Dodge ram truck 3500 1996 Service Manual page 195

B R
BRAKES 5 - 11
DIAGNOSIS AND TESTING (Continued)
electrical side of the switch is connected to the red
brake warning lamp.
The low vacuum switch monitors booster vacuum
level whenever the engine Is running. I f booster vac­
uum falls below 8.5 Inches for a minimum of 10 sec­
onds, the switch completes the circuit to the warning
lamp. The warning lamp is designed to differentiate
between a low vacuum condition and a hydraulic cir­
cuit fault.
MASTER CYLINDER/POWER BOOSTER
(1) Start engine and check booster vacuum hose
connections. A hissing noise indicates vacuum leak.
Correct any vacuum leak before proceeding.
(2) Stop engine and shift transmission into Neu­
tral.
(3) Pump brake pedal until all vacuum reserve in
booster is depleted.
(4) Press and hold brake pedal under light foot
pressure. The pedal should hold firm, i f the pedal
falls away master cylinder is faulty (Internal leak­
age).
(5) Start engine and note pedal action i t .should
falls away slightly under light foot pressure then
holds firm. I f no pedal action is discernible, power
booster, vacuum supply, or vacuum check valve is
faulty. Proceed to the POWER BOOSTER VACUUM
TEST.
(6) I f the POWER BOOSTER VACUUM TEST
passes, rebuild booster vacuum reserve as follows:
Release brake pedal. Increase engine speed to 1500
rpm, close the throttle and Immediately stop turn off
ignition to stop engine.
(7) Wait a minimum of 90 seconds and try brake
action again. Booster should provide two or more vac­
uum assisted pedal applications. I f vacuum assist Is
not provided, booster is faulty.
POWER BOOSTER VACUUM TEST
(1) Connect vacuum gauge to booster check valve
with short length of hose and T-fitting (Fig. 10).
(2) Start and run engine at curb idle speed for one
minute.
(3) Observe the vacuum supply. I f vacuum supply
is not adequate, repair vacuum supply.
(4) Clamp hose shut between vacuum source and
check valve.
(5) Stop engine and observe vacuum gauge.
(6) I f vacuum drops more than one inch HG (33
millibars) within 15 seconds, booster diaphragm or
check valve Is faulty.
POWER BOOSTER CHECK VALVE TEST
(1) Disconnect vacuum hose from check valve.
(2) Remove check valve and valve seal from
booster.
(3) Use a hand operated vacuum pump for test.
SHORT
CONNECTING CHECK
HOSE VALVE
VACUUM
GAUGE
J9005-81
Fig. 10 Typical Booster Vacuum Test Connections
(4) Apply 15-20 inches vacuum at large end of
check valve (Fig. 11).
(5) Vacuum should hold steady. I f gauge on pump
Indicates vacuum loss, check valve is faulty and
should be replaced.
BOOSTER
CHECK VALVE APPLY T E S T
80316866
Fig. 11 Vacuum Check Valve And Seal
DIESEL ENGINE VACUUM PUMP OUTPUT
Vacuum pump diagnosis involves checking pump
output with a vacuum gauge. The low vacuum warn­
ing switch can also be checked with a vacuum gauge.
A standard vacuum gauge can be used to check
pump output when necessary. Simply disconnect the
pump supply hose and connect a vacuum gauge to
the outlet port for testing purposes. Vacuum should
hold steady in a range of approximately 8.5 to 25
inches at various engine speeds.