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Thread: Aux fan blowing fuse

  1. #1
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    Aux fan blowing fuse

    My '93 525i has a good working air conditioner except for the aux fan. The resistor is burned out so there is no low speed, which is required whenever the a/c is turned on. The fan does work on high speed when the 99C temp switch or the relay are jumpered, so it's just the resistor. (The engine cooling fan is electric and on a thermostatic controller, so when it's not running and the car is stopped, there is no airflow over the condenser. Pressures got very high before I realized this, although not quite high enough to trip the overpressure switch).

    So, I thought I would just temporarily jumper both the 91C and 99C contacts on the temp switch, which causes the aux fan to run (at high speed) when either contact closes or the climate control calls for aux fan. Unfortunately, after a few start/stop cycles the 30 amp fuse blows. I think it's possible that the aux fan motor is intermittently shorting internally, which would not only blow the fuse but probably damage the resistor eventually (which has already opened).
    But another possibility occurred to me - the aux fan would rarely, if ever, go from "off" directly to high speed. It normally starts in low speed (either coolant > 91C or a/c or both), so it's already turning at half speed when the full power for high speed is applied. The surge current when engaging high speed would therefore be significantly lower.

    Eventually I will have to replace the aux fan, which requires removing the bumper and grille and either spending $$ or rigging an aftermarket fan in place. In the meantime I think I'll experiment with a large power resistor in series with the 30A fuse (F25), and leave the jumper across the temp switch in place. That way the fan will only run on low speed, and if the fuse quits blowing I'll have confirmation.

    Thoughts?

    - - - Updated - - -

    After looking closely at a pic of a fan resistor, I could see it was labeled 0.5 ohm. So I just rigged up a 0.5 ohm 80 watt resistor (from two 1 ohm 40W's in parallel) and tried it out. The easiest way to measure current is to read the voltage across the resistor - every 2 amps is 1 volt drop
    That fan takes a few seconds to get going on low speed, and the current is around 20 amps for several seconds until it drops to a steady state 11 amps. (With the 0.5 ohm resistor and approximately 13 volts available, the 14.2 volts less wiring drops, the current can never exceed 26 amps under any circumstances so the 30 amp fuse will not blow on starting to low speed).
    Now to measure the current on high speed from a dead stop!

    - - - Updated - - -

    OK. I didn't have an analog 50 amp meter handy, so I pulled a 50-0-50 uA meter, a 0.005 ohm resistor (intended for just this type of metering) and a pot from my junkbox and made a 50-0-50 amp meter that doesn't care which direction you hook it up Can't use digital meters here, the numbers will change too fast to read. However, step changes are a bit tricky with analog meters too since the needle may overswing a bit, so some windage has to be allowed...

    Turns out the starting surge from stopped to high-speed is 45 amps for about 2 seconds until steady state at full speed, 20 amps. Obviously this will take out a 30 amp fuse (that is not slow-blow) quickly, which indeed it does! However, with the fan turning at low speed (using my 0.5 ohm resistor), the surge is approximately 25 amps. Now I know pretty much what the German engineer was thinking 25 years ago...

    Bottom line is, I could mount my homemade ballast resistor under the hood and use it for a while on low speed only, but since the fan bearings aren't the quietest either, I'll be eventually replacing the aux fan (and resistor) anyway. Going on a long vacation soon, so it can just sit there already

    (Yes, I could have just bought a fan and resistor, but I like to know WHY I'm doing something, especially when it's with my own money) .
    Last edited by DrCharles; 05-25-2014 at 08:33 PM.
    '98 740iL E38 201k, TCG at 190k, 5HP24 at 195k
    '97 540i/6 E39 Dinan blower & stage 1 suspension 114k
    '93 525i E34 "Golf Ball" (hail damaged) 334k

  2. #2
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    "the fan bearings aren't the quietest either"
    Here is the reason for the high draw

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  3. #3
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    I imagine there is some contribution there, but it does not "bind" and stop rapidly when power is removed... I can hear it winding down gradually.
    Do you know what normal current draw on low and high speed is supposed to be for this fan? Obviously less than 30 amps but I don't know how much less. The measured values of 20 amps on high and 11 amps low seem reasonable.
    Once I get a new fan I can measure that one too

    Any recommendations for a replacement? The OEM fan is rather pricey...
    Last edited by DrCharles; 05-25-2014 at 08:57 PM.
    '98 740iL E38 201k, TCG at 190k, 5HP24 at 195k
    '97 540i/6 E39 Dinan blower & stage 1 suspension 114k
    '93 525i E34 "Golf Ball" (hail damaged) 334k

  4. #4
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    I've been able to jump the high speed relay and turn the aux fan on without blowing any fuses. I even temporarily ran my Volvo fan on high speed with a 30 amp fuse and relay with no problems.

