Owner's Problem Description
The owner reports that when the ignition is turned off and keys are removed, the light on the AC knob remains illuminated and the blower continues to blow hot air, draining the battery overnight. They have to manually turn off the AC knob to stop it. The AC clutch sounds like it's engaged, further contributing to the battery drain. Current vehicle condition is deteriorating due to this issue.
Professional Technical Analysis
The 2006 Mitsubishi Galant is experiencing a significant electrical issue where the AC system remains active even when the ignition is off. This problem is likely linked to a malfunctioning blower motor relay or AC compressor relay, which can cause the blower to operate continuously, leading to battery drain. With approximately 120,000 miles on the vehicle, component wear could be a factor. The owner's description indicates that the AC knob light stays on, which is unusual and suggests a failure in the control circuit. System interconnections must be evaluated, as the AC control module interacts with the blower relay. A stuck relay can keep the circuit closed, leading to unintended operation of the blower motor, even while the vehicle is off. Safety concerns arise from the constant battery drain, potentially leaving the owner stranded due to a dead battery. Notably, thermal expansion or relay degradation is common in vehicles of this age, particularly under high humidity conditions. The owner replaced the blower motor relay with success, indicating that the original relay was likely the root cause of the issue, as it could have been sticking in the closed position, allowing current to flow to the blower motor continuously. Additionally, the AC clutch engaging indicates that the compressor was also being inadvertently powered. This scenario aligns with documented failure patterns for the Galant's electrical system, where relay failures are prevalent.
Possible Causes
Most common causes (ordered by frequency):
- Blower Motor Relay Failure: The relay, located behind the left side of the dash, may be sticking in the closed position, causing the blower motor to operate continuously. This relay is responsible for controlling the power to the blower motor, and if it fails to disengage, it will keep the blower running regardless of ignition status. The relay typically wears out or becomes heat-sensitive, leading to intermittent operation. The relay should be tested for proper operation and replaced if found faulty. - Continued operation of the blower after ignition is turned off, visual inspection reveals relay remains engaged, voltage present at blower motor connector when ignition is off.
- AC Compressor Relay Malfunction: The AC compressor relay may also be stuck, causing the AC clutch to engage even when the vehicle is off. This could lead to the compressor running continuously, which would also drain the battery. The relay should be checked for proper function and replaced if it is found to be faulty. A common failure mode includes internal contact welding due to high current draw. - AC clutch engages while the ignition is off, relay shows signs of overheating or damage, voltage readings at AC clutch connector indicate power is being supplied.
- Control Circuit Failure: There may be a wiring issue or a failure in the control circuit that keeps the blower motor and AC clutch powered when they should not be. Inspect wiring harness for shorts or damage, especially around connectors that may be exposed to heat or moisture. A continuity test may be necessary to ensure there are no breaks in the circuit. - Visual inspection of wiring shows signs of wear or exposure, multimeter test reveals unexpected continuity when ignition is off.
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 vehicle's interior, focusing on the AC control knob and associated wiring. Look for signs of wear, damage, or modifications that may affect the circuit. Check for any visible corrosion on connectors and inspect the relays for signs of overheating.
- STEP 2 - Relay Function Testing: Using a multimeter, test the blower motor relay and AC compressor relay for proper operation. Check for continuity in both the energized and de-energized states. A faulty relay should be replaced to eliminate it as a potential cause. Ensure the relays are seated properly in their sockets.
- STEP 3 - Electrical Testing: Perform an electrical test on the blower motor and AC compressor circuit. Use the GeekOBD APP or a professional scanner to monitor voltage at the blower motor connector with the ignition off. Voltage readings should be zero; any reading indicates a failure in the relay or control circuit.
- STEP 4 - Wiring Inspection: Inspect the wiring harness for any signs of damage or shorts, particularly around the connectors. Use a multimeter to check for continuity and resistance values, looking for any abnormal readings which may indicate a problem. Pay special attention to areas exposed to heat or moisture.
