AC / HVAC Servo Motor Repair B6

[Kevin C]

My AC servo motors have been making strange noises under the dash since I got my 2003 A4 more than two years ago. That and the AC would cut out while they tried to figure out their position. A couple of them finally got bad enough that the system quit trying to position two of them. A fellow A4 owner let me pull a full set from an 2004 A4 with about 90,000 miles. Since mine have 154,000 miles on them I figured it had to be an improvement. They tested out fine, I installed them and now the AC works perfectly.

Now that I have a spare set I was curious to take the old ones apart and see what I could learn.

1. The motors on all the servos were good. As others have found it is the trim pots that are causing problems.
2. The trim pots are lubricated with a special grease.
3. The only wear I found was on the tips of the rotating contacts. They were still good, but instead of a rounded tip they were more of a flat. The resistor element was perfect
4. A bad trim pot was easy to spot using an ohmmeter.
5. There is no need to try and clean the trimpot using a solvent and a brush. Since the trimpot is greased and can't be taken apart it really won't clean it and may mess the grease up.
6. All you need to do is remove the trimpot and spin it a few time to let the contacts wipe into the fresh grease just past normal travel through an area that normally never gets used. That's all it takes to get them functional again.

Easy way to pry the latches:
20170708_085706sm.jpg
Total Resistance of pot = 5.16K (two outer legs).
20170708_084504SM.jpg
Pot set to middle left and center legs = 6.52K
20170708_084946SM.jpg
Pot set to middle right and center leg = 7.08K
20170708_084956SM.jpg


The sum of the resistance of each leg is significantly greater than the total resistance of the trim pot. That indicates that the pot is dirty and has high contact resistance. The sum of the resistance of the two legs should be close to the trimpots total resistance.

In this case the total resistance is 5.16K and the sum of the two legs is: 6.52+7.08=13.6K , The dirty contacts added 8.44K ohms of resistance! (13.6K-5.16K).


I used a drill that just fit the middle of the pot to give it a three second medium speed spin.
20170708_085205SM.jpg

The after results were good. With the trim pot in the centered position the sum of the resistance of each leg was close to the total resistance.

Take away:

• The trimpots are lubricated with a special trimpot grease that you don't want to mess with.
• Removing the grease will greatly shorten the life of the pot and you probably won't be able to get much of it out. What actually fixes the pot is it gets rotated into a new area when you try to clean it.
• You can achieve the same or better results with less effort by just rotating the pot through an unused area a couple of times.

[Kevin C]

Final Results:
Center and left pin reading
20170708_105707.jpg

Center and right pin reading
20170708_105653.jpg

The sum of the resistances of each leg of the pot is very close to the pots total resistance.

2.44+2.60=5.04K ohms
Total trim pot resistance = 5.16K
The contact resistance has been fixed.

[Kevin C]

Update: Its been almost 3 years and they still work perfectly.

[blakefife]

Great thread. My recirculating flaps are sticking I think or the temp blend actuator. It works when the car starts up and then on and off while driving as it wants to. I believe this is the fix for me. Thank you.

[milk]

Update: Its been almost 3 years and they still work perfectly.

First, thank you for the detailed, easy to follow, and to the point post.
Secondly, thank you for following up.
You are appreciated.

[Benbvan]

My AC servo motors have been making strange noises under the dash since I got my 2003 A4 more than two years ago. That and the AC would cut out while they tried to figure out their position. A couple of them finally got bad enough that the system quit trying to position two of them. A fellow A4 owner let me pull a full set from an 2004 A4 with about 90,000 miles. Since mine have 154,000 miles on them I figured it had to be an improvement. They tested out fine, I installed them and now the AC works perfectly.

Now that I have a spare set I was curious to take the old ones apart and see what I could learn.

1. The motors on all the servos were good. As others have found it is the trim pots that are causing problems.
2. The trim pots are lubricated with a special grease.
3. The only wear I found was on the tips of the rotating contacts. They were still good, but instead of a rounded tip they were more of a flat. The resistor element was perfect
4. A bad trim pot was easy to spot using an ohmmeter.
5. There is no need to try and clean the trimpot using a solvent and a brush. Since the trimpot is greased and can't be taken apart it really won't clean it and may mess the grease up.
6. All you need to do is remove the trimpot and spin it a few time to let the contacts wipe into the fresh grease just past normal travel through an area that normally never gets used. That's all it takes to get them functional again.

Easy way to pry the latches:
20170708_085706sm.jpg
Total Resistance of pot = 5.16K (two outer legs).
20170708_084504SM.jpg
Pot set to middle left and center legs = 6.52K
20170708_084946SM.jpg
Pot set to middle right and center leg = 7.08K
20170708_084956SM.jpg


The sum of the resistance of each leg is significantly greater than the total resistance of the trim pot. That indicates that the pot is dirty and has high contact resistance. The sum of the resistance of the two legs should be close to the trimpots total resistance.

In this case the total resistance is 5.16K and the sum of the two legs is: 6.52+7.08=13.6K , The dirty contacts added 8.44K ohms of resistance! (13.6K-5.16K).


I used a drill that just fit the middle of the pot to give it a three second medium speed spin.
20170708_085205SM.jpg

The after results were good. With the trim pot in the centered position the sum of the resistance of each leg was close to the total resistance.

Take away:

• The trimpots are lubricated with a special trimpot grease that you don't want to mess with.
• Removing the grease will greatly shorten the life of the pot and you probably won't be able to get much of it out. What actually fixes the pot is it gets rotated into a new area when you try to clean it.
• You can achieve the same or better results with less effort by just rotating the pot through an unused area a couple of times.

Would this work on a C7.5 S7? My passenger side gets hot but not AC cold