WABCO 400 500 320 0, Maintenance Manual

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ABS Questions and Answers
What is an intermittent fault?
An intermittent fault is a problem that usually occurs only under certain driving conditions. For example,
the ECU may detect a loose cable or wire or receive an erratic signal from a wheel sensor. Since
intermittent faults can be unpredictable and may only happen periodically, you can use information stored in
ECU memory to
fi nd and correct the loose cable or wire. An intermittent fault cannot be retrieved using blink
codes.
Is an intermittent fault di ffi cult to locate and repair?
It can be, because you may not be able to easily fi nd the cause of the problem. WABCO recommends that
you write down intermittent faults to help you isolate a fault that recurs over a period of time.
Can the ECU store more than one fault in memory?
Yes. The ECU retains existing and intermittent faults in memory even when you turn OFF the power to the
ECU.
What if the ECU fi nds a fault in an ABS component during normal operation?
If the ECU senses a fault in the system (with an ABS valve, for example), the ECU turns the trailer ABS
indicator lamp on and returns the wheel controlled by that valve to standard braking. Or, if the ECU
fi nds
a fault with one wheel speed sensor in a system that has four sensors on a tandem axle, the ECU uses
information from the other sensor on the same side of the tandem to ensure continuous ABS function. The
ECU continues to provide full ABS function to the wheels unaffected by system faults. However, the ECU
will turn the trailer ABS indicator lamp on to tell the driver a fault has been detected in the system.
4.5 Frequently Asked Questions
What is the crack pressure of the ABS valve?
The pressure at which the ABS valve opens to allow air pressure to the wheel ends is 3 to 5 psi on the
signal port of the valve.
What can cause the trailer to bounce up and down when the service brakes are applied?
It is possible the ABS is getting signal from the sensor and tone ring that it is going into an ABS event. If
the issue is more noticeable when the trailer is unloaded, it is possible the trailer has worn suspension
components that can cause the ABS to react and go into an ABS event. Review with the trailer suspension
manufacturer.
Why are my brake lights on dimly any time the trailer is powered up?
If the trailer is equipped with LED brake lights and there is no resistor in the circuit, you will get unwanted
illumination of the lights all the time. The 12v LED or circuit must have a resistor installed to prevent them
from illuminating all the time.