Dodge RAM TRUCK 1500 1995 Service Manual page 211

BR
I L L - W H E E L
ANTILOCK BRAKE SYSTEM (ABS) § -
3 9
FRONT
ANTILOCK
VALVE
J""L-Tj,
MASTER
hj=(Jf-*
CYLINDER #
RF
WHEEL
POWER
BRAKE
BOOSTER
COMBINATION
VALVE
REAR
ANTILOCK
VALVE
RR
WHEEL
LF
WHEEL
Fig. 7 ABS Hydraulic
ABS OPERATION IN ANTILOCK BRAKING
MODE
The ABS module activates the system whenever
sensor signals indicate the onset of high wheel slip.
High wheel slip can be described as the point
where wheel rotation begins approaching zero (or
lockup) during braking. The antilock system retards
lockup during high slip conditions by modulating
fluid apply pressure to the wheel brake units.
Brake fluid apply pressure is modulated according
to wheel speed, degree of slip and rate of decelera­
tion. A sensor at each wheel converts wheel speed
into electrical signals. These signals are transmitted
to the module for processing and determination of
wheel slip and deceleration rate.
The ABS system has three fluid pressure control
channels. The front brakes are controlled separately
and the rear brakes in tandem. A speed sensor input
signal indicating high slip conditions activates the
control module antilock program.
The solenoid valves are not static during antilock
braking. They are cycled continuously to modulate
pressure. Solenoid cycle time in antilock mode can be
measured in milliseconds.
COMBINATION ¥ALWE OPERATION
Pressure Differential Switch
The pressure differential switch is connected to the
red brake warning light. The switch is triggered by
movement of the switch valve. A decrease or loss of
fluid pressure in either hydraulic circuit will cause
the switch valve to shuttle forward or rearward in re­
sponse to the pressure differential. Movement of the
LR
WHEEL
J9305-113
System Connections
switch valve pushes the switch plunger upward. This
action closes the switch internal contacts completing
the electrical circuit to the warning light. The switch
valve will remain in an actuated position until repair
restores system pressures to normal levels.
Metering Valve
The metering valve is used to balance brake action
between the front disc and rear drum brakes. The
valve meters (holds-off) full apply pressure to the
front disc brakes until the rear brakeshoes are in full
contact with the drums. The valve is designed to
maintain front brake fluid pressure at 21-207 kPa
(3-30 psi) until the hold-off limit of 807 kPa (117 psi)
is reached. At this point, the metering valve opens
completely permitting full fluid apply pressure to the
front disc brakes.
ABS SYSTEM DIAGNOSIS
ABS system diagnosis involves checking only those
components that form the antilock system. Antilock
system diagnosis involves three basic steps which
are:
• observation of the warning light display
• visual examination for low fluid, leaks, or dam­
aged wires
• identification of the problem circuit
Visual examination includes a check of reservoir
fluid level and condition of the system components.
This includes inspection of the sensor wires and elec­
trical connections. Things to look for are leaks, loose
connections, or obvious component damage. Identifi­
cation involves using the DRB scan tool to identify
the faulty circuit.