Chrysler RAM TRUCK 2500 1994 Service Manual page 199

S - 40 ALL-WHEEL ANTILOCK BRAKE SYSTEM (ABS)
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Light Remains On After Startup
An ABS system fault is indicated when the light re­
mains on after startup. Diagnosis with the DRB scan
tool will be necessary to determine the faulty compo­
nent.
Light Illuminates During Brake Stop
A system fault such as loss of speed sensor signal
or solenoid failure, will cause the amber warning
light to illuminate. The most effective procedure here
is to check for obvious damage first. Then check the
electronic components with the DRB scan tool, or
flash codes.
ABS CONTROL MODULE DIAGNOSIS
The electronic module controls all phases of an­
tilock system operation. The module also differenti­
ates between normal and antilock mode braking.
The module monitors and processes signals gener­
ated by the system sensors at all times.
The module operating program includes a self
check routine that tests each of the system compo­
nents. A failure of the self check program will cause
illumination of the amber warning light. The light
will also illuminate i f a solenoid or other system com­
ponent fails during the dynamic phase of initializa­
tion.
I f a system malfunction should occur, do not imme­
diately replace the control module. A blown system
fuse, bad ground, or loss of feed voltage will cause
system faults similar to a module failure. Never re­
place the module unless scan t o o l diagnosis i n ­
dicates replacement is actually necessary.
ABS VALVE SERVICE AND DIAGNOSIS
The front and rear antilock valves are serviced only
as assemblies. Neither valve should be replaced un­
less a fault has actually been confirmed with the
DRB scan tool.
COMBINATION VALVE TESTING
T E S T I N G 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 as­
sembly.
TESTING P R E S S U R E D I F F E R E N T I A L S W I T C H
(1) Have helper sit i n 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 i n 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. Repair as necessary and repeat
test steps (3) and (4).
(5) I f warning light still fails to illuminate, check
brakelight and parking brake switches and wiring
with test lamp. Repair or replace parts as necessary
and test differential pressure switch operation again.
(6) I f warning light still does not illuminate, switch
is faulty. Replace combination valve assembly, bleed
brake system and verify proper switch and valve op­
eration.
ABS VALVE AND CONTROL MODULE REMOVAL
(1) I f desired, battery can be removed for improved
access to ABS valve and connecting lines.
(2) Disconnect ABS harness at valve (Fig. 9).
(3) Unlatch and disconnect harness at ABS mod­
ule.
Fig. 9 ABS Valve Harness Connections
(4) Disconnect lines to front brakes at ABS valve
(Fig. 10).
(5) Remove stud nuts and bolt that attach ABS
valve to inner fender panel (Fig. 11). Then remove
valve from engine compartment.
(6) I f valve is only being removed for access to an­
other component, cover brakeline fluid ports with
tape or plugs to prevent dirt entry.
(7) Remove bolts attaching module to valve and re­
move module.