How does a transmission "know" when to shift?

Is there a sensor that senses Crankshaft speed and compares it to road speed? Or is it throttle position vs. Crankshaft speed ?

Big question as you have hydraulically controlled and electronicaly controlled transmissions they both rely on pressure differentials at the sides of a shift valve to hold or change a gear.

Page after page of inputs to control shifting. Some transmissions even monitor the fluid temp (either hot or cold) and alter the shift schedule.

Throttle position is fed to both the engine controller and transmission controller,the trans controller is on a CCD BUS with the engine controller

You really are going to have to get the book and plan on spending a lot of time.

In the old low tech days, it was engine rpm and manifold vacuum. The higher the manifold vacuum (more closed throttle), the lower rpm the shift point is. The upshift points are higher than the downshift points to prevent annoying gear hunting at certain speeds and hill grades. Some had mechanical throttle position inputs too.
The thing that measured manifold vacuum had a diaphragm and if that diapragm developed a leak, ATF would get sucked into the intake manifold and you would see a car laying a real smoke screen, on the magnitude of a stunt airplane that turned the smoke system on for an airshow.

A lot of today’s transmissions use computers to control shift points but they still go mostly by engine rpm and manifold vacuum or throttle position and perhaps with engine temperature and other inputs.


Here’s a start:

It will depend on the transmission itself. Yes, input and output speeds are one of a few things the computer uses to command the shifting. TPS readings, Temp readings, MAP sensor readings are other things used. Are you needing information on a specific transmission??


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