Engine slow to idle down

I have a 97 Ranger 4cyl manuel transmission. When shifting gears from 1st to 2nd and 3rd it takes 10 or 15 seconds for engine to idle down. Does same thing when stopping at red light. I suspect it is a sensor that controls idle especially with A/C on. Don't know which or wher sensor s are. Dont think it is corroed throttle cable. Need help