98’ 528i Bmw Aux Fan not coming on

[Young 23]

I have searched through many threads on why the auxiliary fan in an e39 won’t come on but I have a question which the threads haven’t quite answered. If I replace the fan itself, which I did, do I have to replace the so-called resistors as well? Or do they come on the fan itself? I replaced my fan bc it wouldn’t turn on and was seized (very difficult to turn by hand). I changed the temp switch and the fan is an oe labeled Behr fan from FCP euro (made in china). The auc sensor is connected but I don’t really know if it works and whether or not it’s failure will prevent the fan from coming on. But just to reiterate are the resistors independent from the fan assembly or do they come together in the fan with people simply replacing the resistors if the fan motor is still good? Hopefully someone can please clarify this as I don’t know what’s next to do to get my fan working. Thanks!

[Mandagon4d]

I believe that the resistors are standalone, my aux fan ONLY runs on high, never low, why? I believe the first resistor is no good, I have thought about bypassing it completely and letting the high fan come on in all occasions but have not had any problems thus far.


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[Young 23]

I believe that the resistors are standalone, my aux fan ONLY runs on high, never low, why? I believe the first resistor is no good, I have thought about bypassing it completely and letting the high fan come on in all occasions but have not had any problems thus far.


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So if they are independent, do you know where they are and how to access them? But okay I get you. As long as it works right?

[Mandagon4d]

So if they are independent, do you know where they are and how to access them? But okay I get you. As long as it works right?

Yup bahaha, they are on the inside of the fan cover, if I remember correctly, it could also be that your relays have gone bad, that is a common problem
and the relays are not exactly easy to get to. They are behind the glove box, there’s one for the high speed setting and the low speed setting.

When I sat behind the glove box I don’t mean up in the top, I mean taking out the liner in the top of the passenger side footwell and reaching up there into the tiny space, (that’s where mine are anyway, 1998 528i, not sure if it varies by model but both of my e39’s are like this)

Hope this helps! I will try to get a photo of their location tomorrow if I can remember, lad.

[Young 23]

Yup bahaha, they are on the inside of the fan cover, if I remember correctly, it could also be that your relays have gone bad, that is a common problem
and the relays are not exactly easy to get to. They are behind the glove box, there’s one for the high speed setting and the low speed setting.

When I sat behind the glove box I don’t mean up in the top, I mean taking out the liner in the top of the passenger side footwell and reaching up there into the tiny space, (that’s where mine are anyway, 1998 528i, not sure if it varies by model but both of my e39’s are like this)

Hope this helps! I will try to get a photo of their location tomorrow if I can remember, lad.

Wait so are you saying that they are inside the fan cover or behind the glove box? Or both? But hey if you could I would greatly appreciate it brother!!!

[Mandagon4d]

Wait so are you saying that they are inside the fan cover or behind the glove box? Or both? But hey if you could I would greatly appreciate it brother!!!

The relays are behind the glove box, resistors behind the fan cover.


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[Young 23]

The relays are behind the glove box, resistors behind the fan cover.


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Ohhhh okay I misinterpreted you sorry. But cool that’s much more clear!

[JimLev]

One relay is in the DME box and the other one is behind the glove box.
The resistors are mounted on the aux fan metal frame.

See pic of DME box here.
https://www.bimmerforums.com/forum/s...l-speed-always

[CSMBlack-540i]

Here is a detailed writeup I used at one point to fix my Aux Fan.

https://www.bimmerforums.com/forum/s...ervice-READ-ON

[Young 23]

One relay is in the DME box and the other one is behind the glove box.
The resistors are mounted on the aux fan metal frame.

See pic of DME box here.
https://www.bimmerforums.com/forum/s...l-speed-always

How can I test the relay in the dme box? And are relays basically fuses?

- - - Updated - - -

Here is a detailed writeup I used at one point to fix my Aux Fan.

https://www.bimmerforums.com/forum/s...ervice-READ-ON

Nice thank you!

- - - Updated - - -

One relay is in the DME box and the other one is behind the glove box.
The resistors are mounted on the aux fan metal frame.

See pic of DME box here.
https://www.bimmerforums.com/forum/s...l-speed-always

And thanks too!

[Mandagon4d]

Thanks to everyone that responded overnight! Especially Jim, you’re a legend man!


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[Young 23]

Okay I checked the fuses/relays behind the glove box and they are fine, not blown. The relay/fuse in the dme box is there, I just don’t know how to test it as I don’t have a light. Let’s say that it’s good, could the auc sensor stop it from turning on if it’s bad?

[JimLev]

Young 23, your going to need a test light or meter to do some testing.
A relay is just an electrical switch, it consists of a coil that when energized with 12 volts makes the mechanical part of the switch to move to complete the circuit.

There are a few relay drawings you can click on that shows the internal guts of it.
Terminals 85 and 86 are the coil, when the coil isn't energized 30 is connected to 87a.
When the coil is powered 30 then connects to 87.
Pretty simple isn't it.
Class is over.
https://www.rallylights.com/hella-hl...-40a-spdt.html

[Young 23]

Young 23, your going to need a test light or meter to do some testing.
A relay is just an electrical switch, it consists of a coil that when energized with 12 volts makes the mechanical part of the switch to move to complete the circuit.

There are a few relay drawings you can click on that shows the internal guts of it.
Terminals 85 and 86 are the coil, when the coil isn't energized 30 is connected to 87a.
When the coil is powered 30 then connects to 87.
Pretty simple isn't it.
Class is over.
https://www.rallylights.com/hella-hl...-40a-spdt.html

Yea makes complete sense! And I’ll be getting one soon to test mine out. Thanks a lot!

[Young 23]

I hope you guys see this but I tested my fan but jumping the leads on the temp sensor switch and it turned my fan on in both low and high speed. So the fan is good! However, it is worth noting that the ambient temp sensor is missing (that’s how I got it). Could that prevent the fan from turning on? Also, upon further inspection, I found that the high side pressure switch for the ac is broken off, with the sensor in place but the female and male connector attached to the harness. I have no idea how either happened. Could this be the cause of my issue?

[Young 23]

Just in case anyone sees this, upon the reinstallation of my ambient air temperature sensor and high pressure refrigerant sensor, my fan started to work as soon as the ac is turned on. I’ve yet to confirm if it turns on from idling and reaching hot temperatures but still, at least my ac is working fine now. Thanks to everyone

[Mandagon4d]

Happy to hear that! If anything else comes up, good or bad, feel free to respond to this thread!

[Young 23]

Happy to hear that! If anything else comes up, good or bad, feel free to respond to this thread!

Of course! Thanks!

[CSMBlack-540i]

Great work! Always nice when an OP with a particular issue comes back to report the solution to help future members troubleshoot theirs.

[Young 23]

Great work! Always nice when an OP with a particular issue comes back to report the solution to help future members troubleshoot theirs.

That’s exactly why I came to post it despite no further replies. Hopefully it will help someone in the future. Thanks for the shout out!