If installing a 3-post solenoid, there are two different kinds depending on how the coil is configured inside. All coils need +12V and ground to work, on a 3-post solenoid there is a missing terminal because one end of the coil is connected internally to either ground or to the "Batt" side of the solenoid. Which type you have determines whether the solenoid requires +12V or ground to turn on. Wood-Mizer solenoids (3-post hydraulic ones) require ground (one end of the coil is attached internally to Batt or +12V). These are typically referred to as an isolated or insulated ground type solenoid. One way around this (if your not sure what type 3-post is available) is to buy a 4-post and run a jumper wire from 1 of the small posts to the "Batt" side of the solenoid, then hook your wire from the micro-switch to the remaining small post.