R129 500SL Can't change direction of air, stuck on windscreen

MarkGolf

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Hello!

Me again, having used my newly acquired 500SL for a trip to work today, I discovered that the blowers only blow up at the windscreen, no matter what position I turn the dial to, it just blows at the windscreen, although I do get some air out of the drivers side vent, nothing what so ever comes from the centre.

I assume this is going to be a vacuum problem but where do I look first??

Also, the rest button appears to do nothing, there appears to be a small led on the button but it doesn't light up. I have the manual control btw. Temperature, recirc and speed work fine, just direction and rest.

For info, found various problems due to blown tracks in a seat module, mirror/steering module and ovp relay so if there is a module which controls this, I wouldn't be surprised if that has suffered also.

Like this

apijho72h__84820.1515506348.jpg


Thanks for any help!
 

LostKiwi

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The vents are controlled by vacuum. The default no vacuum position is on the windscreen.
The usual cause of no vacuum is either a broken hose (the take off is a white hose from the back of the engine inlet manifold) or a broken vacuum manifold.
The manifold sits in the module coffin (where the engine ECU is - just in front of the windscreen, passenger side). Remove the three (four?) screws and lift off the cover. underneath near the leading edge of the coffin is a plastic manifold like this:
airmainfold1.jpg

Unbolt the manifold from the car and lift it up (you don't need to disconnect the vacuum lines). If you look you can see there is a plastic body with a ball bearing pushed in the hole through the middle to seal it. The plastic around the ball bearing will be split.
Mix up some araldite and reseal the hole. Allow to set and it'll be good again.
 
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MarkGolf

MarkGolf

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I really owe you one!!!!
 
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MarkGolf

MarkGolf

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So I araldited the manifold, it was cracked just as you described but no joy.. I’ll investigate further tomorrow by checking the white hose..

I can’t turn off passenger or driver vent, there is a medium flow of air out of those, the centre vent is totally off but the main air flow is the windscreen.

Will check the grey hose and listen to the manifold with the car running to see if I can hear any air leak sounds..
 
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MarkGolf

MarkGolf

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Just to add, not looked further in to this as yet but I can hear a hissing noise from behind the driver side vent... I’m hoping this is the cause of my problem
 

LostKiwi

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Just to add, not looked further in to this as yet but I can hear a hissing noise from behind the driver side vent... I’m hoping this is the cause of my problem
Good chance. The system is entirely vacuum operated. Any leaks and it defaults to outer vents and screen.
 

flowrider

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Ed China on Wheeler Dealers did a fix for this on one of the shows, you might be able to find a YouTube video.
 
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MarkGolf

MarkGolf

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Fingers crossed. Just been out shopping and I noticed I can totally stop the noise when I adjust the headlight height to 0, as I climb to 3 the air noise increases... The headlight height does still move
 
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MarkGolf

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Investigated the light switch but nothing appeared out of place there.

I rechecked the vacuum manifold and noticed a crack I hadn’t sealed so I sealed that up, one of the little plunger things appears to be stuck open but it does normally having a vacuum line in place so I wouldn’t imagine that matters.
I am getting a vacuum via the grey feed line so I know I have vacuum.

What’s the next check? Does anyone know what colour lines go where so I can apply a vacuum direct to one of lines outside of the vacuum manifold to test if I can open/close vents.

Any advice appreciated of what I can smdo next would be great. I was thinking to pull the glovebox out to see if that gives access to something..

Cheers
 

LostKiwi

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Behind the glovebox is the HVAC vacuum control unit. Essentially a line from the manifold under the bonnet goes to the control unit which then under electronic control operates the vent motors.
You could check for vacuum at that point where the feed from the under bonnet vacuum line goes in.
 
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MarkGolf

MarkGolf

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Perfect, found that! I haven’t tried pulling any vac lines off as don’t know what goes where. When adjusting the direction and closing vents I can hear relays clicking but nothing happens.

I want to work out where the fees goes into the big block, I’m assuming position 1 but I’m about to have a google and see what’s what.
 

LostKiwi

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Ok if you get all the clicks eyes suggests you still have a vacuum leak. Follow the grey hose back to the engine. If you detach it from the manifold atvtfecengine and suck you'll soon see if it leaks.
 
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MarkGolf

MarkGolf

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This is resolved.

I swapped the green pipe which feeds inside the car into a free port on the engine bay distribution block. All came to life!
 


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