In any position except defrost, does the heat blow out the defrost and up to
If yes, you have a vacuum leak somewhere in the dash.
By law, all automatic climate control systems must default to defrost if a
vacuum part fails.
Do you 5 buttons horizontal or 5 buttons vertical?
Thank you very much.
First of all, five buttons vertical.
And yes, I do get warm air out the windshield defrost vents when the
heater control is selected to "Bi-Level". And for what it's worth,
there are other vacuum problems, because prior to my acquiring the
car, someone altered the vacuum hose arrangement where a number of
hoses join at a place under the hood just forward of the driver. One
vac hose has been plugged off. As a consequence, the cruise control
doesn't work, and only the rear vaccum door locks work. I believe this
was done due to a vac problem that probably caused the engine not to
shut down with the ignition switch.
I do not as yet have the MB shop manuals for the W123 300D, and so I
cannot diagram what should be in place with respect to the vacuum
hoses, so in order to completely restore the system back to its proper
configuration, probably requiring a shop procedure.
In any event, is it your belief that the vacuum is not opening the
lower heat vents?
It definitely could be a vacuum problem, as that is what is used to
move the actuators that put the flaps in the proper position to
control where the air goes. That is where I would start, especially
given that the vac system has other serious problems as well.
I take it this car has automatic climate control? If so, then the
next candidate unfortunately is the servo and servo amplifier. The
servo is the plastic widget about the size of two fists, located under
the hood, near the passenger side firewall. It has 2 electric
harnesses, 4 coolant hoses, and a bunch of vacuum lines connected to
it. (Begin to see why it's failure prone? :) Take a look at it,
and if you see any coolant leaking from the lower portion of the
servo, it's either shot or on it's way out.
Google here for AC servo if necessary and you'll find plenty
discussion about it.
One tip to prolong the life of the servo. About once a month, put
the temp all the way to max for a few mins and let the car get fairly
warm inside. Then move it to max cooling, and leave it there for a
couple mins. That moves the servo from full cool to full heat and
cycles it through it's full movement. It helps keep the rheostat
inside from developing dead spots, keeps the gear lubricated, etc.
When you do that, if the system is working correctly, you will hear
the blower slowly drop down through it's various speeds from max heat
speed to lowest, then cycle back up to max cooling speed. The highest
cooling speed is one speed higher than max heat speed. At the same
time, the air should switch from coming out the footwells in heat
mofe, with some bleed air to the side dash vents to all coming out
from all the dash vents in cooling mode.
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.