My Volvo 940 needs the a/c system charged but i can't get the compressor to start. I guess the freon is too low. Does anybody know how to put a jumper or by-pass the low freon switch, (if their is one).

 

You're putting the cart in front of the horse. Assuming there is a vacuum in the system, R134a will dispense into the system of its own accord. When pressure rises to a predetermined point, the compressor will cycle. Thus, there is no need to jumper anything. If you do not pull a vacuum first, you won't remove any moisture from the system and it will neither charge properly nor cool efficiently.

 

And you should also pull a vacuum to check for leaks, ultrasonic leak detectors are dirt cheap and will pick up any small leaks when the system is under vacuum. It really pays to have a shop do this if you don't have a good vacuum pump, the cost is pretty low compared to buying one off tools. They also have the parts on hand to simply replace that leaking O ring you don't know about, instead of having to recharge the system again in six months.
Also add a dose of leak detection dye.
If the system has been opened at all, replace the receiver/drier.

Regards, Andrew.