what do servos do?
 

How do they work, what do they change?

I believe servos control upshift...I'm pretty sure. Someone back me up or correct me?

saervos basicly control how hard the up **** is with a spring.the stiffer the spring the firmer the shift

servos are a type of motor that applies the bands to the drums causing upshifts. There are 2 in a 3 speed trans.

a servo is a movable piston inside of a drum or housing its job is to apply either a band or a clutch pack , every auto trans has 5 /6 or even more a powerglide 2 speed has 3 servos, low speed band , high clutch, and reverse piston, as its called - but it is a servo as it applies a clutch and A/C heating systems use servos to open and close all the doors around the dash for air travel -- but they are vacume servos not hydraulic pressure -- the vacume pot on a distributor is a servo -- i think you get the idea now - they are used in all kinds of applications -- jz