I know this is going back to basics, but what makes my 4Runner ('98 SR5 AT) shift? I know the RPMs trigger the shifting, but what physically happens? Does the computer get a signal that RPMs are at certain number and then it tells the transmission to shift? Previous cars I've worked on shifted mechanically or through levels of vacuum, but I figure my 4Runner must be more advanced than that.

My 4Runner has recently started up and down shifting strangely. The CEL then came on and the code read P1600, indicating the computer was malfunctioning. So, perhaps what I thought was a transmission problem may just be the computer sending bad info?

 

The computer uses a variety of inputs including speed, rpm, throttle angle and probably others to determine shift points. The "kick down cable" actually only controls line pressure not shifting. The computer controls different solenoids in the valve body to direct fluid pressure to the proper clutches, brakes in the the trans.
P1600 means no back up power supply to the ecu. You need to check for power at terminal "BATT" on the connector to the ecu. Also check your EFI fuse