Faulty Demister Fan On Vauxhall Astra.

The fan switch merely diverts the electric power through three different resistors to achieve the 'less than full' speeds. the fourth speed is direct 12volt to the fan, this why your fan still works on flat-out. Probably your resistor pack has failed, although it may be repairable. I can't remember where your one is, but usually the resistor pack is with the fan or in the airflow from the fan.

First place I would look is under the plastic cover at the back of the engine bay (on a vauxhall) or have a look in the manual.

HTH mrcheerful

Fan speed is controlled by a resistor. The fan switch connects different value resistors into the circuit in turn to limit current through the circuit and hence adjust the speed of the motor. On setting 4 the resistor is bypassed and the full battery voltage is applied to the motor.

It sounds as though either there the connections to the resistor have corroded or the resistor pack has gone open circuit from your description. It is mounted near to the heater motor, which is under a water shield at the back of the bonnet at the base of the windscreen. Remove this plastic panel (no tools required) to access the pollen filter (rectangular white filter). Motor and resistor are behind this. Resistor is connected to a plug with 5 wires and is held in by a couple of torx (star shaped) head screws. If the resistor connections are OK (not corroded or distorted through overheating), then the resistor has probably burnt out due to excessive current draw by a partially siezed fan motor. Try spinning the fan by hand - it should spin very freely and keep spinning for a couple of seconds by itself after flicking it over.

HTH

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