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jumping the aux fan resistor

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    jumping the aux fan resistor

    Hi There,

    New to working on cars in general!

    I have come to find out my Aux Fan on my 91 325i is not turning on at all. Spoke to a shop and they suggested replacing the fan all together, but after some research it seems the common answer to fix the issue is to either a) jump the aux fan resistor (located under the license plate area) or b) completely replace the resistor if that fails to work. I've also heard it may be just a blown fused (fuse 19,20) but I am not sure how to tell if the fuses are blown.

    Given my luck and skill with anything electrical when it comes to cars I am not sure what "jumping" the resistor means and how you are supposed to do that.

    Any advice would be greatly appreciated! In addition, what kind of tool do you need to test or jump the above?
    If I had to live my life over, I'd live over a Saloon" - WC Fields

    1991 325i
    1972 R75/5 Cafe

    The FAQ/DIY section is for staff-approved instructional posts only. Please do not ask questions there. If you don't know where to ask your question, post it in General Technical and we'll move it to the appropriate subforum if necessary.
    Originally posted by kronus
    would be in depending on tip slant and tube size


      A bad resistor will only cause the fan to not turn on at low speed. It should still come on at high speed.

      There is a thermo switch on the passenger side of the radiator. With the key turned to the on position, connect the brown wire to each of the other wires. The fan should turn on. If it does not, you either have a bad relay or bad fuses.

      Use an ohm meter to check the fuses. You should see no resistance, not an open circuit. If you don't have a meter, just replace the fuse with a new one. You can also generally see if it is blown. BMW fuses have the actual fuse part of the wire on the top and you can see if it is melted. Aftermarket ones you can see from the side like so:


        here's how to tell if your fuse 20 works: turn on the hvac blower motor. If it comes on, fuse 20 is good! Yay!

        Also you can buy just the resistor and replace that. You just have to cut out the old one and solder in the new one. Not hard if you have any electronic experience whatsoever. Or if you have access to youtube.

        Jumping the resistor would be counter intuitive as the fan would operate in "high" all the time. It is a quick fix and not as the car was intended to operate.

        it's a Kenny Powers quote on wheels