A few days, for the first time ever, my A/C began to intermittently stop working. Specifically, the air blew cold for a few minutes until the car warmed up, and then the compressor kicks off and warm air follows. After a few minutes of pushing all the HVAC control buttons, I found that turning on the front defroster results in cold A/C coming through the defroster and side dash vents. If I cycle from the defroster back to normal A/C modes, the compressor kicks off and no A/C. If I start in the defroster mode though, I get normal hot air. I have worked under this approach for a few days to maintain A/C in these last hot summer days.

Again today, A/C worked for a few minutes upon startup and then shut down (no cold air, fan automatically went from high (auto mode) to low). Again, I switched over to defrost mode and had cold air for a few miles, and then it stopped producing cold air. No A/C no matter what setting today. Lastly, when under low engine speed crusing into the neighborhood, I again turned on the A/C to see what would happen and could feel the drag on the engine of the compressor kicking on and off, on and off, repeatedly every few seconds. I have since stopped using any of the HVAC system as I don't want to burn anything up.

I read some forums regarding issues with the auxiliary water pump drawing too much amperage and triggering similar results. I have unplugged the auxiliary pump with no luck...no cold A/C.

I replaced the KLIMA relay a few years ago when the A/C stopped working. That fixed the problem then and I have had no problems since.

It comes across that something (relay, switch) is getting overloaded and is causing the current issue, but otherwise I have no clue. Any thoughts or suggestions?

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1993 300D 2.5T
1968 280SL - sold

Klima relay is a suspect if the compressor engages for a while then cuts off until the engine is restarted. Did you replace the Klima relay with a new one a few years ago?

Check the low side pressure switch. It's the switch without pigtails on the receiver/dryer. Set ACC to normal (outline arrow and solid arrow) or total ventilation (two solid arrows), thumbwheel to MIN and blower to MAX. When the compressor cuts off, check for continuity across the switch with the wires disconnected. If there's no continuity, you might be low on refrigerant. You can also check for bubbles in the sight glass but checking the switch is more definitive.

Sixto
87 300D

A bad evaporator temperature regulator can cause that to occur.

The receiver/drier is not located on the low side. Neither is the switch.

Oops, I meant low pressure switch.

Sixto
87 300D

FIXED. Thanks for the suggestions guys, but it turned out to be the auxiliary water pump after all. I realized yesterday evening that I had unplugged the wrong electrical connection associated with the aux water pump (I unplugged whatever the other device is just inline of the aux water pump). Once I figured out my error, I reconnected the other device and unplugged the aux water pump...problem solved. I now have constant and correctly operating A/C. I guess the aux water pump was drawing too much amperage and tripping the entire HVAC system. I am just really glad I searched this forum and found some information and an easy fix....I'm sure a trip to the shop would had been a disaster cost-wise.

Thanks to Christopher Pikus and his A/C troubleshooting post at 00 Miscellaneous Reference Articles as discussed and buried somewhere in this forum.

__________________
1993 300D 2.5T
1968 280SL - sold