I have a 90 and 91 740. I am trying to learn about the AC before doing the R134a conversion. I have the low and high pressure gauges, the Volvo kit, and a vacuum pump and believe I have almost enough knowledge to do the job. I would like to put a high pressure fitting on the high pressure side an charge monitoring the high pressure side. I have found a pressure switch on the high pressure side just below the receiver/dryer. I would like to put a T on this fitting for the switch an put the switch on one leg and a R134a fitting on the other leg to measure the high pressure side.

1) What is the switch for.

2) Would this be a correct place to monitor the high pressure side since it is after the condensor. The high pressure side of the compressor goes through the condensor, then through the small tubing where this switch is located. When the high pressure side is measured is it usually right out of the compressor and would it be okay to monitor on the proposed T-addition?

3) What kind of compressors do the 90 and 91 have? I am asking because on a couple types of compressors the o-ring seals need replacing according to the FAQs on the brickboard. I couldn't find a name on my compressors but may not looking in the correct place.

Thanks