Hi all, well, it finally happened; the A/C in my '92 400E has stop working, it's really of no surprise since I have not touched the A/C system ever since I've owned the car, which has been about 7 years. I bought the car with 120k on the odometer and now it has 230k.

The specific issue is that no cold air blows out (guess that's a bit obvious), but that's all I can say. When A/C or normal mode is activated (button in the middle) the air that comes is the same as the outside, and when I switch to economy mode the air feels the same, although it blows at a higher rate. I checked under the hood to see if the compressor activates and it does.

I've never worked on an A/C system before so I have no clue where to start, my guess is that it might need to be recharged. I really don't want to bring it into a shop, I'd like to tackle this issue myself. I searched to forums but too many posts came up on the subject that where about to many different things. Help is appreciated.

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500

Ok, compressor working. Most likely due to low freon or a leak. Do not charge it yet.

The first thing I would do is to check the pressure of both High and Low pressure port in the morning and with the ac on MAX setting, and and go from there.

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Ok, so I testing the low pressure port and it reads 95psi, the temperature outside was 60F. This was with the A/C on full with the engine at idle, the compressor did not engage though.

I don't know where the high pressure port is so I didn't test it; I think it might be on the dryer? I then waiting for about 2 hours to let the engine cool and tested the low pressure port again with the engine off and it still read 95psi.

Tomorrow I plan on checking the fuses and relays and jumping both the AUX fans and compressor sensors on the dryer to make sure that they're all working.

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500

That low pressure reading indicates that the clutch is not engaged. You MUST have a high and low side reading to have any indication of a/c performance. Servicing an a/c system based on low side pressure alone is an a/c disaster waiting to happen.

Back at it again. I checked the pressure again in the system, this time in the morning before turning on the engine and it still read 95psi on the low side. I then let the engine warm up and turned the a/c on and to my surprise the compressor started to cycle, the reading came out to 30psi. After the system was on for about 3 minutes the compressor shut off and the reading then jumped to 95psi and thats the where it's at now.

I need to know where the high pressure nozzle is.

I also tested the AUX Fans, they do come on at high speeds, but do not come on at low speeds. When I jump the terminals on the dryer they didn't come on but I did hear the relay click. I then ran a wire from the 30amp fuse located near the fusebox to the top of the low speed resistor located near the ABS pump and the fans turned on, indicating that the resistor and fans do work. I either have a bad relay or something else is bad. I did take the relay out and popped the cover off to have a look and it looked very clean inside with no burn marks anywhere, does that mean it good?

I also tried to jump the compressor at the dryer and nothing happened, included not hearing any type of relay click, if there is one?

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500