1983 300SD Turbo Diesel - The heater control valve keeps shutting off while I am driving. If I stop and put the car in neutral the valve will open and the heat will come back for awhile. The circulation pump seems to work, (I can hear it running from under the hood). I put a used climate control unit in but it made no effect. (Possibly another bad unit?) Also the A/C compressor wants to stay on all the time, adding to the coldness!
Could this be a vacuum problem?

 

I assume you have the ACC servo unit as my 79 450SLC has, my advice to you would be to contact Performance Analysis Cmpany, George Murphy is the owner's name, they are in Oak Ridge TN, telephone 1 865 482 9175. When I sent my servo down to him it wasn't working at all. I got it back rebuilt and have plenty of heat and AC. George is an expert on these units and if it can be fixed, George can fix it, if no he will tell you straight up and give you a better price on a rebuilt unit. My experience is limited but one thing I have found out, is if you don't take care of the cooling system, these servo's have a 2 to 3 year life span. I received instructions with the fixed servo, which said to disconnect it and back flush the cooling system every two years as well as the heater core, which can be done very easily with a scrape piece of hose, Also, after you have reconnected the servo, bleed the air out of the system by opening up the IN hose to the heater core. Never flush your cooling system with the servo connected as junk from the engine and radiator lodge in the servo and cause premature failure. Also, as I have found she I reinstalled the servo, I have been topping off the anti freeze most everyday, with plain old water. This is normal as the heater core alone hold a lot of antifreeze.If you really lucky all you might need is a new heater control valve, $95 from Performance. I replaced that also, just to be on the safe side.

 

Sounds like you might have a Defective MONO-VALVE or possibly a Contaminated HEATER CORE

A defective mono-valve is indicated by good output at low engine speeds and/or immediately after warm engine start-up that decreases after a few minutes at higher rpm (i.e. intermittent heat).

The mono-valve can be quickly checked as follows:
1. Relieve coolant system pressure at cap.
2. Remove electrical plug and mono-valve insert mounting screws.
3. Pull insert out of valve and plug valve opening with shop rag.

Closely inspect the diaphragm for cuts while pulling down to its extended position. Also, check that after being extended, the valve completely retracts to the "fully open" position. If damaged, exchange entire insert with a new one from a new mono-valve. The mono-valve body need not be removed.

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Also When dealing with any Monovalve ACC problem, first check the monovalve Plug connection for tightness or corrosion, and correct as necessary.
It is not necessary to replace the mono-valve insert when the problem is only a poor connection. Complaints of erratic ACC operation such as intermittent heat from center outlets, etc. may be caused by a poor electrical connection at the two pole mono-valve plug, pin number 2 (left plug) or 4 (right plug) of the pushbutton switch, at the fuse, or main chassis ground. corroded, new pin sockets can be soldered-in and pinched tight for a better fit.


hope this helps