Mercedes-Benz: 1984 300TD Stubborn Climate Control Problem...

Hi,

I have a 1984 300TD station wagon that belonged to my mother. I got the car
four years ago. Overall the car is in great shape, 105,000 miles,
practically everything works, etc. The only major problem was no cooling. It
took only $100 in parts to make it pass the Maryland vehicle inspection. My
wife and I decided, rather than getting an SUV or van for our newish family,
to get the AC fixed and use this one.

The climate control controls all seemed to work fine, i.e., the air came
from the right vents at the right time, and the heating and cooling (without
chilled air, of course) came on at appropriate times. In the winter, there
was no need to touch the controls. All this car needed was the means to
chill the air. So we took it to a garage and they basically renewed the heat
and AC.

They replaced the evaporator, heater core (since they were already in that
compartment), AC hoses, expansion valve, filter drier, and belt. The
compressor seems fine. In fact, the chilled air is now all you could ask
for.

The problem is that now the cold air comes only when the temperature setting
knob in the control cluster is set to maximum cooling (or "Min"). Whenever
the knob is moved away--however slightly--from "Min" the heat comes on full
tilt, the vents change from cooling (all four dash vents) to heating mode
(side vents only) and the AC compressor continues to run. It appears that
both heating and cooling are going full steam and the heating is winning.
From the feel of the temperature control knob as it moves, the "Min" (and
"Max") settings seem to trip some sort of override circuit that forces
maximum output. Evidently, the "Min" setting forces the heating to shut
down. When the control knob is set to the extremes, the fan speed usually
increases if it wasn't already running at that speed. I don't know if the
"Max" setting increases the fan speed now. It did in the past.

The mechanic has been scratching his head for over a week now. He has
replaced the climate control control cluster, the thermostat (whose sensor
is positioned between the evaporator and heater core), the auxiliary pump in
the heater hose under the hydraulic leveling fluid reservoir under the hood
and has verified that the heater control valve attached to the firewall
under the hood is opening and closing as it should. Nothing seems to make
the slightest difference and he has reinstalled the original equipment. He
has accepted that the heating functioned correctly when I brought the car to
him and that something must have gone wrong during the repairs so I can't
complain and I'm not. In checking the car after getting it back the first
time, I discovered that the fuse for the climate control, et al, had been
replaced. I suspect that he didn't disconnect the battery at first and
shorted something. The Mercedes garages around Baltimore would probably have
disconnected the battery but they would have charged me 5 times as much and
are clearly too snobby to have been willing to accept any responsibility for
a snafu like this if they had caused it.

I keep thinking that the problem is with the thermostat in the plenum
housing of the evaporator and heater core--it's not positioned
correctly--but I have no practical way to prove anything.

The point is (thanks for staying with the narrative) that there seems to be
nothing left to account for this malfunction. I am hoping for someone who
will read this and say, "No problem. There's a blue and green control module
just behind the glovebox that has gone bad and you just replace it. Happens
all the time." or something like that. :-)

Thanks very much in advance for any light you can shed on this problem. If
there is any further info you might need or want please don't hesitate to
say so. Also, if you respond to the list would you please cc to me
personally?

Thanks again,

Ken

Amen to that. My 4.5 hours of labor that I paid the dealer to Dx my Climate
control system was well spent. I actually sat and spoke with the mechanic
for 10 minutes after he spent four and a half hours testing and swapping
parts to find the problem. No small local repair shop would have the
resources to do that. I didn't pay for any of the parts. He then told me
where to go buy parts locally for the car at better than dealer prices. It
turned out that the coolant recirc pump was intermittently shorting out and
causing the system to shut down. Recycling the key would fix it temporarily.
Just unplugging it (pump) fixed the problem and made everything work. I then
spent 90 bucks on a new pump and put it in myself. I just redid my Fords
entire ac system and converted it to 134, that whole system was a mess and
full of black death crap. It didn't even get a new evaporator, just a
thorough flushing.
KH

"Bill Ditmire" <com> wrote in message
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