I searched the threads for an answer to my issue but really couldn't find anything like my problem...Have an 83 300CD and there seems to be no regulation of temperature in the cabin...Heat seems to work but only if the temperature control is clicked into the full on position...Could this be the temperature sensor in the dash or is the control unit subject to the same bad solder joints that the cruise control is (recently resoldered and works now)subject to. I haven't even started looking at air conditioning but it seems to work the same way in that cooling only is full on or nothing. Blower seems to work fine and the vents seem to work OK and it will change from floor to defrost etc...Thanks to all who contribute to this forum as I have solved many problems just by reading and applying what I have learned...

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83 300CDTurbo 307,000
2005 Honda Element 266,000
56 Nash Ambassador Country Club Special 34,000(under restoration presently)

http://www.peachparts.com/shopforum/showthread.php?t=125099

My 85 TD climate control system is similar. I believe that these systems require small adjustments in order to have the time to sense the cabin temp and adjust the mono valve (heat) or expansion vave (A/C). It can be frustrating. The system does respond quickly to a full heat or cooling demand but anything in between takes time. You could easily check the sensing line integrity by removing the passenger side kick panel. Lay on your back and with a good light look up for a foam hose. If this is deteriorated then replace it. Not much fun but that may be the problem. Or as you sad could be the ACC head unit in the dash. E Bay is a source or Fast Lane, lots of on line sources.

I noticed a similar problem shortly after I got my 82 300CD. I'd have a/c up untill it 'clicked' to max heat. My problem was the acc unit. I swapped it with another and it resolved the problem.

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1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350