The control of the coolant flow is accomplished through valving or by adjusting the pumping speed of a water pump and a water motor, or a combination of all three elements. During normal operation, where engine cooling is not required, the speed control coupling maintains a slow and constant water pump speed at all engine-operating speeds. The valve is maintained to stop coolant flow from entering the radiator while allowing coolant to flow through a heater. If engine cooling is required, the valve is actuated such that coolant is circulated to the engine and through the radiator. If air conditioning is required, the speed control coupling simply increases the water pump speed and the fan speed while the valve is set to bypass coolant flow to the engine. If air conditioning and engine cooling are required, the valve is actuated to allow coolant flow to the engine.