How does the AC system modulate the amount of cooling delivered to the car?
 

I am trying to figure out how to install a manual HVAC control system to replace the ridiculous "automatic" system in my 1985 300D. The service manual on CDROM is of no help. I am trying to find out how the AC system modulates the amount of cooling delivered to the passenger compartment. Other cars (those with HVAC systems that actually work) have a bypass arrangement that puts all of the air through the evaporator when the blue/red dial is set for maximum cooling and then bypasses some of the air around the evaporator when the dial is set for less than maximum cooling. Does my 300D work that way? If so, is there a name for the air flap or damper that makes this happen?

I've heard it called a ....
 

blend door...

I'm sure, however that Mercedes has come up with some incredibly complicated way to do that though...:D

Look around your local salvage yards and find a 240D from '77 to '83. Take the a/c controls out of it and transplant them to your car. This will involve the instrument panel pieces and the blend door parts. Analyze both cars closely so you can see which parts to change. You will have to rewire the compressor controls which will involve the cooling control and filter/drier switches.

Good luck,

D,

I too had an issue with the A/C on my '98 E320. Couldn't seem to get the car cool. Seemed like the fan was shuuting down too early or the thermostat was about 10 degrees off. I took it in to the shop and I found out what the problem was.

I do not like the A/C air blowing on me nor do I like drafts. As a result, I usually have the A/C vents pointed at the ceiling. I've always done this in all the cars I've owned. Unfortunately, in that model car, the thermostat was located in the same panel that held the sunroof controls (namely the panel on the ceiling immediately behind the rear view mirror). As it turns out, the A/C vents were pointed straight at the thermostat reducing the blower speed prematurely.

My trusty mechanic simply redirected the air flow down a little, away from the thermostat, and walla, the car started to get cool.

While I do not know where the thermostat is on your model car (the vents might not even point up that high) you might want to check, just in case.

My experience has been that MB thermostats are indeed the best in the business next to the units put into Japanese cars (much quieter fans) IMHO....

Best of luck.

The thermal sample input grill is on the top of the dash on the '85 300D, so redirecting the vents would have no effect.

BTW. Have you checked the air sample hose behind the glove box? Remove the glove box and there is a hose that runs from the temp sensor at the center top of the dash to the right. This was a foam hose originally and deteriorates. You may see no trace that there was ever a hose there. If it is gone, go to Home Depot and get a length of foam pipe insulation. Cut to length and slip it over the two connections.

If the hose is missing, replacing it will most likely make your problems go away.

Good luck,

I will have to get behind the dash this weekend and check that out. I also noticed yesterday that it took a long time for the engine to shut off after I removed the key. The door locks were also slower than usual, so I think I have a vacuum issue somewhere. I don't know whether that might affect the climate control system.

Even if I get the climate control working, I think I will convert to manual control. I just think that, in a car, manual control is superior to automatic, even if the automatic system is working properly.

I doubt that the vacuum issue is related, but it could be.

Nine times out of ten you will find your vacuum leak near the oil filter. There are a number of vacuum lines that can get knocked loose when changing the oil filter. Check them out.

Good luck,