Owner's Problem Description
The owner reports that the air conditioning blower stops working intermittently. Initially, it blows air but then ceases to function completely, leading to no airflow inside the cabin. The owner has not performed any previous repairs and notes that the problem occurs randomly, without a clear pattern. The vehicle is in otherwise operational condition but lacks adequate cabin comfort due to this A/C issue.
Professional Technical Analysis
The 1991 Dodge Dakota, equipped with a standard A/C system, is experiencing an intermittent blower motor failure, which is crucial for cabin air circulation. The vehicle has approximately 150,000 miles on the odometer. Symptoms include complete stoppage of airflow from the blower motor, which can occur suddenly while driving or idling. This issue could stem from a faulty blower motor, a bad blower motor resistor, or wiring/connectivity issues. The blower motor requires a stable 12V supply to operate, while the resistor regulates the voltage to the motor. Common failure modes in this model include deteriorating electrical connections due to age or corrosion, as well as potential overheating of the blower motor leading to thermal shutdown. When the blower stops, diagnostics should include checking for power at the blower motor connector using a multimeter. If power is present but the motor does not run, the motor is likely defective. Conversely, a lack of power indicates a need to inspect the blower motor resistor and associated wiring. Given the vehicle's age, it is prudent to check for any signs of wear or damage in the electrical system, including the resistor plugged in under the passenger side dash, which is known to fail over time. Safety concerns arise if air conditioning is essential for comfortable driving, especially in hotter climates. Addressing this issue promptly is critical to ensure passenger comfort and to prevent further electrical failures.
Possible Causes
Most common causes (ordered by frequency):
- Blower Motor Failure: The blower motor may have internal failures such as worn brushes or a burnt-out winding, preventing it from running when powered. When tested, the motor should show continuity and resistance values of approximately 0.5 to 2 ohms. This can be confirmed by applying direct battery voltage to the motor terminals and observing its operation. If the motor fails to run, replacement is necessary. Typical replacement costs range from $100-200 for an aftermarket unit. - No airflow from A/C, blower motor fails to respond when tested directly with battery voltage.
- Blower Motor Resistor Malfunction: The resistor regulates the voltage supplied to the blower motor. A failure can cause the motor to receive no voltage, resulting in it not operating at any speed. Testing the resistor involves checking resistance values, which should be within the range of 5-15 ohms. If open or shorted, replacement is necessary, typically costing $30-60. Confirm functionality by checking for power at the blower motor connector when the A/C is activated. - No speeds available on the blower, resistor shows open circuit when tested.
- Electrical Connectivity Issues: Loose or corroded connections at the blower motor or resistor can lead to intermittent failures. Checking connectors and wiring for corrosion, fraying, or disconnection is essential. Voltage drop tests should be performed to ensure robust connectivity, with acceptable voltage drop being less than 0.5V under load. Repairing or replacing damaged connectors can resolve this issue, with repair costs varying based on severity. - Intermittent blower operation, signs of corrosion on connectors inspected during diagnosis.
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 Visual Inspection: Begin with a thorough visual inspection of the blower motor and resistor connections located under the passenger side dash. Look for signs of corrosion, loose connections, or damaged wires. Ensure that the connector pins are clean and secure. This initial step can often reveal obvious issues before proceeding with electrical tests.
- STEP 2 - Power and Ground Check: Using a multimeter, check for power at the blower motor connector when the A/C is engaged. The voltage should be around 12V. If there is no power, perform a continuity test on the wiring leading to the motor and resistor. Ensure that the ground connection is intact as well. Document any voltage drop readings found during the tests.
- STEP 3 - Blower Motor Function Test: If power is present at the blower motor connector, apply direct battery voltage to the motor terminals. Observe if the motor operates correctly. If it runs, the issue may lie with the resistor or wiring. If it does not run, replacement of the blower motor is necessary. Document the results of this test for reference.
- STEP 4 - Resistor Testing: If the blower motor functions normally when powered directly, test the blower motor resistor for continuity and resistance values. The resistor should show resistance within the specified range. If it fails the test, it must be replaced. Reconnect all components and retest the system to ensure proper operation.
