Speed encoders

Hey, guys.

with my 4.2 automatic the speed is no longer less than 800 u/min in idle. I've already cleaned the idle control. That's not it. The speed also drops much slower, as before. But the car is running perfectly. No hot/cold start problems no jerking or anything else. Lambda's new. Air filters, candles new.
I'd have to know if the speed encoder is only controlling the rev indicator or if it's also responsible for idle before I switch it. I also read about the intake air temperature sensor. Am I going to trade that next?

Welcome

Matthias K.