I also have a 90 V8 quattro. The fan works but the heat control is
It's trying to find the median range but it either cycles
too cold and then too
hot. I believe it may be the sensor by fan in the
dash that draws in cabin air
past the sensor. I replced the instrument
cluster bulbs and had to remove the
dash. I believe I may have disturbed
the sensor that controls the regulation of
the electronic module. Anyone
have other ideas?
The type 44 series has a more common failure that can cause this. The
vacuum lines that control the heater flaps may be broken or the vacuum
motors that actuate the flaps may be failing. These are located under
the dash on the center heater vent box. It could also be due to the
heater control module that is located under the dash on the passengers
side an can be reached by removing the glove box. It has vacuum lines
and a cable running to it. The cable operates the heater flap in front
of the fire wall just in back of the blower fan.
Another common issue to check is the outside air recirc flap which is
also reached on the fire wall right next to the controller (behind the
glove box). This has two typical failures. The vacuum lines break / fall
off the vacuum motor that operates the flap and the return spring breaks
and needs replacement or its plastic mounts fail and need to be fixes by
improvising a fix.
The heater control unit in the dash is rarely the problem.
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