For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | 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 | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Rear Blower Motor Speed Control | 2 | 2 | 2 | --
Aux. Blower Motor Control Module Ground | -- | 2 | -- | --
Blower Speed Feed Back Signal | 2 | 2 | 2 | --
Auxiliary Blower Motor Low Reference | 1 | 2 | 2 | --
Auxiliary Blower Motor Ground | -- | 2 | 2 | --
Auxiliary Blower Motor B+ | 2 | 2 | -- | --
1. Always ON 2. Inoperative

### Circuit/System Description

#### Auxiliary HVAC Control Module

The front HVAC control module must be in the ON position for GMLAN communication to allow the rear HVAC control module to operate.

#### Auxiliary Blower Motor Control Module

The following are auxiliary blower motor control module circuits:

• | Rear blower motor speed control circuit--When the auxiliary blower switch is placed in any speed position this circuit is a pulse width modulated signal that is sent from the Auxiliary HVAC control module to the auxiliary blower motor control module.

• | Ground--A constant body ground is supplied to the auxiliary blower motor control module.

• | Low reference--This circuit is grounded through the auxiliary blower motor control module only when the auxiliary HVAC control module is in high speed. In all other blower speeds this circuit is a pulse width modulated signal (PWM). The longer this circuit is pulsed the ground the faster the blower speed will be.

• | Blower speed feed back signal--This feedback circuit displays in percent on the TECH 2 the commanded output blower speed that the auxiliary blower motor control module is sending to the auxiliary blower motor.

#### Auxiliary Blower Motor

The following are auxiliary blower motor circuits:

• | Ground--A constant body ground is supplied the blower motor.

• | Ignition voltage--Ignition voltage is supplied to the blower motor from the underhood fuse box.

• | Low reference--This circuit is grounded through the auxiliary blower motor control module only when the auxiliary HVAC control module is in high speed. In all other blower speeds this circuit is a pulse width modulated signal (PWM). The longer this circuit is pulsed the ground the faster the blower speed will be.

### Reference Information

#### Schematic Reference

HVAC Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Air Delivery Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Testing

1. Ignition OFF, disconnect the auxiliary blower motor Connector X1 terminal 1.
2. Test for less than 1.0 ohm between the low reference circuit terminal 1 and ground.

⇒ | If greater than 1.0 ohm, test the ground circuit for an open/high resistance.

3. Ignition ON, verify that a test lamp illuminates between the B+ circuit terminal 2 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 is open, replace the aux blower motor.

4. Ignition OFF, disconnect the harness connector at the auxiliary blower motor control module.
5. Ignition ON, verify that a test lamp does not illuminates between the control circuit terminal X2 pin 3 and ground.

⇒ | If test lamp illuminates, test the control circuit for a short to voltage.

6. Test for less than 1.0 ohm between the ground circuit terminal X2 pin 5 and ground.

⇒ | If greater than 1.0 ohm, test the ground circuit for an open/high resistance.

7. Connect the auxiliary blower motor connector.
8. Ignition ON, verity that a test lamp illuminates between the control circuit terminal X2 pin 3 and ground.

⇒ | If test lamp does not illuminate, test the control circuit for a short to ground or an open/high resistance. If circuit test normal replace auxiliary blower motor.

9. Select the AUTO position on the front HVAC module.
10. Test for 12-27 mV between the control circuit terminal X2 pin3 and ground while adjusting the rear auxiliary HVAC control module blower from min to max.

⇒ | If less than 12 mV, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal replace the rear auxiliary HVAC module.

⇒ | If greater than 27 mV, test the control circuit for a short to voltage. If the circuit tests normal replace the rear auxiliary HVAC module.

11. Test for above 10 volts while adjusting the rear auxiliary HVAC blower switch to max setting.

⇒ | If less than 10 volts replace the rear auxiliary HVAC control module.

12. Select the AUTO position on the rear auxiliary HVAC control module if equipped or press the AUX button on the front HVAC control to have the rear blower controls mimic the front control.
13. Test for 12-145 mV between the control circuit terminal X2 pin 3 and ground while adjusting the front HVAC control module blower from low speed to high speed.

⇒ | If less than 12 mV, replace the front HVAC control module.

⇒ | If greater than 145 mV, replace the front HVAC control module.

14. Place the front and rear HVAC modules in low speed.
15. Test for 0.2-15 mV between the signal circuit terminal X2 pin 3 and ground.

⇒ | If less than 0.2 mV, test the signal circuit for a short to ground or an open high resistance. If the circuit tests normal replace the rear auxiliary HVAC module.

⇒ | If greater than 15 mV, test the signal circuit for a short to voltage. If the circuit tests normal replace the rear auxiliary HVAC module.

16. If all circuits test normal, replace the auxiliary blower motor control module.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Auxiliary Blower Motor Replacement

• | Auxiliary Blower Control Module Replacement

• | Control Module References for HVAC Control Module or Auxiliary HVAC replacement, setup and programming