BimmerFest BMW Forum banner
1 - 20 of 183 Posts

·
Registered
Joined
·
51 Posts
Discussion Starter · #1 · (Edited)
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. :)

 

·
Registered
Joined
·
51 Posts
Discussion Starter · #3 ·
sorry but i don't have other pictures (using silicon, fingers and camera is not very compatible lol) and sorry for language mistakes but if someone understands the mean of all this, then a pure and correct recipe could be written. :)

The main idea is releasing the plastic thing, plastic or resin poured on the electronics assy is by the time passing, getting too hard, plastic loses it's elasticity and by the way pulls up components from the main board.
The issue is well known on Porsche 928's ECU with dying boards due to resin which becomes too hard by years.

Removing plastic thing releases components which are "glued" on the pcb.
When plastic is removed, components which were pulled up and no more in contact or making false contacts are back in contact.
Hot air gun helps to restore contact points (soldering points).

Keep in mind to not over heat components and plastic, use cutter to remove plastic rather than only hot air which would help remove plastic better but which would probably destroy some components in the same time. Of course the target is not making a massacre of the board, be gentle and respect pcb and it's components.

Once it works prefer to fill the missing plastic with some silicon, the same used for heatsink, white silicon please, no silicon with metal particles within which would definitely damage the fsr.
silicon won't harden and will be enough to protect from dust.
 

·
Registered
Joined
·
6,925 Posts
A great DIY! I will try it the next time the FSR fails. Better than paying the $70-$90 for a new one! :thumbup:
 

·
Registered
Joined
·
750 Posts
it's a very interesting writeup. so you basically cleaned up the dirty stuffs on your circuit board. did you re-solder or re-circuit anything? and i wonder if it helps to restore any fsr that is working intermittently and draining batteries overnight.
 

·
Registered
Joined
·
51 Posts
Discussion Starter · #6 · (Edited)
It looks dirty but in real those are plastic traces i couldn't remove properly and i prefered to let plastic as is rather than putting circuit and components at risk.
I'm pretty sure that removing the plastic which was making unwanted constraints made some too fragile solderings to recover their contact.
I also applied hot air again after i removed the plastic but i thought that it could then damage components so i didn't warmed that much the circuit and i don't think it was enough to restore as they should some failing solderings.

Maybe someone will find a better way to clean and fix the circuit who knows and btw have a better overview of where it usually fails.
In my case, making an unexpected contact with finger between the brown coloured capacitor in the middle of the circuit and the aluminium body, the fan ran at full power.
This first life sign gave me hope that something could be saved and led me to clean a bit more with the idea to refresh soldering points with an iron.
Contact of skin between capacitor and body acted as a true resistor.
My opinion is that only removing the plastic around the integrated circuit and neighbour components had been enough to restore function of the fsr.
I remember that when i removed at first time the fsr, i almost burnt fingers on the aluminium body.
After removing of the plastic, the body remained cool even at full speed.
This not normal heating of the aluminium body was probably indicating that it was draining battery, i never had troubles with battery it is new, maybe the energy draining was hidden by the very good condition of the battery.

.
 

·
Stimulus Package
Joined
·
4,245 Posts
Very interesting. I put a link to this thread in my FSU thread in the DIY forum.
 

·
Registered
Joined
·
51 Posts
Discussion Starter · #9 ·
I know i know, black box, plug and play, buy and drop in trash etc...but this doesn't forbids to be curious isn't it ? :) ;)
 

·
Registered
Joined
·
51 Posts
Discussion Starter · #11 ·
i think that recycling parts has something beyond the money it represents for a new part, it helps lowering polluting, less need to produce parts, less fumes, less chemicals, less energy consumption for parts production... Ok this poor crappy part won't save the planet but to attempt saving old parts is fun.
Consuming as usual, just as in the good old time when they were no constraints, no limits, buying, using, throwing away...those times are gone :(
 

·
Stimulus Package
Joined
·
4,245 Posts

·
Registered
Joined
·
51 Posts
Discussion Starter · #13 · (Edited)
If it works or fails for you please report it with details if possible.
It will help other users and also to confirm whether if it helped or not.
This will at least define if what i did was random or if the trouble was really the one i thought about.
Thank you. :)
 

·
Registered
Joined
·
221 Posts
Thank you for posting this information. I replaced the fsr last year, but this might come handy some day :D
 

·
Seek to understand,^Value
Joined
·
25,199 Posts
Let's ask everyone to post resistance numbers & do an autopsy on the old BMW FSU

Doru pointed me to this thread from here and it's very intersting!

