09-04-2008, 03:59 PM
Location: Europe - France
Join Date: Dec 2006
Mein Auto: E39 TDS
failed blower resistor (final stage resistor) not really dead
This a well know trouble with the A/C computer talking to a deaf unit, the final stage resistor.
Today i found something that you may find interesting, at least for those who have this issue, fan not working anymore or working for a moment and then stop or with a fan that is not that efficient as it should be.
My final stage resistor was allowing only 10 minutes of fan with cold car and btw cold resistor.
So the usual path is remove and replace the resistor with a new one...but curiosity didnt't killed the cat and i decided to have a look inside this piece of aluminium. First trying to identify or define of what is made the material covering all the electric things within the resistor. By experience i know it won't be removable with some solvents, so i thought for a post mortem studying, thermal gun (hot air gun) would be good enough to remove the plastic thing. moulded in the aluminium part. Started to heat the plastic but without letting temperature to raise enough to not melt the plastic. This help to make the plastic a bit more soft, i used blade to remove plastic carefully to not damage some electric parts under plastic, at this moment i had no idea of what is under the cover of plastic.
With patience i removed plastic bit after bit and a small part of circuit appeared. Bad luck i guess, the circuit is built with cms components. I tried to connect the final stage resistor back to it's circuit....and nothing.
I lost my time, i was on the way to remove it when i touched the capacity (the brown part in the middle of the circuit) with finger and in the same time the aluminium body of the final stage resistor.
The fan started blowing at 100%, so i tried to change speed from the A/C control but nothing was changing.
I removed the fsr with the idea to clean a bit more the circuit and refresh soldering.
I started cleaning more surface of the circuit and i remembered one thing from my Porsche experience, the 928 was known to have terminal ECU failures due to resin which retracts while getting old with the result of breaking hybrid circuits and removing soldering from some cms components from mainboard.
Bmw seems to have same issues, later i plugged back the fsr, tried and bingo.
I filled the space where was the plastic with some silicon paste designed to be used with heatsinks. The paste i used is the white one, never use silicon with metal particles in it, it would fry everything still valid in the fsr.
Silicon won't harden like resins and it will protect from dust. This solution is not the best but it's probably the most neutral at the mechanical point of view compared to resin.
Now my fan works as it never worked since i bought my car...fsr was probably faulty from the beginning but i wasn't in condition to notice it as i always thought that this smooth working fan was normal.
So it worked for me, if you heat gently the fsr without attempting to soften too much the plastic and use some patience with the blade to remove plastic without scratching the components or the circuit then you will probably get a good surprise.
If it doesn't work please don't complain i can't predict if it would fix your fsr but if you believe it is dead you can give a try.
Last edited by kraft; 09-05-2008 at 02:24 PM.
Reason: horrible errors, they are still some of them remaining but can't find them :)