Has your Explorer been converted to R-134a refrigerant? If you add R-134 to an R-12 the two are not compatible, and all the seals in the old R-12 system will leak out the R-134 refrigerant. R-12 is not sold anymore but you can sometimes find old cans for sale by owner.
Another note. If your system was completely empty due to a leaking seal, you could have air in the system. Filling an empty system requires putting it under vacuum first.
If the refrigerant level is correct and the system is properly filled, without air in the system, there are a couple other things to check. Check the AC WOT cutoff relay, that it is functioning correctly. Also check that the AC light is illuminating when you push the AC knob in to turn it on.
There is a sensor screwed into the drier that senses the system pressure. If this switch is unplugged or malfunctioning, it will not let the system engage the compressor, to avoid system damage. Keep in mind that unscrewing this sensor may release refrigerant pressure.
The AC compressor clutch can be tested by applying 12v to the contacts. It is possible to simply run a 12v source to the compressor to make it engage the compressor manually, but this is not recommended with engine running unless you know for sure that the system is properly charged and the compressor is in good condition. Running the compressor on an empty, undercharged or overcharged system, or with a damaged compressor, can be very dangerous and risks damage to the system or personal injury.
AC work is one of those things that you can spend way too much money trying to fix yourself, and not do it properly unless you have the know-how and equipment required. Sometimes it is a lot cheaper and easier to have an AC shop diagnose and repair it the right way.
