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Old 07-10-2013, 01:35 PM
Arthur530SE Arthur530SE is offline
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Location: Burton U.K.
Join Date: Jul 2013
Posts: 1
Mein Auto: 530i SE
Wow 2013 and still helping with the best tread on the net

Hi All,
A little about me: I have a background in the electronics industry of about 30 odd years, so as a result I repair electronics modules for my local Independent BMW garage.

I'm simply amazed how long this thread has been going. These E39's are getting old hat now and finding out any information on them is starting to get very hard.

There are some very knowledgeable people here, and this thread in particular provides a very in-depth solution to a potentially bank breaking problem.

I have only just (last month) bought my E39 and I love it, no question the best car I have ever owned. As it seems these cars have their share of niggles and faults and as a result of reading this thread I'll be acquiring as spare FSU for the tool box.

Just as an addition to this wonderful thread here is a little extra info I don't believe I have read any where here... From the pictures found elsewhere in this forum of a stripped out FSU link:

I hope I don't bore anyone too much... Here goes...

These Final Stage Units are basically a PWM controller and not a resistor. The main AC control unit sends a variable voltage signal to the FSU much in the same way the brightness is controlled in a laptop or similar to the way you would adjust the lighting in your house with a dimmer switch. Basically inside your AC control unit is a digital potential divider (Variable Resistor), so with this in mind one could use a simple variable resistor modified to output the same control voltage as the AC unit. That way we have a way to test these units on the bench.

The IC you see on the board labelled Elmos 10901D seems to be an OP-Amp. Even though no datasheet is available for this IC, some searching found a Russian site claiming this IC is not special but just an LM711. It can't be anything to out of the ordinary as BMW GMBH would not release a schematic, but aftermarket pattern FSU's are manufactured by several different companies.

I would be very interested in getting my hands on an FSU in the same state as the one I have linked to so I could have ago at reverse engineering one and maybe we could all have a better understanding of why these units fail, how we could fully test one (above), suggest some sort of "beefing up" of the circuit to make it more reliable.

After all to BMW I would assume this part is basically a consumable that they can charge lots of money for. Even 40 quid is too much for something that is about 15 quid's worth of electronics, probably including the custom heatsink.

Anyway that's my ten penn'orth worth and I hope it helps.

P.S. If anyone has an FSU in the state as the linked picture and want to donate it for experimentation just let me know.

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