Climate Control Vacuum Problem - Another Victim - Mercedes-Benz Forum
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post #1 of 4 (permalink) Old 06-05-2019, 09:39 PM Thread Starter
BenzWorld Junior Member
Date registered: Jul 2007
Vehicle: 1998 SL500
Location: Mexicali, Mexico / Calexico, CA
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Climate Control Vacuum Problem - Another Victim

1998 SL500 - I too have been victimized by the 'no air from the center vents' problem. In recent days I have followed some of the recommendations on the many threads related. The diagnostics on the Climate Control showed reasonable values of 0.8 - 4.2 volts on each of the thumbscrew controls but no reaction from the associated feedbacks. It seems reasonable that if the diagnostic program is working and the display does show when a button is pressed, then the electrics are probably ok and the most likely problem (as is with most) is with the vacuum part of the system.

I searched high and low, front and back, and any place on the car have been working on lately. I have not found any vacuum leaks or disconnected vacuum hoses. I do not have access to a smoke machine but that would be my next step.

I did find a crack in the Vacuum manifold mounted near the computer box, under the hood on the passenger side of the LHD car. I put my finger over the crack as best as I could expecting to feel vacuum pulling my fingertip but I felt nothing. Maybe I did not have a good enough angle to get a good seal against my skin.

After all of that looking I was going to throw in the towel for the day and come back to further consult benzworld when I thought of simply changing the port where the green vacuum line connects to the manifold. I moved the vacuum line from the lower right in the picture to the upper right. It took 10 seconds for me to reach the interior of the car after the change and the center vents were once again blowing their winter like treat (in the 102dF work space). I could only sit in the car and bask in the frosty breeze enclosed away from the summer heat of the desert SW.

I don't think that is the ultimate solution however. Clearly the Climate Control now has ample vacuum to run the system but soon after I made the change on the manifold, the SRS warning light came on. So, if the SRS uses vacuum in some capacity I might have sucked away some of the vacuum from other parts of the car. First, I will have to get that manifold changed out. Also, I loose vacuum control when I accelerate so there is likely a bad check valve someplace in the system. Does anybody have info about how the check valves are arranged in the system and where they are located? I will have to get that figured out.

So far, there are no other warning lights. If I am in a crash and my airbags don't blow, just know that I was happy in my frosty heaven.
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post #2 of 4 (permalink) Old 06-06-2019, 02:24 AM
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Date registered: May 2012
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I had a similar problem. In the end it was a broken line/ pipe on passenger side around airbag and lower part of dashboard.
Good luck with your troubleshooting.
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post #3 of 4 (permalink) Old 06-06-2019, 04:51 AM
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Where's the clear line that runs back to the vacuum pump?
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post #4 of 4 (permalink) Old 06-07-2019, 06:04 PM Thread Starter
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Date registered: Jul 2007
Vehicle: 1998 SL500
Location: Mexicali, Mexico / Calexico, CA
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Those 4 vacuum lines were the only ones going to that manifold near the computer box. The only difference is that previously, the green line plugged into the top upper port was 2 ports lower.

Is there a vacuum pump on this car? I thought the only source of vacuum in this car was coming from the intake manifold or the crankcase...
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