one pound R12=how many pounds R134a?
 

I have an 87 300E and it normally takes 2.4 lbs of R12 to charge the AC in these cars. I have been told verbally by a good mechanic at an MB dealer to put 80% or about 2 lbs of R134a in. (My car has already been converted to R134a) Stu Ritter in the E Class Owner's Bible says R12 get 2.4 lbs and R134a get 2.2 lbs. So should I put 2.2 or 2 lbs in? Does it make much difference anyway? Is it safer to underfill than overfill?

Yes it is safer to be under than overcharged. The standard rule of thumb is 80%, but I use the pressures to decide that last quarter pound.

My pressures were 240-250 over 39

The reason I didn't state pressures is that they are so dynamic. They depend on numerous things: ambient temp, rpms, fan speed (both inside and outside). Your pressure sound about right depending....

Also want to add that a tech helped me charge the system. The system seemed to perform better last year with 215/40 pressures. Outside temp about 80F.
So what is the algorithm on using pressures to optimize the system?
Thanks.

AC refrigerant level
 

Did you fix the leak? Head pressure sounds a little high, but maybe it was hot that day or your condensor is dirty.

If the system had lost the charge completely--it MUST be vacuumed and the drier replaced or the compressor will soon fail. The lube oil for R-134 does not like even trace moisture or air. I always vacuum down to less than 100 microns (29.995" Hg vacuum) and hold that for 30 min to ensure all moisture removal.

It was down to about 1.08 lbs per the machine. Then a 15 min vac was done and then recharge. This was done at a dlr. Temp was 80F. Do not know exact location of leak. Saw some dye another time at the upper right connection of condensor. I will hose off condensor. So would more R134a decrease or increase head pressure?

wouldn't more = more on either side?

I just topped mine off; was at 25 psi, topped off to 40 psi, 91 deg ambient but this was at idle, not 1500 rpm, and the system had been running pretty warm....

The pressures will be SIGNIFICANTLY different between idle and 1500 RPM. You should let it stabilize at 1500 RPM for about five minutes. It is also best to use a HIGH VOLUME fan to simulate ram air while you are doing this. I have an old squirrel cage fan that moves LOTS of air.

There is no magic formulat to use on every car. Your best bet is to go by manufacturers charging instruction. Once converted, you are on your own and need to undercharge then add in small amounts and proceed cautiously. The high side pressure in a 134 converted system can "run away" easily.

Good luck,

LarryBible,
Thanks for the reply.
My vent temps are maybe 50 degrees, but that's after only a 5 minute drive.
Sounds like I need to get a set of proper guages and check the high side charge as well.
If I have to vent some, can I get the low side valve to safely vent some pressure by pushing on some part of it?
As you can tell, I'm new to this AC stuff, but willing to learn.
I'm just very glad I added straight R134, not some magic fix-it stuff.

Help me here. I'm still unsure what the best way to determine the exact point where enough refrigerant has been charged. I thought one way to determine proper charge (after vacuuming) was to charge until a steady vent temp and pressures are reached, then keep adding until the high side pressures just begin to climb. This would fill the accumulator and allow for any slow seepage without losing cooling performance any sooner than necessary.

I have basically the same question...since I"m not evacuating I need to know that exact point where the system is happiest.