Dodge ram truck 3500 1996 Service Manual page 188

5 4 BRAKES B R
DESCRIPTION AND OPERATION
(Continued)
vacuum. The rear portion (area behind the dia­
phragms), is exposed to normal atmospheric pressure
of 101.3 kilopascals (14.7 psi).
Pressing the brake pedal causes the rear push rod
to open the inlet valve. This exposes the area behind
the diaphragms to atmospheric pressure. The result­
ing force applied to the diaphragms is what provides
the extra' boost in apply pressure for power assist.
POWER BRAKE VACUUM PUMP (DIESEL MODELS)
The power brake vacuum pump and the power
steering pump are combined into a single assembly
on diesel engine models (Fig. 3). Both pumps are
operated by a drive gear attached to the vacuum
pump shaft. The shaft gear is driven by the camshaft
gear.
The vacuum pump is a constant displacement,
vane-type pump. Vacuum is generated by four vanes
mounted in the pump rotor. The,rotor is located In
the pump housing and is pressed onto the pump
shaft.
The vacuum and steering pumps are * operated by a
single drive gear pressed onto the vacuum pump
shaft. The drive gear is operated by the engine cam­
shaft gear.
The vacuum and power steering pump shafts are
connected by a coupling. Each pump shaft has an
adapter with drive lugs that engage in the coupling.
. The vacuum pump rotating components are lubri­
cated by engine oil. Lubricating oil is supplied to the
pump through an oil line at the underside of the
pump housing.
The complete assembly must be removed in order
to service either pump. However, the power steering
pump can be removed and serviced separately when
necessary.
The vacuum pump is not a serviceable component.
I f diagnosis indicates a pump malfunction, the pump
must be replaced as an assembly. Do not disassemble
or attempt to repair the pump.
The combined vacuum and steering pump assem­
bly must be removed for access to either pump. How­
ever, the vacuum pump can be removed without
having to disassemble the power steering pump.
I f the power steering pump requires service, simply
remove the assembly and separate the two pumps.
Refer to the pump removal and installation proce­
dures i n this section.
LOW VACUUM WARNING switch
A vacuum switch is used to monitor output of the
vacuum pump. The switch is in circuit with the red
brake warning light.
A vacuum hose connects the switch to the power
brake booster. A wire harness connects the switch to
the brake warning light.
VACUUM
GEAR ADAPTER
J9105-94
Fig. 3 Diesel Vacuum & Power Steering Pump
Assembly
The switch is mounted on the driver .side Inner
fender panel just below the hood hinge (Fig. 4). The
switch is located just under the front antilock valve
on ABS models.
Fig. 4 Diesel Vacuum Switch Location (Without
ABS)
VACUUM PUMP OPERATION
Vacuum pump output is transmitted to the power
brake booster through a supply hose. The hose is con­
nected to an outlet port on the pump housing and to
the check valve i n the power brake booster.
Pump output ranges from a minimum of 8.5 to 25
inches vacuum.
The pump rotor and vanes are rotated by the pump
drive gear. The drive gear is operated by the cam­
shaft gear.