Technical information
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10
Technician Guidelines for Antilock Braking Systems
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Technicians can communicate with the ECU through a
standard SAE J1587 diagnostic connector (See Fig. 1).
Technicians can read and clear fault codes stored in the ECU
and run various diagnostic tests with this connector.
The type of ECU used and its location (in-cab or frame) vary
by manufacturer and application. A detailed description of all
the different ECU types used today is beyond the scope of this
manual. Consult either the vehicle or component
manufacturer’s service information for specifics.
B. Modulator Valves
ABS modulator valves regulate the air pressure to the brakes
during ABS action. When not receiving commands from the
ECU, the modulator valve allows air to flow freely and has no
effect on the brake pressure. The ECU commands the
modulator valve to either:
• change the air pressure to the brake chamber, or
• hold the existing pressure.
However, it cannot automatically apply the brakes, or
increase the brake application pressure above the level applied
by the driver.
The modulator valve typically contains two solenoids. The
modulator valve and relay valve may be incorporated into a
single unit. The modulator valve may also be separate, inserted
into the service line to the brake chamber(s) after any relay
valve, located as close as practicable to the chamber(s) itself.
When the modulator valve is separate, it has to control
more air flow and, therefore, includes two larger diaphragm
valves which are controlled by the solenoids. It usually has
three ports: the supply port, the delivery port and the exhaust
port.
• The supply port receives air from a quick release or
relay valve.
• The delivery port sends air to the brake chambers.
• The exhaust port vents air from the brake chamber(s).
Typically, when an ECU controlling a separate modulator
valve detects impending wheel lockup, it activates the solenoids
to close the supply port and open the exhaust port. When
enough air is vented to prevent wheel lockup, the exhaust valve
will close and the ECU will—depending on the situation—either:
ABS Modulator Valve