The issue of a Mercedes blower motor staying on even after the ignition is turned off is a common problem that can be frustrating and potentially drain the vehicle's battery. This problem typically affects various Mercedes models, including C-Class, E-Class, GL-Class, and ML-Class vehicles.

Common Causes

Faulty Blower Motor Regulator

The most frequent cause of this issue is a malfunctioning blower motor regulator. This component controls the speed and operation of the blower motor. When it fails, it can cause the blower to run continuously, regardless of the ignition state.

Moisture Damage

Moisture infiltration is another significant factor. In many cases, dampness can lead to corrosion or short circuits in the blower motor assembly, causing it to operate erratically.

Stuck Relay

Although less common, a stuck relay in the blower motor circuit can also cause the fan to run continuously.

Diagnosis and Solutions

Check the REST Feature

Before assuming a malfunction, ensure that the REST feature is not activated. This feature allows the climate control to run for up to 30 minutes after turning off the ignition. If the REST light is illuminated on the A/C controls, simply press the REST button to deactivate it.

Inspect the Blower Motor Assembly

If the problem persists, the next step is to inspect the blower motor assembly:

1. Locate the blower motor (usually behind the glove box)

2. Remove the assembly carefully

3. Check for signs of moisture or corrosion

4. Inspect the regulator board for damage

Clean or Replace Components

  • If you find corrosion, carefully clean the connections with electronic cleaner spray
  • Pay special attention to the spade lugs and connectors, as these are often the culprits
  • If cleaning doesn't resolve the issue, you may need to replace the entire blower motor assembly

Professional Diagnosis

For those uncomfortable with DIY repairs, it's advisable to seek professional help. A Mercedes specialist can perform a thorough diagnosis and repair, which may include:

  • Testing the blower motor circuit
  • Checking for CAN bus communication errors
  • Replacing faulty components

Preventive Measures

To avoid future issues with the blower motor:

  • Regularly check and clean the vehicle's cowl drains to prevent water accumulation
  • Consider applying a protective coating to the blower motor assembly if you live in a humid climate
  • Address any water leaks in the vehicle promptly to prevent moisture damage

When to Replace the Blower Motor

If cleaning and minor repairs don't solve the problem, a full replacement of the blower motor may be necessary. While this can be costly, it's often the most effective long-term solution for persistent issues.

Remember, a properly functioning climate control system is crucial for comfort and safety in your Mercedes. Addressing blower motor problems promptly can prevent more extensive damage and ensure your vehicle remains in top condition.