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Old 08-07-2007, 07:38 PM
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Location: South Mississippi
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Another AC Vent cool issue

I've been searching through the post and hope to find some help, but maybe you guys can point me in the right direction. I am a newbe and just bought a 1990 300d with a 2.5.

AC has been changed to R134. 40 psi low side 90 degree ambiant
180 high side all this is at idle and of course the compressor is running.

I may have two problems anyway I have air going out defrost and side vents but not center vents and the air is not cold. Not sure if it is Hot either just ambiant I believe. Looking for a vacume hose problem I found a hose going into the firewall on the passenger side behind the battery that just ends under the glove compartment. There is vacume on this line. But I dont know where is suppose to go. Also there is a black plastic plenum looking thingamajig that has a number of vacume lines to it. All the outputs looks like they are stopped up with something. Maybe a brass screen?

Anyway where does the orphan vacume line go. I'll try to post a pic. Its the hose on the top....

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Another AC Vent cool issue-dcp_2419.jpg  
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Old 08-14-2007, 05:02 PM
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Location: South Mississippi
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Any of you guys know where the top vacuum line goes. I can't seem to find its mate. Thinking about plugging it. Inside the back of glove box is a manifold but it has a separate vacuum imput to it.
1990 300d 2.5 new to me.
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Old 08-14-2007, 05:22 PM
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Not a W124 owner but Mercedes usually uses a black vacuum line as a "fresh" air supply. They place it into the cabin so contaminants do not enter the vacuum system. Having it not connected to anything is normal.

Your vent problem is usually a vacuum element problem inside the dash. Very common on the W123. Hopefully someone with a W124 will chime in.

The W123 use this concept.

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Old 08-14-2007, 05:30 PM
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Location: South Mississippi
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I am assuming the worst in that the the vacuum element is out and is located behind the evaporator. Whole dash removal thing. It has a vacuum on it, so It goes somewhere. Maybe back to head rests on passengers????
When I finish getting the turbo rebuilt, I'll be chasing the A/C down. Thinking the warm air is the monovalve. The vents are a different fix unless this vacuum is somehow related.
1990 300d 2.5 new to me.
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Old 08-14-2007, 05:58 PM
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Location: Brandon, Mississippi
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from member gsxr

Yesterday I replaced all the vacuum actuators (pods) for the climate control system in my sister's 1987 300D. Of the 6 total, only two are accessible without removing the dash - the pod for the floor vent flaps (pull the pushbutton unit to access this one), and the one for the defroster flaps (behind the glove box.) The other four - 1 center vent, 1 diverter, and 2 recirc - require pulling the dash. That means yanking the instrument cluster, steering wheel, p/b unit, glovebox, both lower trim panels, and then the whole dashboard. And then you need to pull off the top of the heater box to do the center vent & diverter pod, which is also a pain. This car (with 155kmi) had 4 bad pods: center vent, diverter, recirc (small lift), and defrost (small lift). Needless to say the system was NOT functioning as intended, although the ONLY symptom was a lack of air from the center vents with the A/C on. W124's are sneaky that way.
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Old 08-14-2007, 08:26 PM
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The hot/cold thing, ... what is the temp of the return/cold line? Cold? Should be a little cold, also the electric fans on the radator/condensor should be running.

The vacuum line, what does the engine end connect to? I can look in an '87 for the end, but not sure it would be the same as yours.

The vacuum manifold/distributor that you mention is run by the CCU and controls all of the vacuum flaps and valves for the Clmate Control. It should have vacuum.

The no air through the center vent (assuming the manual flap is opened) is a common problem, could be the vacuum actuator or "pod", could also be the CCU (pushbutton unit). My experence is that the non-bosch unit fails and runs the compressor at random, non-bosch has the silver tape squares above the pushbuttons on the plastic body, ... bosch has black tape.

There is a better write-up on the center vent problem, tells which one to apply vacuum to on the vacuum manifold to actuate the vent flap, I think it's in the DIY forum.

Gone to the dark side

- Jeff
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Old 08-14-2007, 11:06 PM
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Join Date: Jun 2007
Location: South Mississippi
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Thanks for the not so good news. Not sure if the center vents is worth tackling. MIght go for a easier fix. Maybe a wire holding the vent open.
Condensor fans running. WHen I get the engine running I am goint to clamp close the hoses to the aux pump and monovalve. That should take all the heat issue out of the equation right? The compressor runs and therefore the evap should cool right? Is there theromstat controler near the evap? What sensor tell CCU to add heat? Does the climate control just add various heat from engine to keep temps? In other word compressor usually runs....

1990 300d 2.5 new to me.
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