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Old 05-24-2012, 09:57 PM
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W124 HVAC Vacuum Supply Issue

1990 E300 D 2.5... A/C and heat work but regardless of the the pushbutton control unit setting, the vast majority of the air is directed to the defroster vents.

The HVAC system derives it's vacuum from the brake booster vacuum line, via a brown plastic hose. Vacuum is present in the booster vacuum line.

The brown vacuum supply line enters the cabin through a firewall grommet on the driver's side. Where does this brown vacuum supply line connect to the HVAC system? On the right side of the heater box? Its not visible with the instrument cluster, PBCU and glove box removed.

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Old 05-24-2012, 11:09 PM
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you likely have several Dash vacuum pots out of sorts. Your supply line goes to an electronic gang valve if it's the typical W124 Setup. the main line should be green... brown is generally they key cyl if I recall right. the default system with errors is Defrost

The electronic valve should be on the right hand side of the HVAC box where the glove box is, theres a retention pin in the bottom and a place where it snaps into place on the box, but it may have jarred loose, I will post several pictures in a moment
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Old 05-24-2012, 11:22 PM
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Old 05-24-2012, 11:24 PM
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Note, I have the HVAC box top OFF so your setup should look a little more complete then this. chances are it's your footwell pods or the Central Control Valve. Buy a mighty vac, It will soon become your best friend diagnosing the system. if you have to do the central Pod, you might as well replace all of them while your there, it's about 100.00 or so in parts, but it's also peace of mind
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Old 05-25-2012, 09:10 AM
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Thanks for the photos... exactly what was needed. The M-B W124 ACC service files consist of photocopies of the original documents and image quality is rather poor.

I stand corrected on the brown vacuum supply line - that is for the fuel shutoff element.

What needs to be checked first is the green/yellow vacuum supply line. The entire heater box was renewed a couple of years ago to address a leaking evaporator problem. The new box included all new vacuum elements, so I'm hoping that the elements are not the issue. The electric gang valve is factory original (1990) so that will been inspected next should the vacuum supply prove to be intact.
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Old 05-25-2012, 09:18 AM
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The main HVAC vacuum line is green and is teed a yellow funnel looking piece behind the brake booster, check there first.
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Old 05-25-2012, 10:39 AM
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Fixed!

Traced HVAC vacuum supply line to 3 way splitter from vacuum pump; isolated HVAC supply circuit and HVAC worked properly. Added remaining vacuum circuits individually until HVAC reverted to Defrost mode. Traced vacuum circuit that resulted in the HVAC defrosting and found cracked rubber vacuum line connector below air filter housing bleeding air into the system. Renewed connecter, vacuum level returned to normal and everything is functional again.

Guys, thanks for your help!
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Old 05-25-2012, 03:54 PM
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Quote:
Originally Posted by jgl1 View Post
Fixed!

Traced HVAC vacuum supply line to 3 way splitter from vacuum pump; isolated HVAC supply circuit and HVAC worked properly. Added remaining vacuum circuits individually until HVAC reverted to Defrost mode. Traced vacuum circuit that resulted in the HVAC defrosting and found cracked rubber vacuum line connector below air filter housing bleeding air into the system. Renewed connecter, vacuum level returned to normal and everything is functional again.

Guys, thanks for your help!
Awesome! Congrats on the fix!

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