Would be better with photos. I think I know the answer, but you can help me to make sure I don't hurt myself.

As part of my normal PM process, and because I have three quarters of a can of R134a left over from my other car that needed a boost:

If I'm tracing the schematic in the ROM correctly, the low pressure fitting on a '91 (pre-facelift) XJ-S is the "smaller cap" which in fact sits on a line over the top of the compressor, coming from the firewall (and the evaporator). The high pressure fitting is the one with the larger "cap", on a "can" looking thing next to the compressor, leading towards the condenser up in front of the radiator.

Is this correct?

And, for the record, while the car is new to me, there is a sticker in the engine bay stating that the car has successfully been converted to R134.

Thanks!

 

Yes, the low pressure hose comes down over the top of B bank, through the fuel cooler and into the compressor.
Greg

 

If you are going to put a gauge on to fill it, why not just read the gauge?

If you don't have gauges, which is why you are asking, how do you know the car needs more refrigerant? Adding too much can cause problems.

 

I thought R134a system had colour coded connection ports : blue and red (blue low pressure, red high pressure)

 

I have an inexpensive low side gauge, but did not want to risk opening the high side, on the concern the pressure may be more than the gauge can handle. Was not going to add unless its needed.

I thought it would have color connection ports as well, but whoever converted it left black caps on it.

I was just trying to be safe. I thought I had it figured out (turns out I did) but wanted to be sure before I tried the "I wonder what this does?" school of mechanical engineering.

 

R134 low and high side ports are different sizes. Low side is smaller diameter then the high side....i.e. A low side gauge/connector won't fit on the high side port.