The fsu from behr doesnt have that resin and it was easy to open.the new mo
dule has the transistor on a copper plate to make the heat transfer fast bu
t the contact with the aluminium its not so good.I did lost the waranty whe
n i did this,but i'm not giving up in building an fsu from scratch.I have s
ome pictures of the fsu.
miercuri, 5 februarie 2014, 01:43:49 UTC+2, firstname.lastname@example.org a scris:
module has the transistor on a copper plate to make the heat transfer fast
but the contact with the aluminium its not so good.I did lost the waranty w
hen i did this,but i'm not giving up in building an fsu from scratch.I have
some pictures of the fsu.
with this new fsu i can see the circuit bord and i can make an ideea about
elmos 10901d and find a replacement
On Tuesday, February 4, 2014 7:27:52 PM UTC-5, email@example.com wrote:
It looks like the chip, the transistor and a bunch of
caps, resistors, diodes. Anyone know what that funny
looking copper strip is with glue or whatever under
I would think that adding a little fan to the heatsink
might be the most productive approach. If you can find
a power source that's on when the HVAC on that would be
great. But you could also power it from anything that has
12V when the ignition is ON, which could be a lot easier
to find. Any chance of tapping into any of the blower
airflow to cool it somehow? And is there even room in
there for the additinal fan? Where the part goes under
the dash, I can't tell what room there is behind it, but
I would doubt there is enough for a fan.
One difference I see with your new part and Sitronic one
from BMW is that your Behr one uses an aluminum housing
all around the part, while the BMW part uses plastic.
I noticed that difference in some of the pics before,
but couldn't tell which ones were all aluminum and
which just had aluminum for the heatsink. The all
aluminum should be better because it can help conduct
more of the heat away.
Another perhaps minor point. IDK how the thing is fastened
into your car, but on the X5, it kind of snaps in, but
from the removal instructions, there are supposed to be
two small bolts that hold it down. The one here had
been replaced previously and had no bolts. I can see
why, given where it goes it would be hard to get to the
bolts and the clip seemed to hold it well enough.
But.... If it's bolted to metal and it's all aluminum
like yours, that would help take some more of the heat
away. But then it looks to me like most of the ones
out there have plastic frames anyway, so in that case
I don't think bolting it down is going to make much
difference in transfering heat, as plastic doesn't
conduct that well.
I also don't know how productive figuring out what's
hooked up to what on that board is going to be. No
one has been able to find a datasheet on that Elmos chip.
No one is even sure what it really is.
If the intention is to build a new design, my approach
would be this. There are only 5 wires to this FSU.
Signal ( very small wire)
The missing piece here is what's on that control
signal wire that sets the speed? If I was going to
make a new design, I'd put a scope on it and find out
if it's an analog signal, digital, etc. If it's just
analog and varies with the set speed, then you can
design a new circuit. Of course getting it into the
right form factor, etc is going to take a lot of work.
IMO, it's not worth it.
Oh, and regarding that single wire control, they use
a similar single wire to control the variable speed
auxilliary fan for the radiator. That fan has electronics
in it too that can vary the aux fan speed, based on a
control input. Simnple on/off like a billion other cars
have was too simple. And it's in the worst place possible
right next to the radiator.
The reason I bring that up, that fan fails frequently
and can drain the battery. The car here, the fan stopped
working, but the AC still works fine except when stopped
in traffic. I was worried about it draining the battery,
which they are known to do when they fail. So, I unplugged
it. It's just 3 wires, 12V, gnd, control.
And..... the AC no longer worked at all. The only
way I could explain what was happening was that somehow
the HVAC control knew the fan was disconnected, leaving
me to wonder if that single wire control to it was
bi-directional. IDK why anyone would design it that
way, not 100% sure it's done that way, but thought I'd
mention it in the sense that the single wire into the
FSU might not be so simple.
Hi.I managed to fit the fsu right behind the glove compartiment.Added a little fan ,but the hvac doesn't have a +12 so i made a circuit with an 10k ntc that allows the fan to start only when it gets hot
The copper strip is just a bypass for the +12 of the blower .
The radiator that i used is all copper and a had to isolate it because when it touches the ground(-) the blower comes on at full speed.
I made the test on low speed half hour and it stays warm.
The fsu gets really hot at low speeds so the air flow its not enough to cool it down.At hi speed its cold .So that why i decide to add the fan .A cpu cooler will do just fine.
I did drill a hole were the transistor has that hole to be placed on a heatsink and added a screw so that the heat transfer is better in that area.
On Wednesday, February 5, 2014 11:47:31 PM UTC-5, firstname.lastname@example.org wrote:
Saw your pics and sounds like you have a good design there that
should work. The idea of just making the fan go on when it gets
hot was a good one, so you could then just tie it to the 12V that
is already there. If you tied the fan to the 12V for the blower,
you might want to put an inline fuse in. That blower circuit is
30A? I've never seen a CPU fan like that cause a problem, but
just a thought. The fan may have some kind of protection of it's
own though, so manybe it's not an issue.
Good idea to screw the transistor down. I noticed that in the
photo, it wasn't clear exactly how they had it secured down.
And it's also near one edge of the heatsink, instead of being in
the middle. Before you modified it, was there thermal paste
between the back of the board and the heatsink?
This is just a really bad design. If they had correctly bonded
the heatsink directly to that transistor where all the heat is
being generated, I bet it would work fine.
The original fsu board was tied down only on the edges by that black plasti
c which was clipped by some small fins of the aluminium radiator .It did ha
ve a thermal paste i also used a paste .
The +12 i took from a main +12 and used a 2.5 amps fuse .
The transistor i think disipates about 50w to 80w of heat at low speeds tha
t makes the heatsink to small to dissipate that heat and the airflow of the
blower at that speed is too little.Its true the transistor is made for 250
degrees but lets be true they dont make like they used.
