1982 300D - heater issue
 

I have searched for a similar set of symptoms without success.

When I leave work each day I have about an 18 mile trip home. I put the heater on mostly using the basic setting without the A/C compressor on, but it doesn't make any difference if I set it with A/C.

It takes a while for the heat to come on (does it rely on engine temp?), but by then I am on an interstate for about 12 miles. It never reaches a really warm temperature (I have it set on 26; outside temps vary between -5 to +5 C at the moment). It is not uncomfortably cold but not anywhere near the heat that comes out later...

12 miles later I slow down for the off ramp (from ~70 to ~30 mph). The heat immediately comes on strong and generally remains good for the next few miles to home - all slow suburban driving.

Any ideas as to why the heat is weak at high speed but good when I slow down? Where is the air intake for the a/c and heat?

Thanks in advance for any help.

Your symptoms point to a small tear in the monovalve diaphragm affecting coolant flow to the heater core.

One of these and 10 minutes will solve your problem:
http://catalog.peachparts.com/ShopByVehicle.epc?q=1982-Mercedes--Benz-300d-Climate--Control&yearid=1982%40%401982&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6190%3AMBC%7C1504%3AED%7C1 0000135%40%40300D&catid=242213%40%40Climate+Control&subcatid=242328@@Mono+Valve+Repair+Kit&mode=PA

Also verify the aux water pump is working.

And yes, if the unit is working properly, it will not come on until the engine temp is greater than the inside temp of the car.
Does it come on when on defrost? That will bypass the temp system.
I am thinking someone drilled holes around your t-stat, and it is taking a long time for the engine to get warm enough to turn the system on.
Could try cardboard in front of the radiator, but watch temps really close.
I read this as the unit is not comming on until you slow down. If it is on, and the air is not warm, then I would also start with the mono-valve.

Thanks for the advice
 

Already placed the order, thanks!

Now I have the opposite problem...
 

I replaced the monovalve and it immediately fixed the original problem of no heat at high speed. Trouble is, I now have very little to no heat at slow speeds and at idle. Any thoughts?

Now that everything is vice versa, you have to drive backwards to get heat at idle.

Seriously, though, now you may have an auxiliary pump problem. Set your heat at maximum and see if you've got 12 volts at the pump (car not running and key in position II). Or you can just listen for it. It's an obvious noise under the hood. If you hear the pump then something else is wrong. If you don't hear the pump and you've got 12 volts, your pump is seized. Been known to happen, and if so, at least disconnect the pump until you get it fixed. A bad aux pump can fry the climate control.

Engine operating temperature
 

It started with a coolant hose rupture, and eventually I ended up replacing the lower radiator hose, thermostat (EOM from dealer), and the coolant head gasket. Now, after all that, the operating temperature is higher... around 85C before, around 92C now.

Is that too high? What would cause the difference?

In my opinion, 92 degrees is not to worry. Every time I do something to my cooling system it changes the operating temperature somewhat.

Just watch it on hot days with the AC on. That will be the tell if there is a real problem.