'90 V8 Quattro heater fan mostly not working

The high tomorrow is 5F here in St. Paul and the fan of my heating system only works intermittantly. It usually runs when I first start the car totally cold in the morning, but it shuts itself off after about 15 minutes
even though the interior temperature is no where near the set point. After this, the fan won't come on for the rest of the day, except for very occasional and too brief moments.
I suspect the car thermostat is stuck open since the water temperature stays around 60-70C and I know the fan isn't supposed to come on until the water gets hot. However, the "high" setting on the heater fan is supposed to override this and the fan does work when the car is first started and very cold.
Another clue is that the fan worked okay until the outdoor temperature dropped below 25F as it did recently (and will stay for the next few months).
I'm going to have the coolant thermostat checked out soon, but I'm not convinced that this is the problem.
Any suggestions?
Thanks, Tom
PS: My first car, a 1961 Buick LeSabre, had chronic problems with the heater. I finally replaced the temperature control with a threaded pipe water valve in the heater hose and a toggle switch under the dash to control the fan. Kind of crude, but it was foolproof.
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In addition to a thermostat stuck open you have a very common T44 problem.
The brushes in your heater fan are shot.
This is a guess based upon a lot of BTDTs on the four T44s that I have owned and a number of friend's T44s. I am a regular poster on AudiWorld forum for the T44 series and this is a question that comes up over and over. The brushes are ALWAYS the problem.
Fix: Replace the fan motor or have brushes refit at a shop or even DIY with a set of brushes from your local hardware store.
The real issue as with many projects is getting to the motor. It is under the plastic shield in front of the windshield (under the hood). The blower is the big white plastic thing in the middle.
It is about a four hour job and it helps to have help at some points. Stuff under the dash needs to be removed as well.
Hope you have a heated garage or it can get fairly pricey to have it done. Do not go to an audi dealer. Their cost will bring tears to you eyes. There are a few very fair and competent independent shops that can help you if you choose to go that way.
It is a very irritating problem. I had to deal with that on a drive to Western Montana from here in Minneapolis. that got cold. One thing I did for temporary heat was to spray WD40 or CRC on the brushes. that would get them working for ten minutes or so. The place to spray is on the driver's side of the white thing that contains the motor there is a black rubber 90 degree elbow. Pull it out and you will be looking right in at the brushes.
Tony '91 100q 5spd former T44s '87q 2:'84FWD
Tom wrote:

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Tony, you are absolutely right. I took the car into the shop and the fan motor needs to be replaced. You were also right about the time. Their estimate is 4 hours. Lots of bucks for a fan, but I don't have the wherewithal to do it myself and Minnesota is too cold to go without a heater fan.
Thanks, Tom

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If you are in Minnesota (I am too - Excelsior) a friend of mine has an Audi shop in Wayzata. He is very good and has very fair prices.
Email me if you want his number.
Remove 'nospam' in email address to contact me.
Tony
Tom wrote:

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