Service manual

Group 59 Anti-Lock Brake System (Bendix) Design and Function
Modulator Valve
The Bendix M-22 modulator valve is a high capacity air
solenoid valve. It has two solenoid coils. One of the coils
is energized to isolate the brake chamber from the brake
system air supply and vent the brake pressure from the
brake chamber. The other coil is energized to isolate the
brake chamber from the brake system supply and hold
the pressure that is in the brake chamber.
During normal braking, compressed air flows freely
through the modulator valves to the brake chambers.
When ABS function is required, the modulator valve for
the affected wheel is energized on and off in order to
quickly, but incrementally, reduce the braking air pres-
sure to the point where the wheel begins to spin freely.
The modulator valve can be used to control the brake
chamber pressure on an individual chamber or on two
service brake chambers. When used to control both ser-
vice brake chambers on an axle or two chambers on the
same side of a tandem axle, the modulator is usually
mounted upstream of the quick release valve, which pro-
vides quick exhaust of brake pressure during normal
applications. In the case of individual wheel control appli-
cations, the modulator is always the last control valve
through which air passes on its way to the service brake
chamber.
The valve consists of a die cast aluminum body and a
solenoid assembly which contains one normally-open
solenoid, one normally closed solenoid, and two inlet
and exhaust diaphragm valves.
W5000533
W5000534
Relay Valve
There is a special type relay valve that is used with the
Bendix ABS system with TCS. Normally, a standard R-
14 Bendix relay valve performs the relay function with
ABS. The inclusion of TCS requires that the relay valve
have extra controls to provide braking air pressure dur-
ing TCS operation.
In TCS equipped tractors, the Bendix ATR-2 replaces the
standard BP-R1 bobtail proportioning valve. The ATR-2
is a specialized brake proportioning valve combined with
a traction control solenoid. It contains pneumatic and
electric components to provide both the service braking
and traction control functions. During tractor bobtail op-
eration, the ATR-2 reduces brake application pressure to
the lighter rear axle brakes to improve vehicle control and
minimize rear axle wheel skid before ABS is required.
All air connections on the ATR-2 are identified with cast,
embossed letters. The designations and their meanings
are as follows:
SUP ......................................... supply from reservoir
DEL ............................. delivery to service actuators
SER ............................ service signal from foot valve
CON ............. control signal from trailer supply valve
W5000536
1) Control 4) Supply
2) Service 5) Traction solenoid
3) Delivery 6) Solenoid connector
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