The anti-lock braking system (ABS) is designed to prevent the wheels from locking when braking. ABS includes:
- a) wheel speed sensors and impulse rings, speed sensors are installed in the front axle cams and in the rear axle flanges. Impulse rings are pressed onto the hubs.
- b) electronic control unit (ECU). The ECU monitors the signals from the speed sensors and controls the corresponding valves (modulators).
The ECU is located on the partition of the driver's workplace.
- c) pressure control valves (modulators). Modulators are installed near the brake chambers.
- Depending on the incoming signal, the modulator regulates the air pressure in the brake chamber, preventing wheel locking.
- d) warning lamp on the instrument panel. The lamp is intended to warn about the serviceability (malfunction) of the ABS and to read diagnostic codes.
ABS is activated every time the wheels become blocked.
Braking with ABS begins at a bus speed of more than 6 km/h and is accompanied by slight pulsation and characteristic noise of the ABS modulators.
The layout of ABS parts is shown in the figure.
For normal ABS operation, the maximum lateral runout of the impulse ring mounted on the wheel hub should not exceed 0.2 mm.
Greater runout may cause the control unit to turn off. The teeth of the impulse ring must not be damaged.
The ECU must have good grounding.
To avoid malfunctions, special care must be taken when laying ground cables.
ABS sensors, modulators, and the ECU itself, if they malfunction, cannot be repaired and must be replaced only with serviceable ones of the same type. Otherwise, proper operation of the ABS system is not guaranteed.
Full diagnostics of the ABS system should only be carried out with a special diagnostic tool.
Carrying out ABS diagnostics
System diagnostics are performed automatically every time the ignition is turned on.
If the automatic ABS test has been successfully completed, the warning light goes out 2 seconds after the ignition is turned on, indicating that the ABS is working properly.
The lamp can be on continuously after erasing errors and if there are sensor errors in the ECU memory, but when the bus speed is more than 6 km/h, the lamp should go out if the system is working properly.
If a malfunction occurs, the diagnostic warning lamp lights up and the malfunction is stored in the computer and coded as a block of light signals.
To determine the malfunction, you need to press the diagnostic button no earlier than 1 second after turning on the ignition and release it. After which the emergency lamp will start flashing.
The first block of lamp blinks indicates the component number, and the second block indicates the error number.
Error codes are shown in the table for decoding light codes.
If the lamp does not light up immediately after turning on the ignition, this indicates that the incandescent lamp is faulty and needs to be replaced.
- Warning lamp pulse duration - 0.5 sec.
- The interval between signal lamp pulses is 0.5 seconds.
- The interval between the first and second block of fault codes is 1.5 seconds.
- The interval between fault codes is 4.5 seconds.
Knorr-Bremse ABS light code table.*
Light code - Description - Remedy:
Component No. - Error No.
- 1 – 1 - No fault
Left steering axis speed sensor
- 2 – 1 - Air gap too large - Install sensor according to instructions
- 2 – 2 - No sensor signal when braking - Install the sensor as required. Replace sensor
- 2 – 3 - Bad impulse ring, ABS service life - Replace worn ones details
- 2 – 4 - Signal instability - Install the sensor according to the instructions
- 2 – 5 - Loss of sensor signal - Install the sensor according to the instructions. Replace sensor
- 2 – 6 - Short circuit to GND or battery, or wire break. - Replace wires
Right steering axis speed sensor
- 3 – 1 - Air gap too large - Install sensor according to instructions
- 3 – 2 - No sensor signal when braking - Install the sensor in accordance with the instructions. Replace sensor
- 3 – 3 - Bad impulse ring, ABS service life - Replace worn parts
- 3 – 5 - Loss of sensor signal - Install the sensor according to the instructions. Replace sensor
- 3 – 6 - Short circuit to GND or battery, or broken wire - Replace wires
Left steering axis speed sensor
- 2 – 1 - Air gap too large - Install sensor according to instructions
- 2 – 2 - No signal when braking - Install the sensor in accordance with the instructions. Replace sensor
- 2 – 3 - Bad impulse ring, ABS service life - Replace worn parts
- 2 – 4 - Signal instability - Install the sensor in accordance with the instructions. Replace worn parts
- 2 – 5 - Loss of sensor signal - Install the sensor correctly. Replace the sensor.
