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Thread: Overheating when AC is on

  1. #1
    Join Date
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    1995 BMW 525it

    Overheating when AC is on

    1995 525i - During non ac usage never a problem whether highway or stop and go but during summer in Arizona when AC is engaged the gauge indicates temperature rise and if I just disengage AC usage goes right back to normal. ANy tests I can perform to isolate issue? The radiator was replaced 15000 miles ago when it was leaking and no issues but since the radiator was replaced it shows on display coolant level low but goes away as soon as car is put into drive so not sure there is a issue with coolant level as it shows as full in expansion tank when cool. Any help always appreciated.

  2. #2
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    Is it when driving at speeds or only when sitting in traffic?

    I know that the aux. fan is supposed to kick on at a certain temp. or when the A/C kicks on, so you might want to check your aux. fan.

    Might also be a weak main cooling fan, so you can search how to troubleshoot that as well.
    Current Fleet:
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  3. #3
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  4. #4
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    One thing that guide doesn't mention Shogun is the low speed relay is a 5 pin - one pin for temp circuit, one pin for a/c operation. If there's a regular 4 pin relay in the low speed aux fan spot then one of those circuits (I forget which) don't get power when it should.

  5. #5
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    Last year I had a problem with my aux fan, did not come on at all. Checked relays and fuses, all o.k., then took a multimeter and tested step by step, in the end it turned out that the 3-wire plug which goes into the aux fan connector apparently did not fit 100% tight, one pin inside the plug housing vibrated and by this contact-no contact the pin terminal got hot and melted a part of the plug housing and then there was no more power on that wire.
    Used a new plug housing, repaired the one wire with a new pin and since then it works again.
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  6. #6
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    I think that may have happened on my car in the past. The insulation of the wiring to the radiator switch plug has some cracks so it must have got really hot once.
    Last edited by fo3; 04-10-2019 at 06:29 AM.

  7. #7
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    Worst case happened on an E32 750 of one of my wrenching buddies some years ago: some magnets got lose in the aux fan motors and the motor really had to work hard to get it turning, apparently just not enough that the fuse melted. Instead of that the complete wire insulation melted up to the fuse box and the fuse holder too. We had to replace all these things incl. a new aux fan.

    The problem is:The fuse can bear overload in a certain range when current fluctuation occurs because of power supply or external interference. When bigger overload current appears in the circuit, the fuse should cut off the circuit in the set time so as to protect other electric parts in the circuit. When very big current appears because of short circuit, the fuse should safely cut off the circuit so as to avoid damage by overload current.

    FUSEOLOGY 4 MB https://www.littelfuse.com/~/media/a..._fuseology.pdf
    Last edited by shogun; 04-10-2019 at 07:23 AM.
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  8. #8
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    I tried leaving a quick reply and it would not allow me to do so so my rather lengthy reply went into oblivion. I will try again.

  9. #9
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    Hope this gets through as my last one would not post. The description shown on your reply was not the same as my wires for the jump connection on the aux fan switch socket. My wire to the top socket which is the one that is completely rounded is brown or tan colored. The socket facing the front of the car where there is an indentation is Black and blue and the one facing the back of car the wire is purple or pink. These appear to be original to vehicle and I am reluctant to try and jump and connect the low and high fan directly together and cause some additional problem. I did check the fuses 25 and 29 and they have no melting or breaks and when I examined the relays they also looked OK. Is there some way to determine which of my wires would be related to the Black and Brown wires that you show in your description?

  10. #10
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    I had the same thing last year as everyone else above. No aux fan activity at all and my testing at the switch did nothing for high speed either and testing at the fan itself showed the fan worked but the switch plug was totally useless. The colors on mine were black, purple and brown so the colors used can vary but from what I recall, jumping between them won't cause any damage (since for example if you jumped the high and low speed pins, nothing would happen since neither one has power).

    Being that high speed doesn't work, I'd assume you either have a problem at the rad switch or the aux fan itself is broken. Neither are very expensive fixes since you can get aux fans fairly cheaply, though their noise level may be quite high. A solution my mechanic and I came up with was to temporarily change the pins on the fan itself for it to kick on high speed when the AC was in use since the low speed was non-functional because of the low speed resistor being blown too. I say temporarily because high speed can wear a fan out more quickly so it wasn't a good permanent solution but it wouldn't be a terrible idea if your car is overheating when using the AC.

  11. #11
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    @springglen6 - Does the the aux fan come on at all when you turn on the AC? It is wire to turn on the aux fan in low speed when the AC is switched on. The aux fan will go into high speed when coolant temp reaches 99C.

  12. #12
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    96 525it A/c overheat

    Quote Originally Posted by shogun View Post
    Last year I had a problem with my aux fan, did not come on at all. Checked relays and fuses, all o.k., then took a multimeter and tested step by step, in the end it turned out that the 3-wire plug which goes into the aux fan connector apparently did not fit 100% tight, one pin inside the plug housing vibrated and by this contact-no contact the pin terminal got hot and melted a part of the plug housing and then there was no more power on that wire.
    Used a new plug housing, repaired the one wire with a new pin and since then it works again.
    I have reached under the car and could spin the fan and then inspected the fuses in the main fuse box and they all are OK so I turned the ignition on without starting car and then jumped with a wire the 3 wire plug between the hot and the low and high speed side and the relays made clicking noises but the fan did not run. Is there another way to connect to the aux fan directly to see if it is truly the fan that is non operable or is there another fuse or connector along the way. Your information was wonderful in my attempts to diagnose the problem but before I throw in the towel and start removing all the front end pieces to replace the fan I want to make sure it is really the problem. It does overheat when it is in traffic and a/c is on but not always and as soon as I shut off the a/c the temp goes back to normal. If it wasn't so hot in Arizona I would not bother with the a/c at all but temps get up to 115 degrees farenheit.

  13. #13
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    Standard auto relay uses pin 87 as output. You hear the relays click so the control circuit is OK (power through pin 85 and 86), fuses are OK so there should be power to pin 30.

    Last steps I can suggest is check there's power to pin 30 (no wiring breaks between fuse and relay plug) or get a 12V source (lead to the battery, jump starter pack etc) and apply that 12V to socket for pin 87

    If that is all good and no fan, then fan is broken or resistor packs. There's some info out there about location and repair resistor packs already given (if not clear then shogun has it) E: There's been so much written about this it is likely to be the resistor packs or the fan itself. If it wasn't common then the info wouldn't be everywhere IMO for both low and high speed not to work it would be the fan motor
    Last edited by fo3; 04-25-2019 at 12:18 PM.

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