Dont mix 134a and Freeze 12 together. They have a much different pressure/temperature relationship. I would advise not to use 134a in the MB anyway. I can get the air out the vent to 28 degrees with Freeze 12 and the pressures are lower than with 134a.

You NEVER charge the system on the high side with the car running. If you have an A/C machine and the system was pulled down in a vacuum like it should have been, then you could do it with the car off, but never while running. Also never put liquid in the low side. The low side line goes to the compressor and it is designed to pump vapor NOT LIQUID. If you get enough liquid in there it will destroy the compressor. Vapor only please. If you have not checked out the pictorial then look it over for some tips.

http://dieselgiant.com/repairyourac.htm

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I was not aware of this. I still don't know what Freeze 12 is...

Absolutely correct.
Not exactly. NEVER dump liquid into the compressor side *(low) while it is running, BUT if VERY SLOWLY metered , putting liquid in on the low side is actually the recommended way to charge... look at the can. it states charge this side up. 134a is what is known as a "blend" and if you use a partial can and you only charge the vapor side of the can, you will not get the correct proportions of chemicals into your system.
John

R 134a is NOT blend. Charging by vapor only is fine.

Blends: Freeze 12 is. Envirosafe is. "Oz gas" is. HC12 is. Duracool is. Others .

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1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

I was under the impression that all the replacement refrigerants were blends... I guess not.
1,1,1,2-Tetrafluoroethane, also called simply tetrafluoroethane, R-134a or HFC-134a, is a refrigerant without an ozone depletion potential and thermodynamic properties similar to R-12 (dichlorodifluoromethane). It has the formula CH2FCF3, and a boiling point of −26.6 °C (−15.9 °F).

Well I got the fitting working, I just needed to take off the R12 to Freeze12 converter that was on the nipple.

So then I started the car up and got the fitting on and started to fill up the system with 134a from the can, into the low pressure side which is located in front of the driver's side in the engine compartment. I realized later that I had the A/C controls set to "EC" which means the compressor was turned off. I was filling it up, and it hadn't even taken a full can of 134a when suddenly the pressured dropped to 0, and now it remains at 0 when the car is running. What is the maxiumum pressure these systems should tolerate? It was still in the blue area on the can's guage, not even in the yellow when the pop happened.... probably around 40-50 psi..... and that is on the low side remember.

HOWEVER, with the engine off the pressure reads really high- over 100 PSI on the high side. So I am at a loss as to what is going on. Any ideas? I hope I didn't ruin anything! I just want some cold air at the vents, and it currently blows hot.

equalized freon in a hot car can be over 140psi, don't worry.
the pop could have been a lot of things, after you realized the compressor was in EC and you turned it on, how high was the High side? charging a 12 system with 134a and no High side gagues is asking for problems. 134 is a less effecient Ref than 12, and it puts out a LOT of pressure if you over charge it. I don't know if MB has a relief valve in it or not. it could be a valve broke in your compressor, give us more details as to what is happening in the system.
John

I don't think the pop was significant now as it happened with very little 134a in the system. And I got it to hold pressure

I determined that I hadn't put enough 134 into the system and now, with the compressor on, I got around 30psi on the low side and 150 psi on the high. Cold air is coming out of the vents! The high side pressure is worrying me though- is that too high? The temp outside was about 20 degrees celisius or 68 degrees F.

Your pressures sound about right to me.

30psi on the low side and 150psi on the high side is about right for the ambient temperature an r134a. They don't call it the 'high pressure' side for nothing!

If cold air is coming out of your vents, and the compressor isn't making a racket when its running, then you've got functional AC. Enjoy it! Keep an eye on the low side pressure to make sure your aren't leaking, and listen for a noisy compressor which could happen if you are leaking in a place that also leaks compressor oil.

peace,
sam

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