Right steering axis speed sensor
- 3 – 1 - Air gap too large - Install sensor correctly
- 3 – 2 - No sensor signal when braking - Install the sensor correctly. Replace sensor
- 3 – 3 - Bad impulse ring, ABS service life - Replace worn parts
- 3 – 4 - Signal instability - Install the sensor correctly. Replace worn parts.
- 3 – 5 - Loss of sensor signal - Install the sensor correctly. Replace sensor
- 3 – 6 - Short circuit GND or battery, or broken wire - Replace wires
Left drive axle speed sensor
- 4 – 1 - Air gap too large - Install sensor correctly
- 4 – 2 - No sensor signal when braking - Install the sensor correctly. Replace sensor
- 4 – 3 - Bad impulse ring, ABS service life - Replace worn parts
- 4 – 4 - Signal instability - a) Install the sensor in accordance with the instructions; b) Replace worn parts
- 4 – 5 - Loss of sensor signal - a) Install the sensor in accordance with the instructions; b) Replace the sensor
- 4 – 6 - Short circuit to GND or battery, or broken wire - Replace wires
Right drive axle speed sensor
- 5 – 1 - The air gap is too large - a) Install the sensor in accordance with the instructions; b) Replace worn parts
- 5 – 2 - No sensor signal when braking - a) Install the sensor in accordance with the instructions; b) Replace the sensor
- 5 – 3 - Bad impulse ring, ABS service life - Replace worn parts
- 5 – 4 - Signal instability - a) Install the sensor in accordance with the instructions; b) Replace worn parts
- 5 – 5 - Loss of sensor signal - a) Install the sensor in accordance with the instructions; b) Replace the sensor
- 5 – 6 - Short circuit to GND or battery, or broken wire - Replace wires
Left steering axis modulator
- 8 – 1 - Short circuit of the reset coil to battery - Replace the modulator
- 8 – 2 - Short circuit of the reset coil to GND - Replace the modulator
- 8 – 3 - Reset coil wire break - Replace the modulator
- 8 – 4 - Wire break on the common pin - Replace the modulator
- 8 – 5 - Short circuit of the lift coil to the battery - Replace the modulator
- 8 – 6 - Short circuit of the lift coil to GND - Replace the modulator
- 11 – 1 - Short circuit of the reset coil to battery - Replace the modulator
- 11 – 2 - Short circuit of the reset coil to GND - Replace the modulator
- 11 – 3 - Reset coil wire break - Replace the modulator
- 11 – 4 - Wire break on the common pin - Replace the modulator
- 11 – 5 - Short circuit of the lift coil to the battery - Replace the modulator
- 11 – 6 - Short circuit of the lift coil to rise GND - Replace the modulator
- 11 – 7 - Break in the lift coil wire - Replace the modulator
- 11 – 8 - Valve configuration error - Check the location of the wires in the modulator connector and install them in accordance with the diagram. Diagonal ground connection pins
- 10 – 10 - Diagonal 1 is shorted to the battery - Check the electrical wiring and replace the faulty one
- 10 – 11 - Diagonal 1 is shorted to GND - Check the electrical wiring and replace the faulty one
- 10 – 12 - All modulators are shorted to GND - Check the electrical wiring and replace the faulty one
Internal ECU malfunctions
- 15 – From 1 to 11 - ECU defective - Replace unit
Power supply
- 16 – 1 - diagonal 1, high voltage - Check the operation of the generator, eliminate malfunctions
- 16 – 2 - diagonal 1, low voltage - Check the operation of the generator, eliminate malfunctions
- 16 – 3 - diagonal 1, wire break - Check the electrical wiring and replace the faulty one
- 16 – 4 - GND_PCV1 wire break or large voltage difference to GND_ECU - Check wiring and replace faulty wiring
- 16 – 9 - U_ECU high voltage - a) Check the operation of the generator; eliminate faults; b) Check the electrical wiring and replace the faulty one
- 16 – 10 - U_ECU low voltage (or U_ECU high) - a) Check the operation of the generator; eliminate faults; b) Check the electrical wiring and replace the faulty one
Special errors
- 17 – 5 - Large difference between front and rear tire sizes - Replace tires
- 17 – 10 - Emergency lamp defective - Replace lamp
- 17 – 12 - Sensor parameter memory problem - a) Install the sensor in accordance with the instructions; b) replace the sensor; c) with the ABS system turned on, conduct a test drive of the bus (acceleration and deceleration) at a bus speed of 6 to 10 km/h.
