Cooling system. General information (VW Passat B4)

The engine uses a liquid cooling system with forced circulation, using all-season cooling liquid. The cooling system includes a radiator, expansion tank, water pump, thermostat and electric fan. On cars with air conditioning or an "ABF" engine, an additional fan is installed, the drive of which is carried out by a V-belt from the electric fan. The radiator with aluminum tubes, with plastic side tanks. The expansion tank is made of transparent plastic. The tank has a plug with a pressure-reducing valve and a coolant level sensor. The opening pressure of the pressure-reducing valve of the tank plug is 1.2-1.5 kgf/cm².

The temperature at which the main thermostat valve starts to open is 85°C.

The full opening temperature of the main valve is 105°C.

The stroke of the main valve piston is not less than 7 mm.

The two-speed electric fan from Bosch or AEG is switched on and off by a sensor installed on the left radiator tank.

The electric fan switch ensures the fan operates at low and high speeds. The sensor contact closing temperature for low fan speed is 92-97°C, high speed is 99-105°C, the sensor contact opening temperature is 84-91°C and 91-98°C, respectively.

On cars produced in 1995-96 with air conditioning, a three-speed electric fan is used.

Depending on the configuration, two electric fan activation sensors are installed, one of which is located on the radiator tank and turns on the electric fan at the first two speeds, and the other is installed on the coolant outlet hose from the cylinder head on the left.



Fan motor switch-on temperature:

• main sensor: at 1st speed 92-97°C; at 2nd speed 99-105°C;
• additional sensor 110-115°C.

Fan motor switch-off temperature:

• main sensor: at 1st speed 84-91°C; at 2nd speed 88-91°C;
• additional sensor 105-110°C.

To replace the coolant, fully open the cabin heating valve. Remove the expansion tank cap. Disconnect the radiator supply hose from the thermostat housing (fig. 2-152) and drain the coolant.



Attach the radiator supply hose to the thermostat housing and tighten the hose clamp. Fill the expansion tank with coolant to the level of the mark on the tank. Close the expansion tank cap, start the engine and let it run to remove air locks from the cooling system. Check the fluid level in the expansion tank and top it up to normal if necessary.

To check the tightness of the cooling system, remove the expansion tank cap and install the VW 1274 testing device in its place. Using the hand pump of the device, create excess pressure of 1.0 kgf/cm² in the cooling system and monitor the pressure. If the system is tight, the created pressure should not decrease. To check the opening pressure of the safety valve in the expansion tank cap, install the expansion tank cap on the testing device.


By creating excess pressure using the hand pump of the device, note the moment of opening of the safety valve, which should occur at a pressure of 0.9-1.15 kgf/cm².

[The article is published on this resource: VWmanual]