How to recharge air in 1988 ford econoline?

I have an 87 e150 which was still R12. If your system has not been retrofitted to R134a it is against the law to recharge it yourself. I retrofit mine myself for under $40. I took it to a A/C service facility and asked them to evacuate any existing R12, which they did for free, then went to Walmart and purchased Interdynamics retrofit kit for $34.99. The kit has easy directions with screw on fittings, pressure gauge and plenty of R134a with oil and stop leak additives included. My A/C has worked great all summer. Previously I had been told by two different a/c shops that my system would cost between $800 to $1000 to repair. You recharge through the low side fitting only, which on my 87 is towards the passenger side of the van up against the firewall (towards the top of the engine compartment) it looks like a larger tire inflation fitting. You will have to remove the air cleaner to get at it. Recharge with the engine running and the A/c on Max.