    I buy my aux fans used for a few bucks and simply take them apart, fix whatever is worn on them and put them back in service. I know the old ones (e28, e23) can be disassembled, not sure on the new ones. That's what's running in my 89 535 at the moment. Resistors are like $20 new any day.
    demet

  5. #5
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    My E23 has a 25 amp fuse for the low-speed power, and two 25's in parallel for the high speed. Never had a problem with them blowing...

    I just looked and can't see a resistor at the top of the fan, so it must be the "Magna" at the four o'clock position which looks simple to change according to this writeup: http://www.bmw4life.com/auxFanR.htm

    I have been looking for new fans online and am amazed at the variety of manufacturers (and prices)... low of $80.79 at RockAuto for a "TYC", who also has Dorman, VDO and Four Seasons in order of increasing price, and the sky's the limit for OEM BMW. Given that the bumper has to come off to replace it, I am not crazy about buying a used fan... do you have access to parts like bearings/bushings to rebuild a worn one?
    '98 740iL E38 201k, TCG at 190k, 5HP24 at 195k
    '97 540i/6 E39 Dinan blower & stage 1 suspension 114k
    '93 525i E34 "Golf Ball" (hail damaged) 334k

  6. #6
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    Ha, I drove an e23 for almost 20 years and I never knew about the 2 fuses. Thanks.

    As far as bearings and stuff, no I don't have access to those parts but the fans I bought were easily cleaned up and greased and they have worked just fine for years now. I did have to throw one out as it was too far gone to fix.
    demet

  7. #7
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    Those fuses are in a separate little box with a knurled knob near (or attached to, can't recall) the main fuse box. At least on my Euro E23

    I'm just torn between: replacing the resistor only, which seems to not require bumper removal; buying a new aftermarket fan; or a good used OEM fan...
    (or even relubricating mine and reinstalling it!)
    '98 740iL E38 201k, TCG at 190k, 5HP24 at 195k
    '97 540i/6 E39 Dinan blower & stage 1 suspension 114k
    '93 525i E34 "Golf Ball" (hail damaged) 334k

  8. #8
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    Quote Originally Posted by DrCharles View Post
    I imagine there is some contribution there, but it does not "bind" and stop rapidly when power is removed... I can hear it winding down gradually.
    Do you know what normal current draw on low and high speed is supposed to be for this fan? Obviously less than 30 amps but I don't know how much less. The measured values of 20 amps on high and 11 amps low seem reasonable.
    Once I get a new fan I can measure that one too

    Any recommendations for a replacement? The OEM fan is rather pricey...
    I have no clue what the current draw is supposed to be. If you'll consider a used replacement I can help. PM if so.

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  9. #9
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    Often the magnets get lose inside the aux fan, that can even cause complete blockage of the motor. Have had that on my E32, even piece of wire going to the fuse was melted. Can be reglued after disassembling the motor housing. Of course the housing is not built for service, so one has to bend some metal.
    http://img104.imagevenue.com/img.php...122_155lo.jpg#
    http://img254.imagevenue.com/img.php..._122_446lo.jpg
    http://img168.imagevenue.com/img.php..._122_239lo.jpg
    http://www.bilder-speicher.de/120812...tzluefter.html

    There are 6 magnets inside like this http://www.pictureupload.de/bild/576...fanrepair4.jpg
    Shogun tricks and tips for the E32 series are HERE!

  10. #10
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    Quote Originally Posted by DrCharles View Post
    Turns out the starting surge from stopped to high-speed is 45 amps for about 2 seconds until steady state at full speed, 20 amps. Obviously this will take out a 30 amp fuse (that is not slow-blow) quickly, which indeed it does! However, with the fan turning at low speed (using my 0.5 ohm resistor), the surge is approximately 25 amps. Now I know pretty much what the German engineer was thinking 25 years ago...
    That's an interesting conclusion. Amazing what these guys think of. Thanks for enlightening all of us to the operation of our aux fans! The low speed wire on the radiator sensor (or whatever it is on the passenger side) is broken, I'll definitely have to get that fixed before summer.
    1992 525i/5

    Quote Originally Posted by Binjammin View Post
    I doubt it was a bomb victim, looks like most members' cars.


