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Thread: E46 radiator fan issues

  1. #1
    Join Date
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    04 330xi

    E46 radiator fan issues

    Ok so here is the rub. Car overheated blowing out the expansion tank. I replaced that and found that the radiator fan would not turn on. I tested voltage at the fan and had 12v bought a new fan and had the same issue. Here is the weird part if I reverse the voltage at the fan it runs fine in reverse but will not do anything when hooked up properly. Please help any info would be great.

  2. #2
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    The fan ALWAYS has 12 volts, and it always has a ground. It's controlled by a pulse-width modulated signal from the DME (engine computer), which tells it how fast to turn, via the small wire.

    I have to say that by powering it backwards, you may have destroyed the circuit board / computer chip which is part of the fan you just bought. Let's hope that's not true.

    The switch in your lower radiator hose tells the DME how hot the coolant is, and when to turn on the fan. Along with that, the air conditioning system pressure will also turn the fan on, also by command from the DME.

    If there's an air bubble in your cooling system, because you didn't bleed the system properly after replacing the tank, then coolant may not flow, and the fan switch in the lower hose won't get hot.

    By the way, is this car an automatic transmission, or manual? The automatic car has an engine-driven fan, which is the primary cooling fan.

    Where did you buy the new fan? If you bought it at a local auto store, it's likely Chinese garbage anyway; return it and buy an OEM part. If you bought it at a reputable BMW specialist supplier, you might want to exchange it under warranty, since you might have destroyed it by hooking it up backwards. Then bleed the system fully, and maybe replace the switch in the lower hose.

    Chris Powell
    Racer and Instructor since, well. decades, ok?
    Master Auto Tech, owner of German Motors of Aberdeen
    BMWCCA 274412
    German Motors is hiring ! https://www.bimmerforums.com/forum/s...1#post30831471

  3. #3
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    It's a manual and once the car gets up to temp it blows good heat. Normally I'm used to that meaning the coolant system doesn't have air pockets but I am new to BMW. I have no voltage readings on the small wire when temp is over half on the guage. What voltage readings should I be getting on the lower radiator hose sensor if it is working properly?

    - - - Updated - - -

    Also the fan does not turn on when you put the a/c on max as well.

  4. #4
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    You won't measure voltage on the small black/green wire to the fan. Again, it is a pulse-width signal.

    Maybe, just maybe, you can look for the signal with your multimeter set to FREQUENCY. Maybe not, I've never tried.

    I use a BMW diagnostic computer to activate the fan, to see if it works. Most good BMW specific diag tools will do this, including the Foxwell NT510, which is about $150 at Foxwell or eBay.
    If I'm not mistaken, the lower rad hose sensor has only a ground and a signal wire. Again, though, I haven't ever tested this; I just hook up a scan tool and read the lower hose temp and engine temp.

    Does your a/c actually work? The a/c pressure switch tells the DME (engine computer) when the system pressure is too high, so that the DME can activate the fan. If the a/c doesn't work, or the pressure switch is bad, the fan won't activate. If everything DOES work, it still usually takes half a minute to raise the pressure enough to require the fan.

    You really need a good BMW specific diagnostic computer / scan tool to do diagnostics. You need to know what data the engine computer sees, so you know whether it's even trying to turn on the fan.

    Again though, you DID deliver power and ground BACKWARDS to a computer circuit board, so your new fan may be toast, and can't even read the pulse width signal from the DME any more. And if it's a cheap Chinese fan, it's quite likely to have been bad when you took it out of the box.

    Chris Powell
    Racer and Instructor since, well. decades, ok?
    Master Auto Tech, owner of German Motors of Aberdeen
    BMWCCA 274412
    German Motors is hiring ! https://www.bimmerforums.com/forum/s...1#post30831471

  5. #5
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    If I have to get a new fan then I have to get a new one, thats understandable. So if I hook the fan back up properly and unpulg the lower radiator sensor and or turn the a/c the fan should start to run regardless in about a minute or so max right?

  6. #6
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    The DME tests the electric fan at each start up by running the fan at 20% speed for a few seconds. The temperature sensor in the lower radiator hose is a negative coefficient thermistor. That is, it’s a resistor that has decreasing resistance as its temperature increases. The DME sends a voltage to the sensor on one wire and senses the voltage drop on the other wire.

  7. #7
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    https://screenshots.firefox.com/fzJP....prodemand.com

    No power; the resistor measures resistance to the ground .

    The screenshot above will disappear in 14 days.


    Jumper the two pins of the plug for the fan switch. This will represent zero resistance, and therefore max temperature. Unplugging the sensor without jumpering it represents negative 40 degrees; no fan activation/

    However, the correct test of the fan is to command the fan "on", using a BMW specific scan tool. If you jumper the plug I mentioned, it will have no effect at all if the circuit is open....if one of the wires has been eaten by a rodent, for instance.

    There are a couple of plugs in that vicinity, too; what color are the wires at your plug?

    Chris Powell
    Racer and Instructor since, well. decades, ok?
    Master Auto Tech, owner of German Motors of Aberdeen
    BMWCCA 274412
    German Motors is hiring ! https://www.bimmerforums.com/forum/s...1#post30831471

  8. #8
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    Oooops! Corrected by the master, again. But, what I wrote about the operation of the sensor (i.e., negative coefficient thermistor) is correct.

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