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Discussion Starter #1
I hit a new issue today that smells like a bad FSU but I wanted to elicit the board's opinion on this.

The AC works fine, but the blower speed is maybe 20% of max. speed, regardless of what fan speed I select through the HVAC controls.

Is this symptom indicative of a bad FSU?

One other thing worth mentioning is that I felt like the fan speed would intermittently go up and down depending on engine load/speed, though not by a significant amount.
 

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Not the usual symptom but if a part is failed who knows. A single speed is the norm but mine was likely 80% and that seems to be what others report. Pretty sure I have heard lower speeds too. There has been a rash of control module failures over the past year or so. I look at it this way FSU will fail so throwing one of those in is not a real waste. If it turns out to be the control I would want a new FSU in anyway, no sense in taking a chance that the FSU is causing old controllers to fail anyway.
 

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sounds like the fsu have seen it many times.
 

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Discussion Starter #4 (Edited)
Swapped out the FSU today, normal service resumed.

Curiosity got the better of me and I went to the trouble of measuring out the resistances across all possible pin combinations to compare the old one with the new one. I don't quite understand what's going on, but it may be a useful reference point for someone who wants to estimate how much useful life is left on their FSU.

This is a legend to indicate the 5 pins on the FSU, to be used with the table:

PHP:
A   B   C
o   o   o
  o   o
  1   2
And here are the measured resistances across all pin pairs:

<table width="241" style="width: 181pt; border-collapse: collapse;" border="0" cellspacing="0" cellpadding="0"> <colgroup><col width="64" style="width: 48pt;" span="2"> <col width="113" style="width: 85pt; mso-width-source: userset; mso-width-alt: 4132;"> <tbody><tr height="20" style="height: 15pt;"> <td width="64" height="20" style="border: 0px black; border-image: none; width: 48pt; height: 15pt; background-color: transparent;">Pins</td> <td width="64" style="border: 0px black; border-image: none; width: 48pt; background-color: transparent;">Old</td> <td width="113" style="border: 0px black; border-image: none; width: 85pt; background-color: transparent;">New</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">A-1</td> <td style="border: 0px black; border-image: none; background-color: transparent;">107.1 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">0.634 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">A-2</td> <td style="border: 0px black; border-image: none; background-color: transparent;">106.1 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">23.22 kΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">B-1</td> <td style="border: 0px black; border-image: none; background-color: transparent;">0.2 Ω</td> <td style="border: 0px black; border-image: none; background-color: transparent;">0.2 Ω</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">B-2</td> <td style="border: 0px black; border-image: none; background-color: transparent;">1.008 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">0.614 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">C-1</td> <td style="border: 0px black; border-image: none; background-color: transparent;">7.94 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">1.212 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">C-2</td> <td style="border: 0px black; border-image: none; background-color: transparent;">6.94 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">0.601 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">A-B</td> <td style="border: 0px black; border-image: none; background-color: transparent;">106.9 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">0.634 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">A-C</td> <td style="border: 0px black; border-image: none; background-color: transparent;">112.8 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">0.607 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">B-C</td> <td style="border: 0px black; border-image: none; background-color: transparent;">7.96 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">1.212 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">1-2</td> <td style="border: 0px black; border-image: none; background-color: transparent;">1.008 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">0.611 MΩ</td> </tr> </tbody></table>
 

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Swapped out the FSU today, normal service resumed.

Curiosity got the better of me and I went to the trouble of measuring out the resistances across all possible pin combinations to compare the old one with the new one. I don't quite understand what's going on, but it may be a useful reference point for someone who wants to estimate how much useful life is left on their FSU.
I'm tempted to try this. My E39 540 FSU was acting up so I swapped it with the FSU from the M. Worked fine. The M didn't experience any symptoms from the 540 FSU. I've switched them twice. I have a feeling hot weather plays a role. The 540 sits outside and the M in the garage.

By the way, my M has the coldest air compared to my 540 and my E60 550. It's odd... but I like it :rofl:

Maybe I should measure it with a thermometer!
 

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I also had these symptoms, only 5 first speeds available i.e 20% like you, after that fan wouldn't change on button request for higher speeds.


I changed the FSU for a EBay one, said to be highest quality (...). Now I have minimum 50% blower speed on first speed increment !!, goes to 100% on 5th fan increment, and from there, no change till max speed is chosen on climate control.


Quite weird...I don't think it can come from the climate control module what you think ? I think my new FSU (30$) is a crap one, but still unsure. May try to order a VALEO one as it's made in Europe...


Robin
 

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Discussion Starter #7
I also had these symptoms, only 5 first speeds available i.e 20% like you, after that fan wouldn't change on button request for higher speeds.


I changed the FSU for a EBay one, said to be highest quality (...). Now I have minimum 50% blower speed on first speed increment !!, goes to 100% on 5th fan increment, and from there, no change till max speed is chosen on climate control.


Quite weird...I don't think it can come from the climate control module what you think ? I think my new FSU (30$) is a crap one, but still unsure. May try to order a VALEO one as it's made in Europe...


Robin
New one on the left, old one on the right.



 

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Yes I have bought the new model with lot of "spikes" but nothing mentionned on it.

As it changed the fan speeds behaviour, I think my problem is still related to FSU, I just don't bought the right one.

Every seller on EBay says its product is at least as good as OEM....lol

Now I Wonder if I let it as is with half highest fan speeds available, or if I try another brand, Behr or Valeo...........
 

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Yes I have bought the new model with lot of "spikes" but nothing mentionned on it.

As it changed the fan speeds behaviour, I think my problem is still related to FSU, I just don't bought the right one.

Every seller on EBay says its product is at least as good as OEM....lol

Now I Wonder if I let it as is with half highest fan speeds available, or if I try another brand, Behr or Valeo...........
Try getting one from a reputable BMW parts source rather than an unknown EBay seller. Your going to be wasting money if they're sending you foul units. I've used Bavarian Autosport before with no problems. Or if you have a friend at the dealer, maybe they could give you a good deal.
 

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I've ordered a new FSU, this time a HELLA/SITRONIC...hope it will work (70$), if not, will look for a used HVAC control module.


However it's not a big problem as I have A/C, but not all speeds (only higher ones with current FSU and only lower ones with previous !)
 
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