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 |
|---|---|---|---|---|
Blower Motor Speed Control circuit at the blower motor control processor | 1, 2 | 1 | 1, 2 | -- |
Battery Positive Voltage circuit at the blower motor control processor | -- | 1 | -- | -- |
Ground circuit of the blower motor control processor | -- | 1 | -- | -- |
Blower Motor Low Reference circuit (between motor and control processor) | -- | 1 | -- | -- |
1. Blower Motor Inoperative 2. Blower Motor Always On | ||||
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.
Reference Information
Schematic Reference
Connector End View Reference
Description and Operation
Electrical Information Reference
Scan Tool Reference
Circuit/System Verification
Ignition ON, command the blower motor ON and OFF with a scan tool. Monitor/feel the operation of the blower motor solenoid relay.
Circuit/System Testing
- Ignition OFF, disconnect the harness connector at the blower motor control processor.
- Test for less than 1 ohm of resistance between the ground circuit terminal and ground.
- Ignition ON, verify that a test lamp illuminates between the B+ circuit of the blower motor speed control processor supply voltage terminal and ground.
- Connect a test lamp between the blower motor speed control circuit and the blower motor supply voltage circuit of the blower motor control processor.
- 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.
- Ignition OFF, connect the harness connector at the blower motor control processor.
- Disconnect the harness connector at the blower motor.
- Ignition ON, verify that a test lamp illuminates between the blower motor supply voltage circuit of the blower motor control processor and ground.
- Connect a test lamp between the blower motor supply voltage circuit and the blower motor ground circuit of the blower motor control processor.
- 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 all circuits test normal, replace the blower motor.
| ⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
| ⇒ | 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/RFA 10-amp) fuse in the instrument panel (I/P) bussed electrical center (BEC) is open, test all components connected to the B+ circuit and replace as necessary. |
| ⇒ | 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. |
| ⇒ | 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. |
| ⇒ | 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 |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for HVAC setup, replacement, and programming |
