Self-Study Program
17
Note
Contaminants can clog the thermostat bypass,
thereby impairing the function or disabling the
thermostat.
This can cause the gearbox to overheat!
At an ambient temperature of 25 °C and during
normal vehicle operation, the ATF temperature
barely exceeds 110 °C.
Note
If the cooling system has been opened to carry
out repairs (thereby draining the ATF cooler),
the ATF temperature must be raised to at least
90 °C by carrying out a test drive in order to set
the ATF level correctly.
This ensures that the ATF cooler is filled. The
ATF level must be set after the ATF has cooled
down to normal test temperature (refer to
Workshop Manual).
367_013
367_014
Thermostat closed
The expanding wax element also acts as the
thermostat slide valve and regulates the ATF feed
to the cooler. In the closed state, a small fraction of
the ATF flows through the bypass, thereby heating
the expanding wax element.
At a temperature of approx. 75 °C, the plunger
of the expanding wax element begins to press
downwards against the force of the spring, thereby
opening the inlet to the cooler (see next figure).
Thermostat open
At a temperature of approx. 90 °C, the thermostat is
fully open.
G
G
K
K
Expanding wax elementBypass
G = from or to gearbox
K = from or to cooler
Seal
G
G
K
K
Plunger