LR3/Disco 3
The speed of the cooling fan is also influenced by vehicle road speed. The ECM adjusts the speed of the cooling fans, to
compensate for the ram effect of vehicle speed, using the Controller Area Network (CAN) road speed signal received from
the Anti-lock Braking System (ABS) module.
Pressure Relief Thermostat (PRT)
The thermostat is exposed to 85% hot coolant from the engine on one side and 15% cold coolant returning from the
radiator bottom hose on the other side. This allows the thermostat to react to the ambient conditions and provide coolant
control for both winter and summer use. Hot coolant from the engine passes via holes in the by-pass flow valve into a
tube which surrounds 85% of the thermostat sensitive area. Cold coolant from the radiator conducts through the
remaining 15% of the sensitive area. In cold ambient conditions, the engine temperature is raised by approximately 10°C
(50°F) to compensate for the heat loss of 15% exposure to the cold coolant returning from the bottom hose. This
improves heater performance and engine warm-up.
The by-pass flow valve is held closed by a light spring and operates to further assists engine and heater warm-up. When
the main valve is closed and the engine speed is at idle, the coolant pump does not produce sufficient flow and pressure
to overcome the spring and open the valve. In this condition the valve prevents coolant circulating through the by-pass
circuit and directs coolant through the heater matrix only. This provides a higher flow of coolant through the heater matrix
improving passenger comfort in cold conditions.
When the engine speed increases above idle, the coolant pump produces a greater flow and pressure than the heater
circuit can accommodate. The build up of pressure acts on the flow valve, overcoming the spring pressure, opening the
valve and relieving the pressure in the heater circuit. The valve then modulates to provide maximum coolant flow through
the heater matrix and allowing excess coolant to flow into the by-pass circuit to provide the engine's cooling requirements
at higher engine speeds. The thermostat then regulates the flow through the radiator to maintain the engine at the
optimum temperature. Maximum opening of the thermostat, and therefore maximum flow through the radiator, occurs if
the coolant temperature reaches 95°C (203°F).
