Prior art solenoid valves are often used to control the flow of hydraulic fluid in automotive vehicle transmission systems. A traditional variable bleed solenoid valve regulates pressure by reducing pressure in a control volume through a variable leak path (exhaust) to a pressure sump. The amount of leakage is inversely proportional to the pressure in the control portion of the valve. This leakage is controlled by a variable orifice, which typically takes the form of a sealing member (flat, conical, ball, or otherwise) that is moved in relation to a fixed annular sealing surface that surrounds a fixed orifice. When the sealing member is in contact with the sealing surface, the leakage approaches zero, and the pressure in the control volume is at its peak value. When the sealing member is moved away from the sealing surface, the leakage increases, and the pressure in the control volume decreases at a rate that is a function of the swept area of the shortest distance between sealing member and seat. The leakage can be significant when multiplied by the number of solenoid devices located within the transmission. Minimizing the leakage of each of the individual solenoid components is desirable in order to minimize the capacity requirement of the hydraulic pump that generates the system pressure and to improve the efficiency of the transmission. A low leak variable bleed solenoid by design minimizes this leakage. Minimal leakage to sump is achieved by integrating a supply shut off (secondary) valve in series with the exhaust bleed (primary) valve. These two valves are actuated simultaneously. When the exhaust bleed (primary) valve is fully open to achieve minimum control volume pressure, the supply shut off (secondary) valve is fully closed to prevent fluid leakage to sump through the control volume. As the exhaust bleed (primary) valve is closed to increase pressure in the control volume, the supply shut off (secondary) valve is opened to allow fluid to enter into the control volume. At maximum pressure in the control volume, the exhaust bleed (primary) valve is fully closed and the supply shut off (secondary) valve is fully open.
A disadvantage of known low leak variable bleed solenoid valves is that the addition of a supply shut off valve in close proximity to the exhaust bleed valve complicates the construction of the solenoid valve. Additional components that re-direct flow often have complex geometries that are difficult and expensive to manufacture. Another disadvantage is that many of the known low leak variable bleed solenoid valves are more sensitive to fluctuations in supply pressure and temperature as they have hydraulic areas acted upon by supply pressure that are not balanced. The invention described herein differs from the prior art by utilizing simpler valve and flow geometry, which also has the advantage of minimizing supply pressure feedback area.