On Tuesday, February 4, 2014 12:02:11 PM UTC-5, Ralph Mowery wrote:
wrote in message

...



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

heat related.



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.

Yes, I hear what you're saying. I assume it's being used like a switch,
all the time being switched on and off in a PWM fashion, like a
light dimmer. But while going from off to on, in the middle, it
will generate more heat. That's why electronic light dimmers get warm too.
And I guess if something is wrong with the motor, like your bearing
example, it could generate more heat in the FSU. I guess a way to look at
it is like what would happen if you put a 1500W load on a triac based
AC light dimmer rated for 700W. At full power, it would probably
be fine. At half power, it could get so hot it fails.





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 ?


Not so far because there isn't an easy way to do so.
I don't have a clamp on DC ammeter and with the connector
they use, no way to get something into/onto the circuit. It
hasn't blown the blower fuse. I guess I could stick the
wire with a pointed probe, but even that isn't easy because
there isn't much access to the cable.

There is one more avenue being pursued. This car uses a cabin
air filter and it's fairly dirty and needs to be changed.
Thinking that might have some effect on it, but on the other
hand AFAIK, it only filters the outside incoming air, which is
like 20% max and the dash controls allow that to be shut off,
or automatically controlled,
etc. So I would think the blower should run fine with just the
cabin air, especially at lower speeds, but who knows. So one
of those is on the way.

After that, I think next is going to be to get another
FSU, from another manufacturer and try it. The BMW one was
$125, there are Behr ones available for $45. As you can see
from the thread, these things are poorly designed and have
a high failure rate. They've been around since 2000 and
they still all suck and fail. Folks can't even agree if
any of the various manufacturers are better. They have
changed the design. The original ones had less heat sink
pins, but they were larger. New ones have at least 2X the
pins, like 40 of them, so clearly they've changed the
design because of heat issues. Because they all suck is why
the guys here have been trying to reverse engineer it and
see if they can do something to make it work. I think just
a larger heat sink would probably solve it, but no easy way to do
that I can see.

Still you think a new one from BMW would work out of
the box, but who knows. The other sad news is that to get to
the blower requires removal of the whole dashboard

If you want to see a pic of what one looks like, just
google BMW FSU.