Climate Control Vacuum Servo Fix
#1
04-05-2011, 11:55 PM
Climate Control Vacuum Servo Fix
So this is not a problem, but I thought it seemed like it might be helpful to others. This applies to my 1989 740 Turbo, but may have applicable information to others.
For the past 2 years the foot vent setting for heat and A/C has not worked and makes a hissing noise. Obviously I have a vacuum leak. I checked through my manuals and the only vacuum line that is unique to that setting is a yellow line that controls a vacuum servo to the defrost. That particular servo has a blue line on the out side and the yellow line into the back of the servo to pull it the other way for a bleed position. The hiss would appear when the yellow side had a vacuum, but not the blue side.
It seemed unlikely that the line had the leak, but since it worked with the blue side under vacuum I knew that there couldn't be a hole in the actual diaphragm. So where could it be? My brother picked up a horse syringe that we have used to pump, gas, oil and brake fluid. It's worked great and has not deteriorated. It's been well worth the 99 cents he spent on it, especially on mountain bike hydraulic brakes. Back to the point, I adapted it to the servo and it leaked on the yellow side only. I figured that it was probably a crack in the housing. I figured that epoxy would be the solution.
It was a pain to get out, but once out I found no crack. I then used the syringe and soapy water and found the back seal to leak. It turns out that the back side needs a seal since the servo arm needs to move in and out of that side. That makes sense. It's really more like a rubber boot. It's not even glued to the housing. The vacuum seals it. So the leak was in the middle part of it that flexes. I figured that it looked like the same material that bicycle tubes are made out of and I tried some self vulcanizing tube patch cement. Then I put a vacuum on that side until I heard it hissing. That way I felt and knew when the cement had been drawn into the hole. I let it dry and threw it on the car. So far no leaks and the foot vent setting works again. We'll see how long it lasts.
Here's some pics of the servo and the syringe.
For the past 2 years the foot vent setting for heat and A/C has not worked and makes a hissing noise. Obviously I have a vacuum leak. I checked through my manuals and the only vacuum line that is unique to that setting is a yellow line that controls a vacuum servo to the defrost. That particular servo has a blue line on the out side and the yellow line into the back of the servo to pull it the other way for a bleed position. The hiss would appear when the yellow side had a vacuum, but not the blue side.
It seemed unlikely that the line had the leak, but since it worked with the blue side under vacuum I knew that there couldn't be a hole in the actual diaphragm. So where could it be? My brother picked up a horse syringe that we have used to pump, gas, oil and brake fluid. It's worked great and has not deteriorated. It's been well worth the 99 cents he spent on it, especially on mountain bike hydraulic brakes. Back to the point, I adapted it to the servo and it leaked on the yellow side only. I figured that it was probably a crack in the housing. I figured that epoxy would be the solution.
It was a pain to get out, but once out I found no crack. I then used the syringe and soapy water and found the back seal to leak. It turns out that the back side needs a seal since the servo arm needs to move in and out of that side. That makes sense. It's really more like a rubber boot. It's not even glued to the housing. The vacuum seals it. So the leak was in the middle part of it that flexes. I figured that it looked like the same material that bicycle tubes are made out of and I tried some self vulcanizing tube patch cement. Then I put a vacuum on that side until I heard it hissing. That way I felt and knew when the cement had been drawn into the hole. I let it dry and threw it on the car. So far no leaks and the foot vent setting works again. We'll see how long it lasts.
Here's some pics of the servo and the syringe.
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