Hey Gang - The HVAC blower motor recently went out and after replacing it, it now acts very erratic. The AC blows cold but the fan speed goes from full blast (even when turned to the lowest speed), then slow, medium, etc. Nothing I do seems to change it.

Appreciate any help you can provide.

Am I crazy in thinking it’s the climate control module that would likely be the culprit?


‘08 Base

 

Are you saying it's stuck at the highest speed and the button does not change the speed ???

 

No, it doesn’t matter what speed I have the fan on, the blower will blow high for a few seconds, then very low for a few seconds, then somewhere in the middle for a few seconds and it does that on repeat

 

Here is how the system operates...having a factory level or equivalent aftermarket scan tool where you can look at the inputs to the HVAC module would be helpful and when you increase fan speed that is an input so the scan tool can see the speed increase...may be a bad module but I can't guarantee that without testing...below I used a labscope to look at the fan circuit with AC on and fan at low speed...blue trace is the PWM signal the gray/black wire and at 8.48 volts...the power B+ red/black is at 6.15 volts and at 68% duty cycle.

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 as a low side control to adjust the blower motor speed.

Reference Information

Schematic Reference

HVAC Schematics

Connector End View Reference
Component Connector End Views

Description and Operation

• Air Temperature 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 Verification

1. Ignition ON, command the blower motor ON and OFF with a scan tool, the blower motor should turn ON and OFF when changing between the commanded states.
2. Observe the scan tool Blower Motor Speed parameter while changing the blower motor switch from Off through to the High speed positions. The display should change from 0–100 percent.
   • If the display does not change, replace the HVAC control module.
   
   

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the blower motor control processor.
2. Test for less than 1.0 ohm between the ground circuit terminal C 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 B+ and the 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 and the B+ circuit fuse is open, replace the blower motor control processor.
4. Connect a DMM between B+ and the control circuit terminal B.
5. Cycle the blower motor from OFF to HIGH. The voltage should increase while changing between the commanded states.
   • If voltage remains greater than 11.5 volts while changing between the commanded states, test the control circuit for a short to ground. If the circuit test normal, replace the HVAC control module.
   • If voltage remains less than 1.0 volts while changing between the commanded states, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the HVAC control module.
   • If voltage is between 1.0–11.5 volts and does not increase when changing between the commanded states, replace the HVAC control module.
6. If all circuits test normal, replace the blower motor control processor.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• Blower Motor Replacement
• Control Module References for HVAC control module replacement, setup, and programming

 

 

Just a wild guess from my experience-
Make sure the heavier 12v supply wires on the motor plug are bright and tight in the mating connector, over time high current can cause damage.
Old motor could show evidence of discoloration on those pins.

 

For fun and those unfamiliar with pulse width modulation, this is what the pwm control signal actually looks like between the control head and motor.
The varying interval of current is cycled on and off according to demand at a constant 35 times per second in our case. Voltage of pulse equals supply voltage, motor speed is varied by pulse width.

 

I changed the blower motor assembly and the pulsating still happens. Regardless of the fan speed it will pulsate from 100% to 0%. The only other logical step I can think of is to replace the climate control module itself.

 

To test control head output; with plug at blower disconnected connect a voltmeter between B+ and b terminals at blower connector, the voltage should vary up/down with adjustment and be stable if all is well at that point.
This image is from 2011 manual

 

Just replaced my blower motor and have the exact same issue. 2012 GS. Did you get your issue resolved and have any feedback I can use to fix mine?

 

yep - do NOT use anything but an OEM blower motor. I got one off eBay for $100 and after trying 3 Car Quest motors, the OEM plugged in and worked perfectly.