Vacuum leak - AC switching intermittantly between windsheild and center vents

[willrev]

I put in a new climate control unit this past year. I am pretty sure it is not the source of the problem.

I am thinking it is a vacuum leak somewhere.

I noticed my fuel door was locked yesterday when I unclocked the car. I cycled the locks again and it unlocked. It is acting intermittantly.

When running my AC on the MAX cold setting yesterday (I was in a suit headed for a job interview) it switches between the windshield and the center vents every 5 to 30 seconds. It did not used to do this. You can hear the relay clicking in the climate control unit when it does.

My parts guy said vacuum leak is likely culprit somewhere on the car.
Any suggestions? I have not done any vacuum work myself on this car.

[SD Blue]

On a w126 the door locks and vacuum system are separate systems.

For the fuel door, it sounds as if that particular actuator is acting up.

On the climate control, the problem sounds more like the CCU head is intermittent and possibly a solder joint. I have had both control head and vacuum issues with mine and cycling was repaired by resoldering. The vacuum issues were usually failed to one mode, namely defrost only.

[Brian Carlton]

I agree.

If the relays are clicking and the doors are changing intermittently, the problem is likely the CCU again. A vacuum loss just means that the defrost functions and nothing else works........like my SD.

I've got a suspicion that the aux. water pump can create problems for the CCU unless it's working perfectly and drawing less than 1/2 amp. I had to disconnect my brand new pump because it was drawing close to 1 amp and causing the CCU to cycle the monovalve. The CCU didn't like it one bit. I don't even realize the pump is disconnected...........I'll proabably never replace it.........heat is perfect without it.

[willrev]

Originally Posted by SD Blue

On a w126 the door locks and vacuum system are separate systems.

For the fuel door, it sounds as if that particular actuator is acting up.

On the climate control, the problem sounds more like the CCU head is intermittent and possibly a solder joint. I have had both control head and vacuum issues with mine and cycling was repaired by resoldering. The vacuum issues were usually failed to one mode, namely defrost only.

I think it is still under warranty, should I hit my parts guy up for an exchange?
I just put this one in last season.

I am assuming you mean the CCU is the control unit in the dash with the switches. I hear it clicking and cycling when it is doing this.

[Biodiesel300TD]

My CCU was making clicking noises a while back. The defrost would work if you didn't change the temp, but as soon as you turned the temp wheel it would make a few clicking noises and then the defrost would stop working. I pulled the CCU out and took it apart. There was a bad solder spot. I resoldered it and it works fine now. So I would try getting a new one if it is still under warrenty.

[willrev]

Yesterday it was much hotter here in NC. It got to 82 degrees and the ccu was not intermittantly switching.

Could it be the temp sensor in the roof of the car above the rear view mirror?