Toyota Repair: 93 Tercel AC, r 134 freon, r 12 freon

Question
My mom's Tercel A/C isn't working. The clutch isn't engaging when the A/C button on the dash is pressed. Therefore, I guess there is a freon leak and a low pressure switch on the compressor is keeping it from engaging. So I'm going to add freon to the system to see if the clutch will start working. I heard that there was a freon change over from R12 to R134 between '93 and '94. Do you know which refridgerant the '93 Tercel takes? Also how do you know when you've entered enough refridgerant? Do you have to hook up a guage to the high or low side? What would the full pressure be?
Thanks
Keith

Answer
It takes R-12 freon which is no longer available at the retail level in the US, only certified a/c repair stations and dealerships are allowed to buy it from the manufacturers.
Considering the cost of one pound of R-12 freon(about $55.00) I would recommend installing a relatively cheap changeover kit which is available at almost all autoparts stores for about $30-40, this changes the system to the R-134 freon which is used in all systems since 1994, the cost of a pound of the r-134 is only about $6.00, follow the instructions included with the changeover kit, this should get the system up and running but it may not solve the root problem which may be a leak in the system, you will need to take it to a a/c repair place or the dealer to locate the leak with either a leak finder or by adding a flouro-dye to the system.