I also have a 90 V8 quattro. The fan works but the heat control is unregulated. 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 c> I also have a 90 V8 quattro. The fan works but the heat control is
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