Patent ID: 7527338

Claim:
A hydrodynamic retarder control apparatus comprising: a source of pressure; a retarder having an inlet and a outlet; a flow valve for controlling fluid flow to said inlet: a regulator valve responsive to an operator demand to control the pressure within the retarder; a multifunction valve connected with said source and being operable in response to the operator demand to supply control fluid to said flow valve and, in response to an abnormal operation by said regulator valve, to control the pressure within the retarder; wherein said regulator valve and said multifunction valve each having a control spring having equal force and a control area wherein said control area of said regulator valve being smaller that the control area of said multifunction valve and said control area and control spring of said regulator valve control the pressure output thereof at a first predetermined range and said control area and control spring of said multifunction valve control the pressure output thereof at a second and higher predetermined range.