My 1981 240D a/c does not work. I have no clue on how to troubleshoot this problem. The person I bought it from said something about getting it charged every so often. I noticed that the compressor does not engage when I turn the dial to max. Is this because of the low charge and is a safety function so the compressor will not burn out? Looking for good input as the DC heat is about to be upon us.

TK

yes there is a low pressure switch that prevents the compressor clutch from pulling in when its low on refrigerant.
having it charged "every so often" is a sure sign there is a leak somewhere. It could be a simple hose or, (if you're real lucky!) just a leaky Shrader valve.
Using a dye test or a Halogen sensing device (beeper) a leak can usually be spotted right away. It's what is causing the leak that will determine the next step and cost.
But WHATEVER you do, don't convert it to R134A

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

The A/C system in the 123 is a pretty easy system to fix and diagnoisis. You do need some specialized tools but they are not that expensive.

http://dieselgiant.com/repairyourac.htm

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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
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