HEAD 4658701 - Caterpillar

Identifying the Heating System
The heating system can be identified as either a water valve control system or a blend door control system. The control system for the water valve regulates the amount of hot engine coolant that is circulated through the heater coil. The control system for the blend door regulates the amount of air that is routed through the heater coil. Both the water valve and the blend door can be regulated through either a mechanical cable or through an electronic actuator.Heating System with Water Valve Control
Illustration 1 g06039335
(1) Supply from engine thermostat
(2) Return to water pump
(3) Heater return to water pump
(4) ESPAR return to water pumpThe hot coolant from the engine circulates through the heating system. This coolant flows from the engine thermostat (1), through heater control valve, through heater coil, and back to the return valve for the water pump. The temperature control knob, which is on the control panel within the cab, regulates the flow of coolant.With the temperature control knob in the maximum cold position, heater control valve is closed. This prevents the flow of coolant into heater coil. When the control knob is moved away from the maximum cold position toward the maximum heating position, the heater control valve begins to open. As the control valve opens, coolant begins to flow from the outlet valve for engine thermostat through the heater control valve and into the heater coil. As the hot coolant flows through the heater coil, the temperature of the air that is flowing through the coil increases.The temperature of the air that is flowing out of heater coil and into the cab is controlled by the amount of coolant that circulates through the heater coil. The coolant transfers the heat into the air stream. By increasing the amount of coolant that is circulating through the heater coil, the temperature of the air that is flowing out of the coil also increases. The coolant leaves the heater coil at a reduced temperature and flows back into the return valve for the water pump.Heating System with Blend Door Control
The hot coolant from the engine circulates through the heating system. This coolant flows from the outlet valve for the engine thermostat, through heater control valve, through heater coil, and back to the return valve for the water pump. The temperature control knob, which is on the control panel within the cab, regulates the flow of coolant.The air from outside of the unit is pulled through evaporator coil. This is through a series of ducts and filters. The incoming air is directed by using a blend door. The air can be directed to flow through heater coil, or the air can be directed to bypass the heater coil. With the temperature control knob in the maximum cold