3
System ground (pin 1 on the seven pin connector—white)
enters the MC-12
™
modulator at pin C.
Each wheel speed sensor sends the MC-12
™
modulator its
AC. signal through a pair of wires. The MC-12
™
modulator
pins for the sensors are L-M and N-P.
The MC-12
™
modulator can send a +12v failure signal to the
optional trailer mounted status light through pin D. The status
light indicates the condition of the trailer antilock system. In
addition, the MC-12
™
modulator can send a grounding signal
through its pin J to an optional tractor, dash mounted status
lamp. During start up, when the trailer brakes are actuated,
trailer antilock immediately runs a self check. The status
light flashes once and then goes off. Should a problem occur,
the antilock system disengages and returns to normal
R-12
™
relay valve operation.
If the status light comes on and remains on when the trailer
brakes are actuated, a system problem has occurred. The
EC-12
™
controller contains a diagnostic window, which
FIGURE 3 - SPEED SENSOR VOLTAGE CYCLE OUTPUT
PEAK TO PEAK
PEAK TO PEAK
HIGH SPEED
LOW SPEED
directs the driver to the problem area. A series of LEDs on
the EC-12
™
controller indicate the status of power, EC-12
™
controller, wheel speed sensors, M-12
™
modulator solenoids,
or voltage level.
OPERATION
APPLYING:
Normal Service Application
When a normal service brake application is made, and the
EC-12
™
controller does not sense impending wheel lock up,
control air pressure from the brake valve enters the modulator
control port. The air passes through the supply solenoid
and acts on the modulator's piston. The piston closes the
modulator exhaust and opens the inlet, delivering supply air
out the delivery ports.
FIGURE 4 - APPLYING: NORMAL SERVICE APPLICATION
DELIVERY
PISTON
EXHAUST
SUPPLY
EXHAUST
SOLENOID
CONTROL
DOUBLE
CHECK
TRAILER
CONTROL
BRAKE
VALVE
SUPPLY
SOLENOID
SERVICE
BRAKE
CHAMBER
