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Discussion Starter · #1 ·
2001 540 a/c works fine most of the time but then takes a notion to quit and will not come back on until it wants to. The illuminated buttons on the dash unit indicating auto, recul, etc. will start acting crazy when this starts. Has anyone ever had to replace the unit on the dash that controls the a/c. Is this an identified problem?
 

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Can you describe the occasion when "... takes a notion to quit ..."? Is it at a stoplight, and resumes after you're underway?

Any observations will help us diagnose.
 

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Discussion Starter · #3 ·
a/c

it will go off for a day or two then come back on to cool great for a few days then go back off and not come back on no matter how much you coax it until it wants to. In other words, it comes back on when it wants to. If anyone has had this problem pleaase tell me what to do before I drive this hellseed into a lake and make an insurance claim.
 

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it will go off for a day or two then come back on to cool great for a few days then go back off and not come back on no matter how much you coax it until it wants to. In other words, it comes back on when it wants to. If anyone has had this problem pleaase tell me what to do before I drive this hellseed into a lake and make an insurance claim.
I recall seeing a post about an AC component that looked like three or four aluminum prongs that was a known issue on our e39's. I cant remember what it was called but my AC could be 100% normal or has some funny behaviors, here is what mine does:

- Turn on car, press AUTO, turn on AC (LED on) crazy air blasts out, fan speed indicates max bars. Nice and cool.
- 3-4 minutes later, the air volume drops in half yet the LEDs stay on max. The air is cold the entire time so I'm happy with the reduced air volume but it never goes back up to that super volume like when I first start the car.

I have not spent any time yet understanding the components at work here, do we have a second AC fan in the system? Would not surprise me because the BT software reads an AC Aux Fan relay code error. Again, have not gotten to the AC on my "list" but it appears there are current draw issues on certain e39 electrical parts that then cause other electrical gremlins totally unrelated to the original failure / symptom.

How is your battery? How is your alternator charge voltage (look at self test if you have it)? Seems like a healthy charging system / battery is a starting point to work backwards from... then hope it is not your heat/ac computer. I'll do a search here on the AC Aux Fan Relay and see what come up and post.
 

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It IS possible that the final stage unit has failed, but the typical failure modes reported thus far are erratic or no blower action. I've had two failures, and they were one of each. In either case, there was no funky light flashing on the control panel.

In some cases, the FSU will drain the battery.

The IHKA panel has been known to fail. You'll need the BMW DIS to interrogate it for stored codes; a total of six can be stored.
 

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+1 Edjack is right on. Your symptoms are not typical of the FSU, but can't rule it out. Yes, FSUs are a well-known BMW issue and have been re-designed with many more cooling fins. If you don't mind spending a few bucks possibly for naught, do the FSU as if yours is original it is not a bad idea to get the new one anyway. Don't expect a miracle as Mark said. I don't think anyone would say investing in the new FSU is a bad idea as long as you don't put a lot of faith in it absolutely curing your problem. At worst, it is good preventive medicine. Mark can fix you up. G/L
 

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Discussion Starter · #8 ·
problem solved!!! my indy ran a code on it and it was the ambient temp sensor ($99). good as new!!! so anyone else having that problem check this first and it is cheap when the temp here in Dallas is running around 100+
 

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problem solved!!! my indy ran a code on it and it was the ambient temp sensor ($99). good as new!!! so anyone else having that problem check this first and it is cheap when the temp here in Dallas is running around 100+
Um...when that sensor goes out, they generally read -40 degrees. Did you not notice this? I mean really, if your sensor was reading such that the car thinks it was cooler outside then what you desired inside the cabin, it won't come on or stay on. Your sensor must have been at least jumping around?
 
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