- 17 – 13 - Axle 1 or 2 sensors are mixed up - Install the sensors in accordance with the ABS diagram
*- to start diagnosing Knorr-Bremse ABS turn on the ignition and press the diagnostic button for 0.5-8 seconds.
After releasing the key, the lamp lights up for 0.5 seconds and goes out. Then after 1 second the codes light up.
You can interrupt the generation of error codes by pressing the diagnostic button again.
WABCO ABS light code table*
Name
Light code - Description
- Remedy:
- 1 – 1 - No fault
ABS solenoid valve
- 2 – 1 - Front right
- 2 – 2 - Front left
- 2 – 3 - Rear right
- 2 – 4 - Rear left
- Check the solenoid valve cable. In the wires to the intake, exhaust valves or in the “common” wire there is an intermittent or constant break, or a short to negative.
sensor: increased air gap
- 3 – 1 - Front right
- 3 – 2 - Front left
- 3 – 3 - Rear right
- 3 – 4 - Rear left
- Check the wheel bearing runout. Move the sensor towards the rotor. Check the sensor cable and connectors for loose contact
sensor: short circuit/wire break
- 4 – 1 - Front right
- 4 – 2 - Front left
- 4 – 3 - Rear right
- 4 – 4 - Rear left
- Check the sensor cable. Open or short to “minus” or “plus” or between the sensor wires
Losing signal/tire size
- 5 – 1 - Front right
- 5 – 2 - Front left
- 5 – 3 - Rear right
- Check the sensor cable for loose contact. Check the rotor for damage. Connect another sensor to check.
Rotor
- 6 – 1 - Front right
- 6 – 2 - Front left
- 6 – 3 - Rear right
- 6 – 4 - Rear left
- Check the rotor for damage. Replace Rotor
System functions
- 7 – 4 - ABS lamp
- Check the cable and emergency lamp. Was the diagnostic key pressed for more than 16 seconds?
Electronic unit
- 8 – 1 - Low supply voltage - Check power cable and fuse
- 8 – 2 - Increased supply voltage - Check the battery and generator
- 8 – 3 - Internal error - Replace the ABS unit if the error recurs
- 8 – 4 - Configuration error - Incorrect electronics / parameterization
- 8 – 5 - Connection to battery negative - Check the ground on the electronic unit and magnetic valves
“- to start diagnosing WABCO ABS, turn on the ignition and press the diagnostic button for 0.5-3 seconds.
After releasing the key, the lamp lights up for 0.5 seconds and goes out.
Then after 1.5 seconds. codes light up.
The interval between two blocks of signals of the same fault is 1.5 s.
The interval between signals of two faults is 4 s. To turn off diagnostics, you need to turn off the ignition.
Erasing error memory
After troubleshooting the system, you need to erase the error from the error memory of the control unit, to do this you need to:
- for Knorr-Bremse - with the ignition off, press the diagnostic key and release only after the ignition is turned on.
In less than 3 seconds the error memory is erased;
- for WABCO - with the ignition on, press the diagnostic key for 3-6 seconds and release.
After 1.5 seconds, eight rapid flashes of the LED lamp indicate erasing errors from the ECU memory.
After 4 seconds, the lamp flashes three times, indicating that the ABS configuration is correct. Then turn off the ignition.
Final ABS check
For final inspection, check and complete the following:
- 1. Check that the ABS configuration is correct. To do this, press the diagnostic button twice with a break of less than 1 second. After which the signal lamp should flash in two blocks: 1-1 (i.e. -12 V) and 2-3 (i.e. - 4S/3K with one modulator on the front axle).
- 2. Erase the memory of errors.
- 3. Count the light codes.
- 4. The emergency lamp should go out after turning on the ignition after 2 seconds.
- 5. Perform a dynamic test. It is necessary to carry out the bus runs (acceleration - deceleration) on a flat area. The emergency lamp should go out when the bus speed reaches 6 - 10 km/h.
If the ABS malfunctions, the functionality of the braking system is maintained, but it is necessary to remember about the possibility of deterioration in the controllability of the bus due to wheel locking.
When significant deterioration in controllability occurs, it is necessary that the ABS be checked immediately by an appropriate workshop in order to eliminate the problem and return the system to normal operation.
Attention! When carrying out welding work on a bus, you should disconnect all electrical connectors from the ECU.