I feel your pain. I have a 1980 MB 300SD. For the AC,
MB used a part from Chrysler that they used on the Imperial.
It's the size of a grapefruit and it controls the entier HVAC
system. In that one widget you have a valve that's in the
path of the engine coolant system that goes to the heater,
about a dozen vacuum hoses that control the various
flaps. You also have the electical outputs for the blower,
AC clutch, etc. And all that is driven by a small electric motor
that is inside the thing. The motor is part of a feedback
servo system that moves in relation to the desired temp
setting vs the actual. It goes from max cooling, to max
heating. Oh, and a critical part of that sytem is the
potentiometer inside that widget, the resistance value
of which changes based on the current position of the
motor that works the whole thing, moving it from max
cooling to max heat.
So, you have hot engine coolant, vacuum, electrical,
a potentiometer, all inside one widget. How smart is
that? The typical failure mechanism is that the plastic
housing cracks and it leaks coolant. They were up to
$600 in the 90s for a new one. Some guy down in
TN started a business rebuilding them. His uses an
aluminum housing for the part where the valve and
hot coolant are. I bought one 10 years ago for $500.
It's been fine until recently. Now it's starting to have
problems. Haven't had a chance to find out what's wrong.
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.