The culprit (was: A/C blower resistor pack)

[Priit Varik <Priit.Varik@xxxxxxx>]

Yesterday I finally got to the A/C fan resistor pack, thanks to the posting
of
mr. Brett Anderson to my original question to this list.
I thought I'd share my experience with this quite common failure of this
part.
Mine was made by Behr (the resistor pack for Siemens heater has different
BMW
part number) and beared a manufacturing date of 34.92. So apparently
it has
served quite a time but couple a weeks ago it finally gave up.
The resistor pack is mounted into the heater with the obvious reason: the
blower has to cool this resistor pack!
The pack itself consists of three wire resistors. It also has a 4-pin
connector.
The wire resistors are connected in series in a following manner:

(3)---/\/\/\---(2)---/\/\/\---(1)---/\/\/\---X---/|---(4)
       1.9             1.1           0.7        temp

The values are in Ohms. As you can see, between pins 1 and 4 there is a
very
small resistance (0.7 ohms) in series with a bi-metal thermo-switch
which
normally is connected and from certain temperature onwards breaks
the
circuit.
As the current thru the 0.7 ohm resistor is quite significant (about 17A),
the
whole thing heats up pretty nicely.
When taking apart the resistor pack, I expected to see some burnt and broken
resistor. Alas, that was not the case. All resistors were in good shape
and
quick measurement with the ohmmeter confirmed they were all OK.
The culprit turned out to be in the point marked with X in the above
schematic.
At this point there is a copper (!) rivet that mounts the thermo
switch
to the resistor and the whole thing to plastic basement of the entire
pack.
It turned out, that due to high temperature, the plastic basement had
melted
from beneath the rivet and the mount had become loose! I measured the
resistance
between the pins 1 and 4 to be somewhere near 40 ohms. This was
obviously
too much for the blower motor.
To fix the thing a gentle hit with a hammer on the copper rivet cured the
problem: after that I measured 0.7 ohms between pins 1 and 4 and the blower
works on all speeds again!
So the bottom line: if you are told, the resistor pack has died and you
need
to buy a new one, I bet a hit with a hammer is actually all you need!
The bulk of the job is instead to get the pack out of the heater box (just
above the throttle pedal).

My 2 cents.

Priit.

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