DAF LF45, LF55 Series. Manual - part 177

©

 200505

3-1

Description of components

BE ENGINE COOLING SYSTEM

ΛΦ45/55 series

2

3

3. DESCRIPTION OF COMPONENTS

3.1 PRESSURE CAP

The pressure cap is fitted on the header tank by 

means of a threaded connection.

To fill the cooling system, there is a filler cap at 

the front of the header tank.

The pressure cap has two valves: a pressure 

relief valve and a vacuum relief valve.

Normally, both valves are closed.

Overpressure
As a result of the rising coolant temperature, the 

pressure (P1) in the cooling system will increase. 

If the pressure in the cooling system becomes too 

high (0.7 bar), the pressure relief valve (1) will 

open against the pressure of the spring.

Negative pressure
If the coolant temperature drops, the pressure 

(P1) in the cooling system will decrease. If the 

pressure (P1) in the cooling system drops to 

approximately 0.1 bar below the ambient air 

pressure (P2), the underpressure valve will be 

opened.

M201151

P2

P1

BE ENGINE COOLING SYSTEM

3-2

©

 200505

Description of components

3

ΛΦ45/55 series

2

3.2 THERMOSTAT

Operation, thermostat
Coming from the cylinder head, the coolant 

passes through the thermostat. Depending on the 

coolant temperature and the corresponding 

position of the thermostat, there are three 

possibilities:

Thermostat closed
The coolant has not yet reached the opening 

temperature of the thermostat (1).

The supply channel (B) to the radiator is 

completely closed.

The coolant flows directly to the water pump 

through a bypass (A) and the water pump returns 

the coolant to the cylinder block.

Thermostat starts opening
The coolant has reached the opening 

temperature of the thermostat (1).

The supply channel (B) to the radiator is opened 

and the bypass (A) is partially closed.

Now coolant will flow both through the supply 

channel (B) to the radiator and directly to the 

water pump through the bypass channel (A).

1

A

B

M201126

M201127

1

A

B

©

 200505

3-3

Description of components

BE ENGINE COOLING SYSTEM

ΛΦ45/55 series

2

3

Thermostat fully opened
The temperature of the coolant has further 

increased. The supply channel (B) to the radiator 

is fully opened and the bypass (A) is fully closed.

The entire coolant circulation now flows via the 

supply channel (B) to the radiator where it is 

cooled before flowing back to the water pump.

In the event of excessive coolant temperatures, 

removing the thermostat as an emergency 

solution is not permitted.

If the thermostat is removed from the engine, 

uncooled coolant will flow to the water pump 

through the bypass (A). As a result, the coolant 

temperature will continue to rise.

M201128

1

A

B

BE ENGINE COOLING SYSTEM

3-4

©

 200505

Description of components

3

ΛΦ45/55 series

2

3.3 VISCOUS FAN CLUTCH

The fan is connected to the engine by means of a 

viscous fan clutch. If, under certain 

circumstances, the heat is not sufficiently 

dissipated by the air flow passing through the 

radiator, the fan will have to draw in extra cooling 

air through the radiator. In a viscous fan clutch, 

the drive torque is transmitted by a silicone fluid.

The fan clutch is divided into two chambers. In the 

working area (1) is the rotor (2), which is 

connected to the drive flange (7). The supply 

chamber (3) rotates freely round the drive flange 

(7) and is connected to the fan. There is silicone 

fluid in the supply chamber (3). The opening (4) in 

the supply chamber (3) is closed by a valve (5). 

The valve (5) is operated by a bimetallic strip (6).

If the opening (4) in the supply chamber (3) is 

closed by the valve (5), no silicone fluid can enter 

the working area (1). The silicone fluid still 

present in the working area (1) will flow back to 

the supply chamber (3) through the bores (8). As 

only very little fluid will be left in the working area 

(1), there will be a great difference in rotating 

speed (slip) between the drive flange (7) and the 

supply chamber (3) with the fan.

When the air temperature increases, the 

bimetallic strip (6) will bend and the valve (5) will 

partially release the opening (4) in the supply 

chamber (3). Through this opening, a limited 

amount of silicone fluid can enter the working 

area (1) and flow past the rotor (2). This will cause 

friction, so that the difference in rotating speed 

(slip) between the drive flange (7) and the supply 

chamber (3) with the fan will decrease.

M201039

6

8

7

5

2

3

4
1