Got my AC recharged and working today in the 92 300TE. It was already converted to 134a, so I just loaded it up. Was trying to figure the proper charge, by weight of 134. Just happened to check my parts car, a 93 300E with a factory 134a system.... folks, the entire systems are IDENTICAL, except the compressor oil and refrigerant. Same evaporator, same condenser and fans, same compressor. 92 calls for 2.4lbs of R12, 93 calls for 2.2lbs of 134a.

I’ve never been asked once for my license when I purchased r12 for my 89 300e on eBay. At least 2 different occasions. Further, the “license” is a $15 fee and a ridiculously easy open book test.

True or false: it’s ok to vent more than 10 lbs of refrigerant in to the atmosphere

If you can answer that, you are golden.

I agree, but I still took the test and got my 609 cert.

Remember big brother's always watching :D

Here are the pressures with the motor off
https://farm5.staticflickr.com/4670/...552fbe15_b.jpgUntitled by ttownthomas, on Flickr

And with the car running but the HVAC system off - center of pulley NOT spinning
https://farm5.staticflickr.com/4666/...b49b24a3_b.jpgUntitled by ttownthomas, on Flickr

And with HVAC set to max cool - center of pulley spinning
https://farm5.staticflickr.com/4676/...f2517133_b.jpgUntitled by ttownthomas, on Flickr

What was the temperature outside? You are definitely low. The low side should be like +30, and high in the 200 psi ish range

Definitely very low. For it to be that low, you probably have a leak somewhere too. I'm surprised the compressor is engaging with the pressure that low, the pressure switch on the receiver/dryer is supposed to keep the compressor from engaging when the system is very low.

Around 60f

Am I thinking right here:

Since the compressor dosen't run when the a/c is off I think that means that the electromagnetic clutch on the compresser is good.

If the car was drained completely of freon and the compressor still engages we know that the pressure switch must be bad.

I might as well fill it with 152a and dye see if it cools and find my leaks. Drain it again and Replace the A/C seals and the pressure switch.

If I do this how do I know if the proper amount of oil is remaining in the system? It it as simple as removing the fill hole screw and filling it up?

The clutch is good because it engages the compressor. The controls that engage the clutch are questionable, I'd expect at the pressures you have that the clutch would be disabled. I bring it up because if you run it for long like that, you risk tearing up the compressor. It relies on gas circulation to return oil to the compressor sump. When I got my SDL, the Klima relay was stuck closed, so it ran the compressor with no refrigerant and burnt it up. That happened long before I got the car. The compressor was a write-off and the system was a mess. Don't let yours get to that point, it's a lot of extra work that doesn't need to happen.

If you want to do a quick gas & go, buy a couple cans of R12 and top off the charge. See how long it lasts before it leaks out again.

If you're going to have the system recovered and filled with new gas, you might as well do the job right. Flush the system out, replace ALL of the O-rings (they're a common leak point), replace the expansion valve with a new one (all the new ones are R134a rated), install a new receiver-dryer, and replace the fan switch and pressure switch on said receiver-dryer (the newer switches have pressures matched for R134a). Pull a deep vacuum, dump in some PAG oil and refill with R134a. The small-tube condenser on the Gen II 126 works fine with 134a, and when matched with the correct expansion valve, the system works very well.

I agree with all your points but gassing it up the way it is will not solve the problem with the compressor running all the time right?

Here is my plan:

I want to keep running r12 but I need to fix my leaks and replace all the seals first. but before any of that I want to see if the system is functioning correctly.

I want to pull a vacuum on the system and then fill it with 152a and dye to find all my leaks AND test the rest of the system.

What do I do about oil with the 152? Should I check the level at the compressor when it is empty? Just add a little for good measure? What oil should I use?

If you change refrigerants, you need to flush the system and refill with PAG oil. 134a and 152a both require PAG oil. R12 uses mineral oil. The importance comes with how the oil circulates, R12 is miscible in mineral oil, so it is carried along with the gas charge. 134a and 152a are not miscible in mineral oil, so it will not carry it through the system. You run the risk of starving the compressor for oil, or oillogging the condenser or evaporator.

If you're sticking with R12, stick with R12. I did that originally in my SDL, but after about a month and a half of use, the expansion valve got stuck open and stopped cooling. You can't buy a new R12 rated expansion valve (different orifice size than R134a), so I converted the system at that point. When the conversion is done correctly (flushed, filled by weight, correct expansion valve) there's not much difference in performance on these later W126's. Early 126's and the W123's had a crappy condenser that was barely adequate for R12. The Gen II's have a small-tube double-circuit condenser that's much more efficient with the newer refrigerant. The SDL cools just as good if not better than most modern vehicles in my climate and I don't even have the windows tinted...

By "flush the system" you mean take it apart and flush all the parts manually? No reason not to replace stuff at that point. Are their quality aftermarket parts or will I need genuine MB parts (reciever/dryer & expansion valve) for this?

Flushing the system involves opening up the lines and sending a flush solution through it. It isn't difficult. You'll want to drain the sump of the compressor too.

After flushing, replace the O-rings, pressure switches, and receiver/dryer. There are decent aftermarket parts out there. Modern replacement parts are designed for the pressures and lubricants used in 134a systems, just another reason to convert it if you're going that far.

FWIW I ran the 300E on mineral oil and 152A until it met its demise New Years Eve. That was three years worth on the original compressor.

You would be fine to charge it and get it cooling.