  11. #11
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    Well, I'm not 100% sure I read Hans und Franz's minds correctly, but it does seem to be logical from the measurements and examination of the schematic. My fan may have dried up grease in the bearings causing the initial starting surge to be otherwise greater than intended, too. I have a good used one on the way.

    But you definitely should fix the low speed circuit. Although since you live in Ontario, the engine fan probably creates enough airflow over the condenser to keep pressures within reason I have to fix mine since it has a thermostat-controlled electric fan for the engine, and if it's not turning there is no airflow at all when the car is stopped.

    I'm flattered, you put my what-not-to-do quote in your .sig
    '98 740iL E38 201k, TCG at 190k, 5HP24 at 195k
    '97 540i/6 E39 Dinan blower & stage 1 suspension 114k
    '93 525i E34 "Golf Ball" (hail damaged) 334k

  12. #12
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    The "tested and working" aux fan arrived today, another Siemens that matches the one in my car. It does work, but the bearings are slightly noisier than the one I have, which has now quieted down after some run-time (redistributing the grease in the bearings, probably). So I decided just to swap in the resistor according to the write-up I referenced earlier - just remove the license plate, pull the center bumper cover, access resistor. Don't even have to jack the car up.

    It would have been easy except that the steel screws corroded into the aluminum resistor body on both the old and the "new" fan :P I used a Dremel tool with a small grinding stone to remove the heads from the screws in the car. On the replacement resistor, one of the two screws I persuaded to come out but the other head broke off. Drilled that out but the aluminum threads were toast. A simple 6-32 screw/lockwasher/nut and it was remounted on my existing fan. In my crimp terminal drawer I had a couple of heat-shrink/adhesive crimp butt connectors, so I used those. They cost more than the common yellow ones but very handy when working in a restricted space.

    A/C now works well, aux fan comes on low speed when it's turned on, doesn't blow the fuse
    '98 740iL E38 201k, TCG at 190k, 5HP24 at 195k
    '97 540i/6 E39 Dinan blower & stage 1 suspension 114k
    '93 525i E34 "Golf Ball" (hail damaged) 334k

  13. #13
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    Glad it's fixed. I still have a quiet fan if this one goes south

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  14. #14
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    Will keep you in mind, thanks! Do you have any advice on disassembling the fan motor and refurbishing the bearings/brushes?
    '98 740iL E38 201k, TCG at 190k, 5HP24 at 195k
    '97 540i/6 E39 Dinan blower & stage 1 suspension 114k
    '93 525i E34 "Golf Ball" (hail damaged) 334k

  15. #15
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    Quote Originally Posted by DrCharles View Post
    Will keep you in mind, thanks! Do you have any advice on disassembling the fan motor and refurbishing the bearings/brushes?
    Nope, never done it. Shogun probably has something.http://twrite.org/shogunnew/topmenu.html

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  16. #16
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    I already posted the pics I have above. This fan motor housing was made / built as unservicable, so you have to bend something/use power to get it open.
    And I still assume the magnets are lose and that is the problem, re-glue them.
    All pics that I have from this are here, starting with location of the aux fan in the car, where the fixing points are etc
    http://img158.imagevenue.com/img.php...22_1162lo.jpg#
    http://img45.imagevenue.com/img.php?..._122_725lo.jpg
    http://img104.imagevenue.com/img.php..._122_155lo.jpg
    http://img254.imagevenue.com/img.php..._122_446lo.jpg
    http://img168.imagevenue.com/img.php..._122_239lo.jpg
    http://img196.imagevenue.com/loc783/..._122_783lo.jpg

    http://www.bilder-speicher.de/120812...tzluefter.html
    arrangement of magnets http://www.pictureupload.de/bild/576...fanrepair4.jpg
    Shogun tricks and tips for the E32 series are HERE!

  17. #17
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    Hey, thanks for sharing your effort in diagnosing the possible causes of the fuse blowing.

    I am facing a similar issue with my E34. Discovered the burnt fuse only after I bought the car.

