BENDIX FD-L User Manual

®

Bendix® FD-L™ Fan Clutch

SD-09-8505

FAN HUB

BRACKET

PULLEY

BEARINGS

SPACER

PISTON COMPLETE

HEX NUT

LOCKWASHER

DUST CAP

ISOLATOR

PRESSURE PLATE

SPRING

PISTON HOUSING

BEARINGS

WASHER

HEX NUT

FIGURE 1

COTTER PIN

CAP

SPACER

O-RING

O-RING

O-RING

1

The FD-L™ fan clutch is designed for mid-range diesel engine
applications and can withstand maximum fan torque of 750
inch pounds and a fan weight not to exceed 15 pounds. It
cannot be used to replace heavy duty clutches or hubs that
do not meet the above stated specifications.

DESCRIPTION

The Bendix® FD-L™ fan clutch is an air operated
thermostatically controlled clutch for the engine cooling fan.
Its purpose is to maintain engine temperature by engaging
or disengaging the cooling fan, thereby , providing better fuel
economy , greater engine efficiency , faster warm-ups and a
quieter vehicle.

The FD-L™ fan clutch is produced in several different models
to accommodate the variety of installation requirements
resulting from the many engine, vehicle and accessory
combinations. Its fail-safe design prevents overheating in
the event of air loss for any cause.

OPERATION

GENERAL

The FD-L™ fan clutch replaces the standard fan hubs on the
engine of mid-range diesel vehicles. It is controlled by a
temperature sensitive air valve. (See Figure 4). The same
type of valves used to control radiator shutters are used to
control the FD-L™ fan clutch. Installed in the engine block,
the thermo-pneumatic control valve directly senses coolant
temperature. Provided coolant temperature remains below
the setting of the valve, air passes through it to disengage
the fan clutch. When coolant temperature rises to the valve
setting, the valve closes and exhausts air pressure from the
fan clutch which engages the fan.

FAN ENGAGED

PISTON MOVES

FORWARD AS AIR IS

EXHAUSTED

FIGURE 3

DISENGAGED

When the vehicle is started with a cold engine the thermo­pneumatic control valve is open. As brake system air
pressure is built up, air passes through the control valve to
the fan clutch. Air enters the inlet port in the bracket of the
fan clutch and travels through the drilled passage in the
shaft to fill the piston cavity. When air in the piston cavity
reaches a pressure of 75-80 psi, the piston slides on the
shaft moving the pressure plate assembly to the disengaged
position. In this position the clutch lining is out of contact
with the fan plate and the fan is no longer driven by the
engine (see Figure 2).

ENGAGED

Depending upon optional equipment installed on the vehicle
such as radiator shutters and radiator mounted air
conditioning condenser, dif ferent piping arrangements are
necessary. These will be explained in the paragraph on
control systems.

FAN DISENGAGED

FIGURE 2

2

As the engine coolant rises in temperature, the engine
thermostat opens to circulate radiator coolant through the
engine. When the engine coolant temperature reaches the
setting of the control valve, the valve exhausts air from the
piston cavity. The fan clutch springs force the pressure
plate forward until the lining contacts the fan plate. The fan
is engaged and is driven by the engine (see Figure 3).

CONTROL SYSTEMS

The fan clutch, in addition to its primary function of operating
the fan as needed to maintain the cooling liquid within a
certain temperature range, must be coordinated with other
devices when installed on the vehicle; these are radiator
shutters and radiator mounted condenser for air
conditioning. Four basic configurations are possible as
follows:

FIGURE 4. F AN CLUTCH ONL Y
FIGURE 5. F AN CLUTCH WITH SHUTTERS
FIGURE 6. F AN CLUTCH WITH RADIATOR MOUNTED

CONDITIONING CONDENSER

FIGURE 7. FAN CLUTCH, SHUTTERS AND CONDENSER

BLOCK

THERMO-PNEUMATIC

VALVE

FAN

CLUTCH

AIR

CONDITIONING

SIGNAL

THERMO-PNEUMATIC

SOLENOID VALVES
(NORMALLY OPEN)

VALVE

BLOCK

FAN

OVERRIDE

SWITCH

AIR

CONDITIONER

OVERRIDE

SWITCH

RECEIVER

DRYER

FIGURE 4

FIGURE 5

AIR

CONDITIONING

SIGNAL

THERMO-PNEUMATIC

BLOCK

THERMO-PNEUMATIC

VALVES

BLOCK

FAN

CLUTCH

VALVE

SOLENOID VALVE

AIR COND.

COMPRESSOR

FAN

CLUTCH

RADIATOR

FAN

OVERRIDE

SWITCH

RADIATOR

RADIATOR

SHUTTER

CONDITIONING

OVERRIDE

CONDENSER

AIR

SWITCH

RECEIVER

DRYER

AIR COND. COMPRESSOR

FIGURE 7

FAN

CLUTCH

RADIATOR

CONDENSER

SHUTTER

The objective in the case of Figure 5 is to not engage the fan
until the shutters are fully open. The thermo-pneumatic control
valve should therefore be calibrated to release the air from
the fan clutch when the coolant temperature is at least 10°F
higher than the full-open temperature of the shutter.

In the case of Figure 6, the fan must be controlled to perform
the supplementary function of pulling cooling air through
the air conditioning condenser when required, although the
engine coolant may be below the fan cut-in temperature.
This is most likely to occur at high ambient temperatures
and low road speed or idling. If the condenser is
inadequately cooled, the air conditioning system will start
to build up a higher than normal pressure. Therefore, a
pressure switch is connected into the air conditioning
compressor discharge line, as shown in Figure 6. When
this pressure switch senses a higher than normal pressure
in the air conditioning system (approximately 250 psi), it
closes an electrical circuit which in turn energizes a solenoid
operated air supply valve which exhausts the air supply for
the fan control, causing the fan to engage.

Most vehicles also have an additional dual function pressure
switch connected in series with the air conditioner clutch.
This switch is normally open, closes at approximately 30
psi and opens again at approximately 400 psi. Its purpose
is to prevent operation of the air conditioning compressor if
the refrigerant is lost or to shut off the compressor if the
system pressure reaches a dangerously high pressure
(approximately 400 psi). This switch does not normally have
anything to do with fan operation; however, it is sometimes
combined with the previously mentioned override switch.

FIGURE 6

3