Bendix Commercial Vehicle Systems M-40HF PMV SD - Manual (Page 5)

5
ANTILOCK APPLY MODE
If a service brake application is made and the antilock
system detects an impending wheel lockup, the antilock
controller will make a controlled brake application using
the modulator.
In order to control the brake application, the coils of the two
solenoid valves contained in the modulator are energized
or de-energized in a pre-programmed sequence by the
antilock controller. When a solenoid coil is energized,
and — depending on whether the exhaust or hold solenoid
is energized — it either opens or closes, thereby causing
the exhaust or reapplication of air pressure to the brake
actuator. The solenoids in the modulator are controlled
independently by the Electronic Control Unit (ECU).
Before ABS systems, drivers would sometimes “pump the
brakes” in order to attempt to prevent wheel lock-up and
maintain vehicle control. In the case of an ABS braking
system, the antilock controller is able to apply and release
the brakes using the modulators, automatically, with far
greater speed and accuracy. Depending on the number of
modulators used, some systems are able to apply braking
power to wheels independently.
ANTILOCK EXHAUST MODE
When wheel lock is detected or imminent, the antilock
controller energizes the supply and exhaust solenoids in
the modulator.
Further delivery of air to the brake chamber is prevented,
and air is allowed to exhaust through the exhaust port.
ANTILOCK HOLD MODE
The antilock controller will place the modulator in the hold
position when information from the wheel speed sensors
shows that the correct wheel speed (braking force) has
been attained. The antilock controller will also place the
modulator in the hold position — prior to entering the
reapply mode — when it detects recovery from a locked
wheel condition.
In this mode of operation, the exhaust passage is sealed,
holding the air pressure in the brake chamber at that level.
At the same time, no application air is permitted to flow to
the brake chamber.
The modulator can enter both the antilock exhaust or
reapply mode from the antilock hold mode depending on
the needs of the antilock controller.
ANTILOCK “REAPPLY” MODE
If the antilock controller senses that wheel speed has
increased sufficiently enough to allow re-application of
braking pressure, without further wheel lock-up, it permits
the supply air to re-apply air to the brakes in the same
manner it did during a non-antilock event.
PREVENTIVE MAINTENANCE
GENERAL
Read and follow the General Safety Guidelines on
page 2 of this document.
Perform the tests and inspections presented at the
prescribed intervals. If the modulator fails to function as
described, or if the leakage is excessive, replace it with a
new Bendix ® brand unit, available at any authorized parts
outlet.
EVERY MONTH, 10,000 MILES OR
350 OPERATING HOURS
1. Remove any accumulated contaminates and visually
inspect the exterior for excessive corrosion and physical
damage.
2. Inspect all air hoses and wire harnesses connected to
the modulator for signs of wear or physical damage.
Replace as necessary.
3. Test air line fittings for leakage and tighten or replace
as necessary.
4. Perform the routine OPERATION & LEAKAGE TESTS
described below.
OPERATION & LEAKAGE TESTS
OPERATION TEST
This test uses the Chuff Test (described on page 3).
To properly test the function of the modulator will require
two (2) service technicians.
1. Park the vehicle on a level surface and block or chock
the wheels. Release the parking brakes and build the
air system to governor cut out.
2. Turn the engine ignition key to the OFF position then
make and hold a full brake application.
3. With the brake application held and one (1) service
technician posted at one (1) of the modulators, turn the
vehicle ignition key to the ON position. ONE OR TWO
SHORT bursts of air pressure should be noted at the
modulator exhaust. Repeat the test for each modulator
on the vehicle. If at least a single burst of exhaust is
not noted or the exhaust of air is
prolonged and not
short, sharp and well defined, perform the Electrical
Tests.