Chrysler RAM TRUCK 2500 1994 Service Manual page 301

7 - 46 COOLING SYSTEM •
The viscous fan drive clutch is a silicone-fluid-filled
coupling connecting fan blade assembly to water
pump shaft. The coupling allows fan to be driven i n a
normal manner at low engine- speeds. I t will limit top
speed of fan to a predetermined maximum level at
higher engine speeds.
M O U N T I N G N U T
T O WATER PUMP
Fig. 71 Viscous Fan Drive
The thermostatic spring coil (Fig. 71) reacts to the
temperature of radiator discharge air. I t engages
drive clutch for higher fan speed i f air temperature
from radiator rises above a certain point. Until addi­
tional engine cooling is necessary fan will remain at
a reduced rpm regardless of engine speed. Only when
sufficient heat is present i n the air, will the viscous
fan drive clutch engage. This air flowing through ra­
diator core will cause a reaction to thermostatic
spring coil. Fan speed will then increase to provide
necessary additional engine cooling.
CAUTION: If v i s c o u s fan drive is replaced b e c a u s e
of mechanical damage, cooling fan blades should
also be inspected. Inspect for fatigue c r a c k s , loose
blades or loose rivets that could have resulted from
e x c e s s i v e vibration. Replace fan blade assembly if
any of these conditions are found. Also inspect wa­
ter pump bearing and shaft assembly for any re­
lated damage due to a v i s c o u s drive malfunction.
Regardless of increased engine speed, once the fan
has reached its maximum operating speed, i t will not
rotate any faster. When the necessary engine cooling
has been accomplished causing a reduction i n tem­
perature of radiator discharge air, the spring coil
again reacts. This will cause fan speed to be reduced
to its previous disengaged speed.
T E S T I N G VISCOUS FAN DRIVE
In case of engine overheating, inspect thermal viscous
fan drive for proper operation. Observe movement of
thermostatic spring coil and shaft. Lift end of thermo­
static spring up and out of retaining slot in fan drive
(Fig. 72). Rotate spring counterclockwise until a stop is
felt. The gap between end of coil and retaining clip
should be approximately 12 mm (0,5 inch) (Fig. 73). Re­
place unit i f shaft does not rotate with coil. After test­
ing, install end of coil in slot of retaining clip.
Fig. 72 Disconnecting End of Thermostatic
Spring—Typical
Fig. 73
TeBimc Sfrmc
S l i d Shaft Rotation—Typical
REHMOVAUIHSTALLATION
For removal and installation procedures of the
thermal viscous fan drive, refer to Cooling System
Fans in this group.