The Mercedes Benz fan blower motor regulator is a series of resisters in a printed circuit board about 2-inches square located right next to the blower motor. The Mercedes has a series of sensors that, working in concert with a computer, maintain a constant climate in response to the settings on the console. These sensors even sense when the window is down or the door is open and increase the fan speed as necessary to maintain a constant temperature within the car.
Remove the passenger side, lower dash cover under the glove compartment that conceals the blower motor. A 1/4-inch socket will be needed to remove the screws holding the cover. Locate the blower motor. The motor can be seen hanging down close to the passenger kick panel. Just to the left of the blower motor, an electrical connector connected to a flat 1-inch-by-2-inch plate can be seen. This is the blower motor resister, which controls the fan speeds.
Disconnect the multi-wire connector from the blower motor resister next to the blower motor. Start the car and turn the air conditioning on.
Test the connector for power at the terminals in the connector using the voltmeter. If there is power, reinstall the connector to the resister. If there is no power at the connector, there is a problem with the console on the dash, and the console should be replaced.
Remove the wire connector that can be seen at the blower motor itself. Test this connector for power. If there is power at this connector, the blower motor is at fault and needs to be replaced. If there no power is found, the blower motor resister is the culprit and needs to be replaced.
As a note, 85 per cent of the time when the blower motor does not come on or operates at one speed only, the blower motor resister is at fault. The resister emits substantial heat from the resistance it supplies and eventually burns up. The resister is inexpensive as parts go and is simple to replace. Just disconnect the connector and remove the two screws to replace the resister. To test using the voltmeter, touch a good ground with the negative lead. Test each circuit by touching the positive lead to the circuit.