On Tuesday, February 4, 2014 7:27:52 PM UTC-5, wrote:
And this is the heatsink i want to use and maybe add a little fan which will run when hvac comes on but i need to find a +12v from hvac control

https://www.dropbox.com/sc/h14pr26wq0xgd63/-Zrd9vszho

Good pictures.

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
it is?


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.

12V
GND
Signal ( very small wire)
Motor +
Motor -

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.