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  #1  
Old 07-23-2014, 05:43 PM
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Join Date: Jul 2008
Posts: 12
99 sl500 center vent

Our 99 sl center vent has started to cycle open and closed. The fan continues to run. I noticed that the cycling has started to follow acceleration indicating a possible loss of vacuum. This is the only vent that is having an issue. When I looked at the CD manual for the location of the control and how to remove the first step is to remove dash. First question is where to start troubleshooting? Any experience? Second, if the control is bad is there an easier way to get to it?
Any help is greatly appreciated.
Pete
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  #2  
Old 07-24-2014, 11:11 AM
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Join Date: May 2006
Location: Bradenton, FL
Posts: 286
Quote:
Originally Posted by pryner View Post
Our 99 sl center vent has started to cycle open and closed. The fan continues to run. I noticed that the cycling has started to follow acceleration indicating a possible loss of vacuum. This is the only vent that is having an issue. When I looked at the CD manual for the location of the control and how to remove the first step is to remove dash. First question is where to start troubleshooting? Any experience? Second, if the control is bad is there an easier way to get to it?
Any help is greatly appreciated.
Pete
Pete,
There are two distribution blocks in our system; the one you mentioned and one in the engine bay, passenger side, near the firewall. The block is secured by a single screw to a panel in front of the firewall. You`ll see one vacuum line in, and around 7 out. The vacuum lines are secured by a tiny plastic MB only fitting. Remove the block and inspect the outbound lines. Cracks can be tiny, so use an eye dropper (or similar) to put several drops of water on each line where it comes out of the block. If you find the bad fitting/line, replace. If all the lines check out, the fault could be the distribution block itself. Consider replacing with a new one; they`re not terribly expensive. If the system is still under performing, then and only then inspect the block in the passenger side of the car. I believe, but not 100% sure, you can get to the block by removing the knee bolster and passenger side airbag.
Bob
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Bob Jones
Bradenton, FL
'87 560SL-Sold
'99 SL500 For Now
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  #3  
Old 07-25-2014, 07:45 PM
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Join Date: Jul 2008
Posts: 12
Thanks Bob,
I found the block and checked the lines. All appear to be fine. The problem doesn't exist until the throttle is pressed fairly hard, but it seems to get worse as the car warms up. I suspect a crack some where heat can be causing a problem so I doubt it is the plastic block. Still, I'll pull it out and check it. I checked the rubber tubing going to the valve cover and that area seems to be okay too. The block only had one in and three out. I did find that one line was completely disconnected, but after connecting it there was no difference. I have another issue in that the AC works very well when the car is first started, but as it warms up the AC air gets warmer. Not to the point where there is no AC but noticeably warmer. Don't know if the two problems are related or not.
Pete
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  #4  
Old 07-26-2014, 02:27 PM
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Join Date: May 2006
Location: Bradenton, FL
Posts: 286
Quote:
Originally Posted by pryner View Post
Thanks Bob,
I found the block and checked the lines. All appear to be fine. The problem doesn't exist until the throttle is pressed fairly hard, but it seems to get worse as the car warms up. I suspect a crack some where heat can be causing a problem so I doubt it is the plastic block. Still, I'll pull it out and check it. I checked the rubber tubing going to the valve cover and that area seems to be okay too. The block only had one in and three out. I did find that one line was completely disconnected, but after connecting it there was no difference. I have another issue in that the AC works very well when the car is first started, but as it warms up the AC air gets warmer. Not to the point where there is no AC but noticeably warmer. Don't know if the two problems are related or not.

Pete
Pete, would you please explain what you did (test?) To conclude `All appear to be fine`? Reason, cracks/leaks at the block and/or vacuum connecters are almost invisible to the naked eye. Did you use an eye dropper and water? Re the disconnected vacuum line...how did you determine which connector to use. The overwhelming cause of the problem you`re having is at that block, i.e., bad line(s), connector(s), or the distribution block.

Bob
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Bob Jones
Bradenton, FL
'87 560SL-Sold
'99 SL500 For Now
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