Depending on what year you have, this might help.

1988 and older (maybe newer but them I don't know about) have a receiver/dryer in the system mounted on the right end of the radiator. On top of it is a 2-wire harness connection. That is the low-pressure kick-out switch.

If you lift the connector just a little while leaving the contacts still connected, you can jump the contacts with a paper clip or some such. Do this with the engine running and the controls set so the a/c should be engaged. If you can hear and see the compressor engage, the test is over. Disconnect the jumper - the Freon is too low.

A shop can run a leak test, and would LOVE to sell you the R-12 needed to fill the system at $60+ a pound. Get the test only unless that amount is OK in your budget. Note that the Volvo kit for R-134 conversion runs around $200.

Good Luck,

Bob

:>)