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Section 12: BRAKE AND AIR SYSTEM
22
X3-45 Commuter PA1593 DOB 2400-2489 Section 12 Updated Sept. 2015
WARNING
To prevent personal injuries, brakes should be
inoperative prior to working on any of their
components.
1. Block the wheels to prevent the vehicle from
moving.
2. Safely support vehicle at the recommended
body jacking points.
NOTE
To gain access to a given brake air chamber,
the corresponding wheel can be removed
(refer to Section 13: "Wheels, Hubs and
Tires").
3. Exhaust compressed air from air system by
opening the drain valve of each reservoir.
4. For the drive axle brake chambers, manually
release spring brakes (refer to “
Emergen-
cy/Parking Brake Manual Release" proce-
dure in this section).
5. Remove clamp ring, remove and discard the
existing diaphragm. Install the new dia-
phragm squarely on body.
6. Reverse the procedure for assembly. Tap
clamp ring to ensure proper seating. Check
for proper operation before placing vehicle in
service.
31. ANTI-LOCK BRAKING SYSTEM (ABS)
This device has been designed to ensure
stability and permit steering control of vehicle
during hard braking, and to minimize its stopping
distance whatever the road conditions are. On
slippery roads and generally in emergency
situations, over-braking frequently induces wheel
lock. The anti-lock braking system provides
maximum braking performance while maintain-
ing adequate steering control on slippery roads.
The ABS continuously monitors wheel behavior
during braking. Sensors on each wheel of front
and drive axles (tag axle is slave to drive axle)
transmit data to a four channel electronic
processor which senses when any wheel is
about to lock. Modulator valves quickly adjust the
brake pressure (up to 5 times per second) to
prevent wheel locking. Each wheel is therefore
controlled according to the grip available
between its tire and the road.
With this device, the vehicle is brought to a halt
in the shortest possible time, while remaining
stable and under the driver's control.
Since the braking system has dual circuits, the
ABS is also provided with a secondary system
should a fault develop in the ABS. Anti-lock
systems are a parallel system which does not
hinder brake functioning in case of failure.
Braking system functions in normal, non
anti-lock controlled operation during ABS system
failure.
The ABS system consists of two diagonally
related circuits, only the half of the system which
has sustained damage or other fault is switched
off (i.e. wheels return to normal non-ABS
braking). The other diagonal half remains under
full ABS control.
NOTE
ABS is active on service brake, transmission
retarder, Jake brake, but is inactive on
emergency/parking brake.
NOTE
The ABS system is inoperative at speeds
under 4 mph (6 Km/h). Illumination of ABS
telltale indicator at these speeds is normal.
CAUTION
Disconnect the ECU or pull the ABS fuse
before towing vehicle.
31.1 TROUBLESHOOTING AND TESTING
For troubleshooting and testing of the vehicle's
anti-lock braking system, refer to Meritor Wabco
Maintenance Manual MM-0112: “Anti-Lock
Braking System (ABS) for Trucks, Tractors and
Buses" , at the end of this section. Use
dashboard Message Center Display (MCD)
Diagnostic Mode for troubleshooting and repair.
31.2 ABS COMPONENTS
The main components of the ABS system are
listed hereafter. Refer to each component for its
specific function in the system and for proper
maintenance.