    The previous mechanic who serviced the car for the previous owner probably simply replaced the blown fuse with a larger one, which eventually caused a fire in the fuse box (know this from the burn marks in the fuse box over fuses 25 & 26). The wires connected to fuse 25 were then attached to a 30 amp slow blow fuse, but that one was also blown when I checked with a multimeter. Verified the wiring by replacing a 30 amp fuse, which blew right away when I turned the aux fan on by turning the A/C on.

    Any recommendation on the next test to do? I can't tell if the aux fan runs since the fuse just blows right away. I might get a larger fuse just to test if the aux fan works. If it works, where should I begin searching for the cause of high power draw or power surge?

    Quote Originally Posted by DrCharles View Post
    The "tested and working" aux fan arrived today, another Siemens that matches the one in my car. It does work, but the bearings are slightly noisier than the one I have, which has now quieted down after some run-time (redistributing the grease in the bearings, probably). So I decided just to swap in the resistor according to the write-up I referenced earlier - just remove the license plate, pull the center bumper cover, access resistor. Don't even have to jack the car up.

    A/C now works well, aux fan comes on low speed when it's turned on, doesn't blow the fuse

  18. #18
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    Don't put an even larger fuse in, unless you enjoy the sight of your E34 going up in flames...

    Anything that instantly blows a 30 amp fuse is pretty close to a dead short. Either the wires have rubbed through on the body somewhere (or possibly have insulation melted off and shorting from the previous hack), or the aux fan motor is toast.

    You're quite possibly going to be replacing the aux fan anyway... I'd consider removing the front trim as needed to get to the aux fan wires, disconnect them from the harness and measure resistance to ground. If it's not showing a short, you could try a couple of heavy wires to a spare battery (Briefly!) and see if it spins up. My guess is that the fan motor is toast.
    '98 740iL E38 201k, TCG at 190k, 5HP24 at 195k
    '97 540i/6 E39 Dinan blower & stage 1 suspension 114k
    '93 525i E34 "Golf Ball" (hail damaged) 334k

  19. #19
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    Quote Originally Posted by DrCharles View Post
    Don't put an even larger fuse in, unless you enjoy the sight of your E34 going up in flames...

    Anything that instantly blows a 30 amp fuse is pretty close to a dead short. Either the wires have rubbed through on the body somewhere (or possibly have insulation melted off and shorting from the previous hack), or the aux fan motor is toast.

    You're quite possibly going to be replacing the aux fan anyway... I'd consider removing the front trim as needed to get to the aux fan wires, disconnect them from the harness and measure resistance to ground. If it's not showing a short, you could try a couple of heavy wires to a spare battery (Briefly!) and see if it spins up. My guess is that the fan motor is toast.
    Yeah I agree with him on this one. Larger fuses are never the answer. Disconnect the fan and measure the resistance between the +12v and ground on the car side just like he said.
    1992 525i/5

    Quote Originally Posted by Binjammin View Post
    I doubt it was a bomb victim, looks like most members' cars.


  20. #20
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    Awesome, will take note about the fuse. Last thing I want is to see it in flames after spending half a day replacing the flex disc and bearing...

    Would a faulty resistor be a possible cause of the short? Or should there not be a closed circuit at all if the resistor is faulty?

    - - - Updated - - -

    Ok, think I found the answer to my question after reading up more on the resistor. It simply regulates high and low fan movement speed, right? I'll remove the aux fan and see if I can find the cause of the short circuit. If not, it's time for a new fan. The A/C compressor is so over stressed from not having a working aux fan, it sounds like it's spinning metal bearings around whenever the compressor clutch is activated.

  21. #21
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    AUX FAN Diagnosis E39 540i

    Here are my experiences with the aux cooling fan. First the fan would not come on at all when the AC was turned on. Note that this fan is a straight 12V DC fan and not the later Pulse Width Modulated (PWM) type. I did my research but did not really find a good step by step guide to diagnosing the fan problem with youtube vids being particularly useless.

