Repeat Blower Module Failure or Logic Lock after TSB 05-01-39-001A has been Performed

This PI has been revised to clarify blower control module inspection. Please discard PIC3874.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Some customers may comment on an inoperative or intermittent Heating, Ventilation and Air Conditioning (HVAC) blower motor even though TSB 05-01-39-001A has been performed. Others may comment that the blower motor speed may momentarily drop or that the blower motor continues to run after the key is turned off.

Verify that TSB 05-01-39-001A has been completed on the vehicle, if not perform the repairs per the TSB.

To avoid repeat blower control module replacements it is important to examine the failed blower control module prior to another replacement. By doing so it can be determined whether the blower control module failed due to ASIC issues (transient voltage damage), or Mosfet issues due to high current conditions (stalled motor, water in the motor, foreign objects rubbing on squirrel cage, etc). If the blower control module has failed due to a Mosfet concern (blower run on condition symptom) and the cause is not corrected the vehicle can exhibit another module Mosfet failure.

A simple way to determine the failure mode is to measure the resistance at the blower control module between the ground (terminal A) and Motor ground (terminal D) pins with a multimeter. A good blower control module value will be 1M to 1.3M ohms from terminal A to D.

For low resistance conditions (0.1 to 13 ohms) replace the blower control module. Then remove the blower motor & inspect for a tight shaft (squirrel cage doesn't spin freely by hand compared to a new motor) or corrosion on the end of the shaft that can be seen from the cover end. Replace the blower motor as necessary.

If the resistance values are 4000 to 50000 ohms across pins A to D the blower control module will have to be replaced in addition to the installation of the capacitor from the power circuit to ground (Bulletin procedure from TSB 05-01-39-001A).

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.