just bought a '91 940 turbo, and the a/c doesnt work. the compressor isnt coming on.

how can i tell if it has been converted to r-134a? and where is the schrader valve for the low side?

also, where is the relay located for the a/c and how can i test that?

 

The schrader valve should be next to the dryer.
If there is a funny looking connection at the dryer it should be converted.

Unplug the low pressure switch at the dryer and jump it with a wire with the car running and see if the compressor kicks in. If it does then most likely you are low on freon.

 

can you tell from these pictures if it has been converted to r134a? and what does the low pressure switch look like, can you see it in these pictures?

 

That is still R-12.
The low pressure switch is the thing screwed into the Dryer(Aluminum cylinder) That has the plug that goes to it.