The chiltons wont go into detail on how to charge the A/C on 90 EX.
Is it the pressure tap near the radiator or the pressure tap near the
firewall?

 

You can't recharge R12. You can't even buy it. And you CAN'T mix R134a with R12... Bad joojoo. You have to get the R134a conversion kit ($60 for all the cans and hoses/gauges). But first you have to evacuate the existing refridgerant (using EPA approved methods of course)

Anyway, with engine running and A/C turned ON, you fill it from the low side (should be the larger hose). Filling on the high side is dangerous. You start by emptying the can of oil into it, and then the refridgerant until you get to the recommended pressure. (The gauges you get with the kit have Low, Normal, High, Too High, marks on them, etc.)

Have fun!

 

I am selling a can of R12 12oz if you need one, of course i assume my buyer are liscened professional blah blah blah