1-2 and 2-3 Shift Solenoid Valves

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Fig 1: Identifying 1 - 2 And 2 - 3 Shift Solenoid Valves

Courtesy of GENERAL MOTORS CORP.

The 1-2 and 2-3 shift solenoid valves (also called A and B solenoids) are identical devices that control the movement of the 1-2 and 2-3 shift valves (the 3-4 shift valve is not directly controlled by a shift solenoid). The solenoids are normally-open exhaust valves that work in four combinations to shift the transmission into different gears.

The PCM energizes each solenoid by grounding the solenoid through an internal quad driver. This sends current through the coil winding in the solenoid and moves the internal plunger out of the exhaust position. When ON, the solenoid redirects fluid to move a shift valve.

Important: The manual valve hydraulically can override the shift solenoids. Only in D4 do the shift solenoid states totally determine what gear the transmission is in. In the other manual valve positions, the transmission shifts hydraulically and the shift solenoid states CATCH UP when the throttle position and the vehicle speed fall into the correct ranges.

The PCM-controlled shift solenoids eliminate the need for TV and governor pressures to control shift valve operation.