Most ATVs and other vehicles that use hydraulic steering today use what is called hydro assist. This means that the hydraulics only assist in the steering of the vehicle. They provide a partial amount of force to turn the wheels. The hydraulic fluid is pumped from a chamber into the steering rods, and the force of the fluid filling this chamber turns the steering wheels. The set up involves an additional kit that is installed on the vehicle, while keeping all of the other components of your steering assembly intact. If this unit fails, you can still control the ATV or machine by the traditional factory steering unit. Full hydro means the vehicle is completely powered by hydraulics and no other means to steer the vehicle exist.
The hydraulic steering units make use of several parts. The first is the reservoir where the fluid is stored. It holds the oil that is pumped into the cylinder arm, which is connected to the steering unit. This pushes the wheels left or right. The pump mechanism is what forces the fluid into the cylinder arm. It pumps fluid from the reservoir through a hose line and into the arm. The metering valve is the unit that tells the pump how much fluid to pump and where to pump it to. It is the brains of the unit, and it turns the steering movement into actual fluid disbursement which turns the steering frame.
The hydraulic steering process starts with a turn of the steering wheel. This movement sends a signal to the metering valve, or control unit, which takes this input and turns it into a value. That value activates the pump for a period of time, based on how far the wheel is turned. For a short quarter turn, the value is short, and the pump is only on for a second. For a complete turn, it is on for much longer. The pump sends the fluid into one of the cylinder arms, depending on which way the wheel was turned, which extends the arm and turns the wheels.