    Car: 1999 E39 540i

    1. Check that the fan moves freely by hand. There should be a small amount of resistance. My fan was catching on the upper plastic grille cover so decided to bite the bullet and remove the fan to see what was going on as you cannot get to any of the wiring with the front end in situ.
    2. Remove headlights, front bumper and plastic kidney grille shroud, this give access to the 4 aux fan mounting bolts so it can be removed.
    3. I removed the plastic grille that was catching on the fan blades and checked that the fan moved freely, so far so good.
    4. Visually inspect all of the wiring including the connector plug. 2 terminals in the plug were showing signs of corrosion so some contact cleaner and a bit of cleaning sorted this. I was hoping that this was the cause of the problem but a quick test by turning the engine over with the AC on did not bring the fan to life.
    5. Now for testing. Find a piece of wire that is reasonably thick, a piece of 13 amp mains cable will suffice. Pair back the insulation at the terminal of each resistor on the resistor pack at the top of the fan. Remove the actual fan via a single 10mm nut - take note there are spacer washers under the fan. note their position on the shaft before removing them. Removing the fan will save your fingers should the fan start. Connect one end of the mains wire to the BATT terminal at the top of the engine - take car not to let the loose end short out on the chassis. Touch the loose end of the 12V wire to the left terminal of the left resistor. If the fan runs you have a simple lack of 12V going to the fan connector. Using a DVM check for 12V at the blue black wire on the main plug connector (with engine on and AC on). If you do not have 12V at the connector go to the relay box under the passenger side (RHD model) cabin air filter and look for 2 large wires - one is red the other is red\blue check again for 12V with engine and AC on. If you have no 12V on the red\blue wire then suspect the relay that sits underneath these 2 wires.
    6. My fan did not run during the above test so touch the 12V wire to the right hand terminal of the left resistor. Amazingly my fan motor started and ran smoothly. Repeat the test on the right hand resistor terminals. The fan should run at different speeds. So the 0.5Ohm 85 watt resistor was open circuit, I just ran a simple bypass wire around this resistor as a temporary fix and ordered 2 replacement resistors just in case the other resistor was about to fail. Ebay had similar resistors for 99 pence each rated at 100W

    http://www.ebay.co.uk/itm/183603453219

    Just select the correct resistance and wattage from the drop down menus for your resistors.

    7. If your fan does not run when performing the above test then you have a bad fan motor or a broken wire between the resistors and the fan motor. However all is not lost. There are 2 bolts that hold the back cover of the fan motor in place. Remove the bolts - these were rusted on mine and the bolts sheared when trying to remove the nuts. Carefully and I mean carefully prize off the back cover working around the cover as the bearing seats in a bulge of the cover so take your time easing this cover off. If you go at it too hard you can damage the paxolin baseboard or as I did dislodge one of the permanent magnets from the inner housing.
    8. With the cover removed carefully lift up the paxolin baseboard - the 4 brushes with spring forward but remain in place. Check the brushes, clean the armature with contact cleaner, check the transformer wire windings for burning, melting of insulation and use your nose to smell for any sign of heat\burning. Check the 4 permanent magnets to make sure that are bonded to the case. One of the magnets on mine was loose so gave the case a good clean, cleaned up the concave surface of the magnet of residual adhesive with a file and used araldite - a thin smear to fix the magnet back in place and uses cocktail sticks placed between the armature and the magnet to help it to seat and ensure that there was a gap that allowed to armature to rotate. Ensure that the araldite has fully cured before removing cocktail sticks.
    9. Time to put the motor back together. Check the armature spins freely, re seat the paxolin board with the brushes around the centre armature. Check the bearing spins freely, a bit of wd40 sprayed on the bearing for good measure, carefully replace the back cover using hand pressure only to seat the centre bearing. At this point I realised that the securing bolts has sheared off. So a quick look around the toolshed revealed 2 bolts that were long enough but not thin enough to go through the existing holes so I removed the backplate again and drilled out the 2 holes, then very carefully drilled out the 2 holes on the front of the housing taking care not to get any swarf inside.. With the cover back on check that the spindle moves fairly freely you may have to use and adjustable spanner of the shaft flats to move it around. I actually used a drill connected directly to the shaft starting slow and building up to full speed just to make sure that the magnet would not come loose. Repeat test 5 above and if the motor runs you have fixed your Aux Fan. If it does not run then the windings inside the motor must be bad and you will need a replacement motor. The motor can be purchased on its own for $80 bucks without buying the whole fan assembly for £300 quid or so.
    10. Have a brew at this point before putting the front of the car back together again.

    I have not fitted the new resistors yet so I left of the upper protective fan grille off so hopefully can fit the new resistors without removing the front end of the car although I have my doubts on this and will probably have to remove bumper etc.

    I suspect that the original cause of the problem was the fact that the fan blades were catching on the upper plastic grille causing the fan not to start resulting in excessive current draw through the resistor.

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