The early models done hydraulically and solenoid power went thru a stoplight and vacumn switch to energize it.Transgo offers a kit that makes it hydraulic only but in some cases it dosnt work well.Adjusting governor weights and springs will improve it.I source the wsitch and related wiring with the vacumn delay valve from mid 80s suburbans or you can source an adjustable vacumm switch to dial it in but I would still use a delay valve.As stated earlier 90 models do not have a provision in the valve body for the hydraulic spools,the casting is blocked