Aloha Ed,, The ac on older vehicles are a little easier to figure out and either ac clutch gap gets to big or freon as rk mentioned, sometimes low switch or some electronic. if freon is low it usually means a leak if its old and dirty in lines it will just not get as cold. you can tap on outside of clutch fan to see if it will engage, most time it will engage if issue, truck needs to be running and hit the outside plate, be careful and watch a video, there easy to fix, some ya got to remove clutch or some just adjust from out side but way easy and cheap to do,, ya need a filler gauge to check, but see more of this then freon being low on vehicles with on off intervals, if freon is low its usually a small leak somewheres. and most times it just stops working.