At 09:19 AM 6/4/00 +0100, Theo Jenniskens wrote:
>But this did not answer the question what parameters
>the car uses to change gears... speed, rpm, vacuum,
>throttle, current gear, and the kickdown switch...
>What's the formula here?

Conventional (i.e., non-computer-controlled) automatic transmissions are operated and controlled thru fluid pressures. The fluid pressures are controlled by various input devices and controls.

Road Speed - There is a mechanical flyball governor attached to the output shaft that modifies control pressure according to the road speed.

Engine Speed - Primarily sensed thru pump output pressure.

Engine Load / Throttle Position - Vacuum modulator - diaphragm moved by manifold vacuum moves a plunger that changes control pressure.

Throttle Position - Cable from throttle quadrant that moves a valve to change control pressure.

Current Gear - Fluid flow thru the valve body changes according to the valves actuated - the valves actuated determine what gear is engaged.

Kickdown Switch - Actuates a solenoid valve that changes control pressure.

Selector Lever - Moves a multistage spool valve that determines fluid flow and valves actuated.

Operation (GROSSLY simplified!)
Front pump, driven off the input shaft (which turns with the engine - no disconnect) generates hydraulic pressure, which varies with engine speed.

Selector Lever spool valve is moved, directs pressure to actuators to engage clutches/bands for appropriate gear.

Pump pressure is modified by: Feedback from the position of valves in the valve body (determined by what gear is engaged); the vacuum modulator (driven by manifold vacuum); throttle valve open/closed (driven by the cable from the throttle); governor (driven by output shaft speed); and the kickdown solenoid valve open/closed (driven by the kickdown switch/switches.

This control pressure moves valves in the fantastically complex valve body to actuate the valves, clutches and bands that actually engage the gears. In (overly simplified) general, higher control pressures mean later and firmer upshifts and earlier and firmer downshifts.

NOTE: I am NOT an automatic transmission expert, and DON'T intend to spend the next two weeks answering dozens of e-mails on the subject!!!

Wally Plumley
928 Specialists