1674 DIESEL TRUCK ENGINE Cooling System Caterpillar

Cooling System
1.1. Water Pump
2.1. Fan
3.1. Water Temperature Regulator

This engine has a pressurized cooling system. Coolant is circulated by a belt driven, centrifugal type water pump. A water temperature regulator, located at the left front of the cylinder head, restricts coolant flow through the radiator until the coolant reaches operating temperature.

The water pump has two outlets. One outlet directs coolant through a passage in the water temperature regulator housing to the aftercooler, to lower the temperature of inlet air in the inlet manifold. The other water outlet directs coolant through a water manifold in the side of the cylinder block, to cool engine lubricating oil. Coolant flows from the water manifold around the cylinder liners, into the cylinder head and around the precombustion chambers. Both streams of water join at the water temperature regulator.

The water cooled air compressor receives coolant through a tube that is connected to the water manifold in the side of the cylinder block. Coolant returns through a tube from the air compressor head to the diesel engine cylinder head.

Until the coolant reaches the temperature required to open the temperature regulator, coolant bypasses the radiator and flows directly back to the water pump.

When the coolant reaches the temperature required to open the temperature regulator, coolant is then directed through the radiator.

A pressure relief cap assembly is used to control the pressure in the cooling system, and prevents loss of coolant through the radiator overflow tube.

Pressurizing the cooling system serves two purposes. First, it permits safe operation at coolant temperatures higher than the normal boiling point, providing a margin of cooling for intermittent peak loads. Secondly, it prevents cavitation in the water pump, and reduces the possibility of air or steam pockets forming in the coolant passages. Proper operation of the pressure relief cap assembly is essential. A pressure relief cap allows pressure (and some water, if the cooling system is too full) to escape when the pressure in the cooling system exceeds the capacity of the pressure cap. Loss of pressure will cause steam to form when coolant temperature is above the normal boiling point.

Water Pump

The centrifugal-type water pump has two seals, one prevents leakage of water and the other prevents leakage of lubricant.

An opening in the bottom of the pump housing, allows any leakage at the water seal or the rear bearing oil seal to escape.


The fan is driven by three V-belts, from a pulley on the crankshaft. Belt tension is adjusted by moving the bracket assembly which includes the fan mounting and pulley.

Water Temperature Regulator

The water temperature regulator restricts the flow of coolant through the radiator, until the coolant reaches operating temperature; approximately 165°F (74°C.) The regulator is fully open at approximately 180°F (82°C.)

The shunt cooling system is used in many truck installations. The shunt cooling system provides continuous circulation which helps prevent aeration and pump cavitation by maintaining a positive head of water at the pump inlet at all times. It differs from the conventional cooling system in that the radiator top tank is divided into two compartments (upper and lower) with a small air/coolant bleed tube connecting them. A shunt line located as low as possible in the upper chamber directs coolant to the pump inlet.

When the coolant reaches the temperature required to open the thermostat, coolant is directed into the bottom chamber of the radiator top tank, down through the radiator core to the suction line of the pump and then to the engine. Coolant, which may contain some air will flow from the bottom chamber to the top chamber through the small air/coolant bleed tube. The air will separate from the water in the upper chamber. The water will flow out the shunt line and the air will remain in the upper chamber until excessive pressure is built up, at which time it will vent out through the pressurized relief cap. Air which may accumulate in the engine is expelled from the highest point in the engine cooling system to the radiator upper chamber through an external vent line.

Thermal controlled shutter stats or radiator shutters are an effective way to control the air flow through the radiator for maintaining proper operating temperatures. The shutters are thermostatically controlled to open 5 to 10 degrees above the thermostat when in full open position.