1984 300SD. She drives for awhile, AC works great. then it gradually warms up, then it goes to HOT? What might it be. also, when I turn it to defrost, it is hot, I think by design. Am I right?

Your wife or the 300SD gradually warms up?
You are right about the design of the defroster, it is always hot.
Christian

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1974 240D, 125K mi, B100 now, SOLD
1980 300TD, 115K mi, 4-speed stick, roll-up windows, greasecar with two tank conversion (daily driver)
1985 300D, 220K miles, greasecar with two tank conversion, SOLD
1993 300D 2.5turbo, 158K miles, green/tan, B20-B50

Time for a monovalve repair kit.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1R31DYKZJ23T15CWZG&year=1984&make=MB&model=300-SD-002&category=R&part=Mono+Valve+Repair+Kit

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Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE

did a repair 6 months ago, would the problem be maybe it got unplugged?

Yes, if it is unplugged, it is at max heat.

__________________
Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE

On my 300D, I will get max heat if the #14 fuse is blown because the monovalve goes open (I had a short in the wire from the kick-down switch to the tranny, which is also feed from #14). I don't know the equivalent fuse on the SD, but I would take a look at all the fuses.