On Thursday, February 6, 2014 11:04:17 AM UTC-5, email@example.com wrote:
tic which was clipped by some small fins of the aluminium radiator .It did
have a thermal paste i also used a paste .
hat makes the heatsink to small to dissipate that heat and the airflow of t
he blower at that speed is too little.Its true the transistor is made for 2
50 degrees but lets be true they dont make like they used.
I think it's probably generating more heat than that. The transistor
is rated at 300W. In any case, I don't think it's the power transistor's
power handling capability that is
the problem. It's that the heat causes other parts to fail. I'm sure
you've seen pics of ones where the solder joints came undone. IDK
what causes the one here to basically shutdown once it gets too hot,
but suspect it's something to do with the Elmos chip, either by
intentional design or that it just shuts dowm by malfunctioning from
the heat. Whatever the max operating temp is for that Elmos ASIC,
I'll bet it's being exceeded when these things fail.
The overall design is probably just enough if everthing works like
it should. If the blower winds
up pulling some more current than normal, for whatever reason,
then it pushes it over the edge. I'm thinking as Ralph suggested,
it's probably the blower that's the root cause here. And to fix
that requires pulling the dashboard in the X5. Another great
design. Not something I wanted to do 30 years ago, but today
with all the air bags and God knows what else crammed in there,
it's got to be a lot worse. Wish I could get some oil to those
motor bearings and see what happens.
Going back to what Ralph said about bearings, I'm not convinced
that the blower is putting out full RPM even when it's connected
to 12V directly. There ia a lot of air coming out, but I think
it may have been even more previously, hard to tell for sure.
But if it is putting out less at full, then that could be a sign
of the bad motor bearing thing. Another sign I think is that
it seems even when you first start it up, with a cold FSU, it
seems to me that the airflow at say mid speed is more like
airflow at 1/3 speed, etc.
Good luck with your project at let us know how it works after
you've had it running for awhile.
The heatsink of the unit needs to be where air can circulate past it. The
fan is a good idea. I am not sure how the transistor is fastened to the
heat sink. It needs to make good mechanical contact with it.
When you use the thermal paste just use the smallest ammount you can. The
paste is not really that conductive of heat. It is just beter than air.
You want the transistor to make good mechanical contact with the metal of
the heat sink. There are very small inperfections in the metal that has air
voids in it. The object is to just fill the air voids. Too much paste is
worse than none at all.
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I know that it needs a space with more air thats why im thinking of rellocating the fsu in the engine compartiment,but im waiting to get warmer outside.
The transistor is now fastened very good not like the original were it made contact only on the margins of the copper plate
On Thursday, February 6, 2014 2:30:05 PM UTC-5, Ralph Mowery wrote:
The power transistor is not fastend to the heat sink at all.
If you look at the pics, it's on the top side of the PC board.
The bottom of the PC board has copper over all of it. Then the
bottom of the PC board contacts the heatsink. So, you have the
PC board material in between, which is not a good heat conductor.
And the transistor is closer to one side of the heat sink and no
bolt through the usual bolt location provided.
And to top it off, these things have been failing for 10+ years
and not one of the many suppliers has seen fit to make some of
the simple changes that would fix it.
On Thursday, February 6, 2014 4:29:23 PM UTC-5, firstname.lastname@example.org wrote:
I just discovered something important that I haven't seen mentioned
here or in any other threads on the BMW blower motor resistor (aka final
stage unit) problems. This module is mounted in a location where the
heat sink fins are directly in the blower motor airstream. It relies on
the blower air to keep it cool. From where it's located and how you
remove/install it, it's not obvious that the heat sink fins wind up in
the blower air, because you can't see where they actually are.
When I replaced it, to make sure it worked before putting it together,
I just left it hanging for testing. It overheats as soon as you slow
the blower to about 80% of full speed and stops working. Once I stumbled
on the fact that if it's mounted it gets air flow to cool it and
I mounted it, it's apparently working. Just thought I'd pass that along,
as it seems like something that could trip up a lot of people.
On Tuesday, February 4, 2014 11:11:36 AM UTC-5, email@example.com wrote:
Out of curiousity, how did you open it up? The ones I've seen
are potted in 1/2" deep epoxy, with no obvious way to get that out
without destroying what's inside....
Also, Ive been helping a friend with the FSU problem. His blower
went from working fine to not working at all. Being concerned about
all the apparently crap ones out there, bought one from a BMW dealer
that is marked Sitronic. It appeared to work, but..... With the
blower on high it will run all day, no problem. You can back the blower
off to about 80% and as you do so, the heat sink starts to get very
hot, but the blower still runs at 80%. If you back the speed down
anymore, it gets even hotter and then starts pulsating the blower,
finally stops all together. I tried dipping the end of the heatsink
rods into a container of water when it does that and within a couple
seconds the blower starts working again fine. So, it's definitely
Any ideas on what's wrong? First impression would be that it's the new
FSU because the blower works on full or near full speed. But I guess
the other possibility is that the motor has a problem? Seems odd
though that a single winding type DC motor could run fine at full
speed, but then cause the FSU to overheat at lower speeds.
Could be the motor. While not exectally correct, the motor requires a
cretain ammount of power. When running at full speed the transistor is
acting more like a swithc. At slower speeds it is acting more like a
resistor. Any power that is not used to get the motor to speed is converted
to heat in the FSU. The slower the motor is ran, the hotter the FSU will
get. If the bearings are going bad or there is some other reason the motor
is drawing more than the rated current the FSU will heat up more than
normal, especially at slower speeds.
Have you checked the current the motor is drawing and made sure it is
getting the voltage it is suspose to at full speed ?
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