When I find what I did with my old FSU, I'm gonna see if I can take it apart to see what I can find out myself. I hope others do the same.

One recommendation for others (which I already made in the canonical E39 thread from Elvis) is for all of us to post the resistance measurements from the old and new FSUs. That way, we might find a pattern of interest.

Here is what I think each pin does:
#1 = ground
#2 = 12V power, 40 amp fused
#3 = input to fsu from heat/ac module
#4 = output power to the blower motor? (speed 1?)
#5 = output power to the blower motor (speed 2?)

And, here is what I measured between each pin and ground (the FSU body):
#1: Old FSU = 600Kohms, new FSU = 11.2 ohms
#2: Old FSU = 3.02Mohms, new FSU = 1.22Mohms
#3: Old FSU = 2.22Mohms, new FSU = 608Kohms
#4: Old FSU = 2.14Mohms, new FSU = 602Kohms
#5: Old FSU = 3.05Mohms, new FSU = 1.28Mohms

 

·
Registered
Joined
·
2 Posts
I'm surprised no one has posted about trying this since the original post. It seems to have worked for me and I'm not sure why. Perhaps the filler does stress the components over time and throw the resistance off. Near the circuit board there were some areas that didn't stick too well. Maybe those areas loosen up while the other areas hold on.

Here's the story. I have an E36, 1998 323is. For about a year I have had issues with the AC, the blower would just stop at times and then fix itself. About a month ago it went dead for good. Suspected the blower motor but it checked out ok.

The car has the old style FSU with the aluminum body. I decided to give this a shot since the part was going to go in the trash in a few days anyway. I didn't write down the resistances to the 5 pins but they were all nowhere near what others had listed for new units in the 5 series forum. It might be different specs on E36 models, but the unit didn't work in my car so who knows.

I slowly shaved out the plastic filler from the back using a knife and wood chisels. The chisels worked great. It's deeper than I thought it would be so took over an hour of slow shaving. The picture in the OP looks pretty shallow but it might be 1/4" deep until you hit something. Had I know that I would have been more aggressive and cut the time down a lot.

Once the components started showing up I started testing the resistance of the pins, and as expected there was no change. Then all of a sudden pin 2 resistance was so much different that I thought I had screwed up. It was literally a minute or 2 between checks. But then pins 3, 4, and 5 all seem different as well. The resistances are still nowhere near what they "should" be, but there was at least a change in pin 2.

I tried using a match above the surface to see if I could get the filler to soften and it did seem to smooth out right where I had it. But it took so long I figured I'd end up frying the thing before it would all smooth out. So that was all the heat I applied. I have no idea what happened to cause the change in resistance because all I was doing it taking tiny little slivers off and by that time lots of the things were already showing.

Since it was already midnight, I decided to just hook it to the cable and try it without mounting it. For about 3 or 4 seconds nothing happened and I thought I'd wasted a lot of time. Then all of a sudden I feel some air blowing and then it ramps up over 1 or 2 seconds to some real air blowing.

I should note that I hadn't tried disconnecting and connecting it to make sure the connection wasn't the problem originally. But I had tried pushing the connector in and it hadn't made any difference. Plus the resistance did change. I have no idea why. I did accidentally peel up some of the green circuit board layer so there's a nice shiny copper looking area and I did scratch a few spots up. But odds of me scratching in such a way that it fixed the resistance is pretty low. I'm more a fan of the stress theory.
 

·
Registered
Joined
·
2 Posts
Just wanted to add a picture of the inside. It shows how deep it actually is. It's about 0.18 " until you start hitting the components.

I tried again today with the car sitting in the sun all day and the heat didn't bother the unit. The fan started up just fine. Well it did that few second pause when I hooked it back up after taking the picture, but worked just fine.

Many thanks to the OP. Saved me the cost of a new unit.
 

Attachments

·
Registered
Joined
·
26 Posts
IT WORKED!!!!!!! So my blower stops blowing one day and a couple of days later all of a sudden it worked normal. Than it stopped again and after that it didnt work for 2 straight weeks. Upon searching for the problem I stumbled with this FSU problem and decided to go into the dealer and just replace it. Dealer here in Shillington, PA wanted $150. Kept searching and found this thread and decided what the heck I have nothing to loose. Took Wifies hair blower and a small knife and went around the edges first then kept picking at the rest of the soft plastic part. I cleaned as much an I could but not that much either.... Plugged in and everything was back to normal, I HAVE A/C AGAIN FOR FREE. Thanks to this fellow member I have just saved my self close to $100. Dont know how long this will last but THANKYOU anyways!!
 
1 - 20 of 183 Posts
Top