I have a 300SDL 1987..my A/C is coolish but certainly not cold. I went to add some 134a and found 2 things.

1. I could only find 2 ports, one in front of the driver near the fire wall and one on the dryer.
2. Both are screw on connectors. I'm assuming either it's still r12 or it was converted and the correct connectors weren't put on.

So which of these is the low side and can I add 134a to top it off if it's still r12?

 

I'm aware. I talked to my mechanic and he said most likely it's r12.
I was more asking if it is ok to top off with 134...and specifically what the head pressures should be.