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Outstanding Contributor
450slc5.0cab 280sl5sp 280se4.5 500se+500slAMG +250seStkW108 350sl4spdX3 500secEuro
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Discussion Starter #1
I realized this is a vacuum control. It seems to only stop hissing if it's in the middle, not all the way off, or all the way on (=recirculate). I've found that I like to put the knob in the middle/not-hissing position because it's loud. I think this is near where it appears to switch from fresh to recirculate.

Is this hissing normal? Or have I found a vacuum leak worth fixing?
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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32,357 Posts
Any vacuum leak needs fixing and this is a vacuum leak.
 

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...or you might call animal control to get the snake from behind the dash. :)
 

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450slc5.0cab 280sl5sp 280se4.5 500se+500slAMG +250seStkW108 350sl4spdX3 500secEuro
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Discussion Starter #4
I'm guessing that my AC knob needs replacing if the hissing goes away when the kbo is in the middle.

Have you ever watched the way the little peg slides in and out as the knob is turned (when the cover for the climate control is off)?

I'd bet this control is not a cheap one, eh? Maybe it's repairable.
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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It is not the knob. It is the unit it goes into. Wait till you see the vacuum connections.
 

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Outstanding Contributor
450slc5.0cab 280sl5sp 280se4.5 500se+500slAMG +250seStkW108 350sl4spdX3 500secEuro
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Discussion Starter #6
It is not the knob. It is the unit it goes into. Wait till you see the vacuum connections.
Sorry, I shouldn't have been such a knob and said "knob". I should have said AC control. I do believe I saw the control the other day as I tore apart the Climate Control center trying to figure out when/how the AC knob light turns on. On the right side of the ~2" wide controller, there seemed to be a few vacuum connections, and as I turned the knob a horizontally positioned piston-like shaft would move in-and-out. When in the middle, (not all the way in or out) the hissing would stop.

I'm dreading tearing that apart as I lost some really small parts back in there the last time. I was VERY lucky (IMO) to have found the tiny heat symbol on the passenger floor. No more time for car work this week. Maybe this weekend. I'll be sure to take pictures.
 
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