Detroit Diesel 4000 Series, Руководство по обслуживанию, Страница: 1754 / 2706

SERIES 4000 12/16V SERVICE MANUAL
Because there is no pressure in the pressure cavity, no clamping force is applied to the clutch
plates, and the drive to the fan is disconnected. At this time, the fan rotates at idle speed due to
viscous oil drag of the cooling oil passing between the facing plates and steel clutch plates. Idle
speed is normally 300-350 rpm or lower when the system is at operating temperature, unless the
ECM is designed to control it at a predetermined higher speed; refer to section Optional Fan
Speed Limiting Systems.
Modulated Variable Speed Mode
As the engine coolant temperature increases within the designed operating range of the controls,
the thermal sensor or ECM system responds, causing the solenoid valve to gradually increase
the amount of oil pressure being directed into the fan clutch pressure cavity. Reacting within the
pressure cavity, the increased pressure forces the piston to move into contact with the clutch plates
and begins to create a connection between the input and output of the fan clutch. Unless the
temperature demands require that the fan be operated at full designed speed, the fan clutch, in
fact, slips. Increased pressure against the facing creates increased clamping force that decreases
amount of clutch slippage, resulting in increased fan speed. The fan will continue to increase in
speed until it reaches maximum operational speed, unless the engine temperature stabilizes or
begins to decrease as the result of increased cooling created by the fan.
If engine coolant temperature begins to decrease, the above action is reversed. Oil then is
slowly allowed to exhaust from the clutch, through the ("NC" or "SUMP") port of the solenoid
valve to the engine oil reservoir (sump). The lower pressure inside the pressure cavity reduces
the clamping force of the clutch piston. The spring washer pushes the piston away from the
facing plates and the clutch is allowed to slip more, reducing fan speed. Should engine coolant
temperature stabilize, fan speed stabilizes, but this will rarely occur other than in tightly controlled
test cell environments.
When the lower limit of the designed temperature range of the thermal sensor is reached,
minimum oil control pressure exists in the pressure cavity. The spring washer forces the piston
away from the clutch plates and the fan clutch is allowed to fully release.
Fully Engaged Mode
When engine coolant temperature at the sensor reaches the top of the designed temperature range,
full oil pressure is directed by the solenoid valve into the pressure cavity of the fan clutch. The
control pressure, reacting within the pressure cavity, forces the piston to move against the clutch
plates, clamping the disc stack against the front bearing retainer. When the clutch plates are fully
clamped, the input and output are fully connected constituting a 1:1 drive through the clutch,
unless the design of the system limits the fan’s maximum speed. The fan is thus driven at fan
clutch pulley speed; refer to section Optional Fan Speed Limiting Systems.
Optional Fan Speed Limiting Systems
Some engine or vehicle manufacturers may have designed the fan clutch control system to limit
the fan to a maximum speed below that of full fan clutch lock-up speed. In these instances, the fan
clutch is designed to never go to full lock-up under any operational conditions, and the minimum
fan speed at idle and/or full release may be prevented from decreasing below a prescribed idle
speed. In these cases, the fan clutch modulates 100% of the time in operation.
All information subject to change without notice.
C -1608
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