the low reference circuit terminal B.
8. While monitoring the test lamp, cycle the auxiliary blower motor through the various speeds. The test lamp should not illuminate with the auxiliary blower motor in the OFF position and become progressively brighter as the auxiliary blower motor speed is increased.

• If the test lamp is ON with the auxiliary blower motor in the OFF position, test the low reference circuit terminal B for a short to ground. If the circuit tests normal, replace the auxiliary blower motor control module.

• If the test lamp is always OFF, test the low reference circuit terminal B for a short to voltage or an open/high resistance. If the circuit tests normal, replace the auxiliary blower motor control module.

• If the test lamp does not become progressively brighter as the auxiliary blower motor speed is increased, replace the Auxiliary Blower Motor Control Module.

9. If all circuits test normal, test or replace the auxiliary blower motor.

Component Testing

Install a 30A fused jumper wire between the auxiliary blower motor supply voltage terminal A and B+. Momentarily install a jumper wire between the auxiliary blower motor low reference terminal B and ground. The auxiliary blower motor should turn on.

If the function does not perform as specified, replace the auxiliary blower motor.

Repair Instructions

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

* Auxiliary Blower Control Module Replacement
* Auxiliary Blower Motor Replacement (Tahoe/Yukon/Denali/Escalade) Auxiliary Blower Motor Replacement (Suburban/XL)