Owner's Problem Description
The owner reports that after replacing the blower motor resistor and its connector, the blower motor now cuts out at the maximum setting (5). Initially, the blower was not functioning, but after repairs, it works at lower settings. The voltage at the blower motor connection drops when set to 5. The owner also replaced the heater/AC control module, suspecting it might be the issue, but the problem persists with both the old and new modules. Three different resistors were tested, all yielding the same issue.
Professional Technical Analysis
The 2005 Chevy Trailblazer, equipped with a 4.2L I6 engine and approximately 150,000 miles, exhibits a malfunction where the blower motor cuts out at the maximum (5) setting. After initial repairs involving the replacement of the blower motor resistor and connector, the owner observed that while settings 1-4 function correctly, setting 5 causes voltage drop to the blower motor, leading to its failure. The electrical system's behavior indicates a potential overload condition. Given that the heater/AC control module was also replaced, this points towards a possible issue with the blower motor relay or the blower motor itself. The blower motor relay, typically located near the blower motor housing, should be inspected as it may not be providing adequate current to the motor under full load. Additionally, the possibility of a faulty blower motor causing excessive current draw must be considered. The specification for the blower motor operation requires a consistent voltage of 12V under all settings, with a maximum draw not exceeding 30A. The interconnection between the HVAC control module, blower motor relay, and the blower motor needs thorough investigation, as the relay's failure or incorrect operation could lead to insufficient voltage and current supply to the blower motor, especially on max settings. A comprehensive diagnosis using an oscilloscope and multimeter is essential to assess the voltage and current at different settings, ensuring that the system operates within specified parameters.
Possible Causes
Most common causes (ordered by frequency):
- Blower Motor Relay Failure: The relay may be unable to handle the load at max setting, causing a voltage drop. When the blower motor is set to 5, it draws more current (up to 30A), potentially exceeding the relay's capacity, leading to a premature cut-off. Testing the relay's operation under load with a multimeter is crucial. Relay part number is typically OEM 15146282. - Intermittent operation at maximum speed, voltage drop at the relay output when tested under load.
- Blower Motor Overcurrent: The blower motor itself may be drawing excessive current due to internal shorting, especially under full load. Testing the motor with an ammeter could reveal if the current exceeds the specified range of 30A. If the motor is faulty, it should be replaced with OEM part number 15994436. - High current draw when set to maximum, potential burning smell from the motor.
- HVAC Control Module Malfunction: Although replaced, the new HVAC control module may still have compatibility issues or internal faults affecting blower motor operation. Verifying the module's output signals under different settings is essential. - Inconsistent output signals at varying speeds; possible stored codes from previous module.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial Voltage and Current Check: Begin by using a multimeter to measure the voltage at the blower motor connector at each speed setting. Verify that the voltage remains above 12V at settings 1-4 and drops significantly or falls below 12V at setting 5. Concurrently, measure the current draw with an ammeter to see if it exceeds 30A at setting 5.
- STEP 2 - Relay Testing: Inspect the blower motor relay for any signs of damage or burning. Use a multimeter to test its continuity and operation both at rest and under load. If the relay fails to operate properly, it should be replaced with the specified OEM part.
- STEP 3 - Motor Testing: Disconnect the blower motor and perform a resistance test on the motor windings. The resistance should be within acceptable limits (typically 5-10 ohms). If the resistance is too low, the motor may be shorted internally, necessitating replacement.
- STEP 4 - Control Module Output Check: Use a scan tool or GeekOBD APP to monitor the HVAC control module's output signals at varying speed settings. Verify that the output corresponds correctly with the inputs and that no fault codes are present.
