Dodge ram truck 3500 1996 Service Manual page 196

5 - 12 BRAKES B R
DIAGNOSIS AND TESTING (Continued)
DIAGNOSING LOW VACUUM OUTPUT
CONDITION
A low booster vacuum condition or a faulty low
vacuum warning switch will cause the brake warning
lamp to illuminate. I f the lamp does go on and indi­
cates the existence of a low vacuum condition, check
the vacuum pump, booster and warning switch as fol­
lows:
(1) Check vacuum pump oil feed line. Verify that
line connections are secure and not leaking. I f leak­
age is noted and pump is noisy, replace pump.
(2) Disconnect supply hose to booster. Connect vac­
uum gauge to this hose and run engine at various
throttle openings. Output should range from 8.5 to
25 inches vacuum. I f vacuum is consistently below
8.5 inches, problem is with vacuum hoses or pump
component. I f output is within specified limits, con­
tinue testing.
(3) Check booster operation as described In diagno­
sis section. Replace check valve, vacuum hoses, or
booster i f necessary. However, i f booster operation is
correct but warning lamp is still on, continue testing.
(4) Disconnect vacuum hose at warning switch.
Plug hose and connect hand vacuum pump to switch.
(5) Start and run engine.
(6) Apply 8.5 to 9 Inches of vacuum to warning
switch and observe warning lamp. I f lamp goes out,
switch vacuum hose Is either loose or leaking. I f
lamp remains on, leave engine running and continue
testing.
(7) Apply 20-25 inches vacuum to switch and
observe warning lamp operation. I f lamp now goes
out, switch is at fault and should be replaced. I f lamp
remains on, continue. testing.
(8) * Connect vacuum hoses and replace original
warning switch with known good switch. Run engine
and observe warning lamp operation. I f lamp is now
off, old switch is faulty, i f lamp remains on, problem
is in wiring between switch and warning lamp.
COMBINATION VALVE
Metering Valve
Metering valve operation can be checked visually
and with the aid of a helper. Observe the metering
valve stem while a helper applies and releases the
brakes. I f the valve is operating correctly, the stem
will extend slightly when the brakes are applied and
retract when the brakes are released. I f the valve is
faulty, replace the entire combination valve as an
assembly.
Pressure Differential Switch
(1) Have helper sit in drivers seat to apply brake
pedal and observe red brake warning light.
(2) Raise vehicle on hoist.
(3) Connect bleed hose to a rear wheel cylinder
and immerse hose end in container partially filled
with brake fluid.
(4) Have helper press and hold brake pedal to floor
and observe warning light.
(a) I f warning light illuminates, switch is operat­
ing correctly.
(b) I f light fails to illuminate, check circuit fuse,
bulb, and wiring. The parking brake switch can be
used to aid i n identifying whether or not the brake
light bulb and fuse is functional. Repair or replace
- parts as necessary and test differential pressure
switch operation again.
(5) I f warning light still does not illuminate,
switch is faulty. Replace combination valve assembly,
bleed brake system and verify proper switch and
valve operation.
DISC BRAKE ROTOR
The rotor braking surfaces should not be refinished
unless necessary.
Light surface rust and scale can be removed with a
lathe equipped with dual sanding discs. The rotor
surfaces can be restored by machining i n a disc brake
lathe i f surface scoring and wear are light.
Replace the rotor under the following conditions:
• severely scored
• tapered
• hard spots
• cracked
• below minimum thickness
ROTOR MINIMUM THICKNESS
Measure rotor thickness at the center of the brake-
shoe contact surface. Replace the rotor i f worn below
minimum thickness, or i f machining would reduce
thickness below the allowable minimum.
Rotor minimum thickness is usually specified on
the rotor hub. The specification is either stamped or
cast into the hub surface.
ROTOR RUNOUT
Check rotor lateral runout with dial indicator
C-3339 (Fig. 12). Excessive lateral runout will cause
brake pedal pulsation and rapid, uneven wear of the
brakeshoes. Position the dial indicator plunger
approximately 25.4 mm (1 in.) inward from the rotor
edge.
NOTE: Be sure wheel bearing has zero end play
before checking rotor runout.