I have 1990 300E Gas 6. A few months ago for no apparent reason the heater came on and wouldn't turn off. This happened for about a day and then just fixed it self. However now that it is summer, I notice that my AC isn't working. When I switch on the controls in the cab, the compressor isn't turning. Not sure how to troubleshoot this problem.

I think is might be something other than the compressor in light I had the heater problem. Sounds like maybe a climate control switch, or something along that line to me?

Any help would be greatly appreciated!

If the compressor clutch is not engaging, I'd would fist start with a static pressure test. You hook up manifold gauges to do this. The system does not have to activate for this test, hence the word "static".

I would ref. the following thread for Larry Bibles static figures.

A/C problem with my SLC...

If your refrigerant charge is too low, you'll have to hot-wire the circuit at the low pressure switch to draw in additional charge.

If the static test indicates sufficient charge, things get more complicated.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Most A/C systems leak a little every year starting when new. It is possible it finally hit the point where the low pressure cut off sw. said 'thats enough'.
To do air right you should have the tools. If you dont want to be bothered with getting the right tools, Larry's thread is the way to go. I have enough cars that have A/C to justify me owning guages, a vacuum pump, electronic leak detector (sniffer) and a dye kit.

I like the idea of performing a vacuum test first before doing a sniff test but I like the sniff test before the dye test. Pull a vacuum down to -30 on your system shut the valves off. If it stayes there, its not leaking so charge it up. If it slowly looses vacuum over a peroid of 1min., inject the system with a little R134 and do a sniff test. If you can't find the leak, do a dye test. When ever the system is opened up, you need to place baggies with rubber bands over the opened A/C lines to keep moisture from reaching the dryer and dirt from clogging the expansion valve. Moisture in the system expires the absorbtion of the charcoal pellets in the dryer.

You can always pay a mechanic to do a vacuum/leak test for you, then replace the parts yourself.

Note! when jump starting the compressor you will need to disconnect two connectors on the low pressure cut off sw. Be careful when disconnecting these because they tend to break off after they get older. I always squirt these with WD40 a while before I expect to disconnect them.

Where is the low pressure switch, and how do you "Jump" it?
My sons, 88 300TE A/C is not working and still has R12 I assume, as no sticker show otherwise.

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2009 Mercedes ML320 Bluetec

You really can't assume you have refrigerant. The clutch must engage and if that isn't happeneing, low refrigerant could be the root cause. A static pressure test with gauges is going to give you an idea whether or not your system is low or dry altogether.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

If your only problem is a low charge, you should not have to jump the switch to get compressor engagement. Once you have added some refrigerant the pressure should come up allowing the clutch to engage.