A/C mystery

1984 190D I have a problem I have always associated with low freon; however, I just had a freon charge (after two years it took $18.50 in oil & dye and $15.00
in freon R-134A, which doesn't seem like a fast leak), but the problem persists.
When the car has been in the shade when I start up, the A/C cycles on and off normally (usually), but when it has sat in the sun as today (it was 80 degrees here in Abilene today) it either won't come on at all, or it comes on when I start up and when it cycles off, it won't cycle on again. Turning the temperature wheel to max has no effect (it works beautifully for heat).
When it operates normally and I am at highway speed, it usually continues to operate normally.
Any ideas?
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your fan running? mine does not run all the time but if the head pressure climbs a bit or it hots around 100C it turns on.
also you may have junk in te system<dryer> or the pressure switch needs changing.
my 82SD still funs on R12 god its great.
the case, minus a few cans!
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Does it switch on and off, or does the cooling effect come and go? Richard

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low freon pressure will cause short cycling
the case, minus a few cans!
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I would bet that the shop overcharged it. The compressor will shut down if the pressure gets too high. The only way to charge the AC system correctly is to completely evacuate it and put in the recommended amount. This would also get rid of any moisture in the system which could cause any number of problems. I had similar issues with another car where I added freon and wound up overcharging it. It would work fine when it was cooler. When it got hot the compressor would shut down. I dawned on me what was going on when I put gauges on the system. I picked a hot day and let out a little bit of freon at a time untill the compressor started to cycle normally. Godd luck
Peter

Turning
heat).
to
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My C180 had the opposite problem: the A/C would only work when the car was really hot inside, like when it had been standing in the sun for a while.
It turned out to be (as others have suggested) a faulty pressure switch. This switch (on the C-Class it lives on top of the receiver/drier) protects against both too high and too low pressure in the system.
It could be that your 'too low' part of the switch is giving a false message to the rest of the system.
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Could also be a slipping serpentine belt....
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If you're still on R-12 your low side pressire should be 30-32 psi. I understand R134a is more likely in the 28 psi range. If your system hasn't been opened I see no reason to evacuate it for a total recharge. If there are no leaks the system should operate indefinitely with no moisture build up. However, if your system has been opened and lines exposed to ambient for more than a couple of hours I would evacuate the system, pull it down as close as possible to 30 inches of mercury and then recharge. Don't forget an oil charge.
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