I would check the blower motor resistor. The resistor slows the current going to the blower motor to make the fan run slower for the first three speeds.The high speed runs of full battery voltage.I hope this helps.
I experienced same problem with my bmw 318 1992 model with m40 engine. i just repaired the motor resistor and now its working well. if you have other concerned on how did i repaired it pls email to me at firstname.lastname@example.org
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Re: Faulty Heater Fan
I should guess that four position switch is bad, provided it is a simple resistance type of selection. But the number four suggests it may be a binary sort of thing, 00, 01, 10, 11: off, low, med, high. If that is the case there may be more going on. But either way, you have to open up the instrument panel and see what can be done from there. In either case the switch is the first thing to look at.
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A heater blower motor resistor is an electronic device with three or more progressive resistors that allow the heater blower motor to operate at slower speeds.
The highest speed of the heater blower motor
is not controlled by the resistor, so when the resistor fails, the fan
will often work on the highest speed setting, but not the lower speeds.
To resistor is generally located under the dashboard and under the hood, near the blower motor .
The resistor's electrical connector is disconnected, the resistor is
removed and the new resistor is installed. On some vehicles it may be
easily accessible, and on others it may take partial dashboard removal
to replace the resistor. Look under the hood, generally the wiring for the Blower resistor is attached to the wiring to the blower motor itself It's inboard & below the motor, near the accumulator. You can see
it standing beside the passenger fender. it's the off-white connector
behind the L heater hose above the vacuum reservoir & the white
Hi. This will be the result of a failed resistor switch. the switch controls the current level sent to the blower motor. Once replaced, the motor will operate at your desired rate. This switch is an integrated part of the AC/Heater control cluster, that is mounted in your dash. You will need to remove the trim, and cluster assembly to expose the damaged switch.
This sounds like the blower resistor has gone out. This is the component that allows the proper voltage to the fan to change the speed. Have it checked and replaced. That should get you back in business. Hope this helps. Good luck.
The resistors are in circuit only on the lower fan speed settings.
On the highest speed setting, power is switched directly to the fan motor.
Check fan power supply fuses,
Check power is available to the fan when high speed selected.
If all above OK, apply power directly to the motor to verify fan motor runs- if it doesn't the motor may need replacement/repair.