what is a shift control solenoid valve on an auto transmission?

Answer 1

there are two shift control solenoids on most automatic transmissions. They are identified as "Shift Solenoid A" and "Shift Solenoid B".

An automatic transmission or transaxle gets their different gears by applying hydraulic pressure to or releasing hydraulic pressure from the internal mechanisms that hold gear sets engaged. Hydraulic pressure is also used to apply or release the internal clutches for the gear sets and allows for the shifts between gears.

The solenoids are electrically applied to block pressure lines and un-applied to open the line for pressure. Depending on which solenoids are enegized (blocking a passage) and de-energized (allowing pressure) there are four possible combinations of ways for SSA and SSB to allow fluid to flow (On-On; On-Off; Off-Off; and Off-On). Those combinations give you the separate gears in your automatic transmission based on the routing of fluid through the transmission's internal passage ways to the holding devices or gear sets.

When a solenoid stops working you typically hear that the transmission isn't shifting into a gear.

Hope that answers your question

Answer 2

Looks like you don't know much about ATs.