Antilock Braking Systems help prevent the wheels from locking during sudden braking, especially on slippery surfaces. This helps the driver maintain control.
An antilock braking system is only an "add-on" to the existing base brake system. ABS only comes into play when traction conditions are marginal or during sudden panic stops when the tires lose traction and begin to slip excessively. The rest of the time ASB has no effect on normal driving, handling, or braking.
There are two situations in which an antilock brake system will not provide the shortest stopping distances.
The first involves straight stops made on smooth, dry pavement by an expert driver. Under these conditions, a skilled driver can hold the tires consistently closer to the ideal slip rate than the ABS can.
The other situation in which antilock brakes will not provide the shortest stops is when braking on loose gravel or dirt, or in deep snow. Under these conditions, a locked wheel will stop the vehicle faster because loose debris builds up and forms a wedge in front of the tire that helps stop the vehicle.
Off-car Exercises
Below are some possible causes for damaging an ECU.
- Spiked by careless welding, i.e. MIG welding without disconnecting the battery
- Enclosure seal damaged and with obvious sign of water ingress
- Obvious signs of mechanical damage to the enclosure
- Fautls are much more likely to be with connections or sensors
ABS wiring system
Wiring Diagram Practice
Identify the wheel speed sensors and list their wire colours for each sensor.
Front right: Black(B) and White(W)
Front left: Red(R) and Green(G)
Rear left: Pink (P) and Blue(L)
Rear right: Browns (BR) and Yellow(Y)
In the ABS wheel sensor what is the reason for the braded wire?
To prevent from interference from other cables
Identify and list all the fuses that are used by the ASB circuit
FL MAIN
50A ABS
15A ECU-IG
15A STOP
20A DOME
10A GAUGE
Identify the earths for the ABS control unit and ABS motor their wire colours what pin number.
Earths for the ABS control unit: 10B(W-B, White/Black) and 7B(W-B, White/Black)
Earths for the ABS motor: 1A(W-B, White/Black)
On the wiring diagram for the ABS actuator, identify which solenoids control which wheel cylinder. Then note the wire colours and pin number.
(SFRA: Solenoid Front Right Actuator, SRLR: Solenoid Rear Left Releasing)
Front Right Wheel: Pin number: 2B Wire colour: Red/White(R-W)
Front Left Wheel: Pin number: 3B Wire colour: Blue/Red(L-R)
Rear Left Wheel: Pin number: 1B Wire colour: Brown/White(BR-W)
Rear Right Wheel: Pin number: 4B Wire colour: Green/Black(G-B)
What are the correct conditions of the inlet and outlet solenoids valves under normal braking?
Inlet valve Open and Outlet valve Closed
What are the correct conditions of the inlet and outlet solenoids valves when the ABS is operating to reduce wheel brake pressure?
Inlet valve Closed and Outlet valve Open
What are the correct conditions of the inlet and outlet solenoids valves when the ABS is operating to hold brake pressure?
Inlet valve Closed and Outlet valve Closed
What are the correct conditions of the inlet and outlet solenoids valaves when the ABS is operating to increase wheel brake pressure?
Inlet valve Open and Outlet valve Closed
In the four cases above state when the ABS motor will be working?
When the ABS is operating to increase or decrease wheel brake pressure, the ABS motor will be working.
ABS Demonstrators
Record which ECU wires go to which wheel speed sensors:
Left front ECU Pin#4 and Pin#5
Left rear ECU Pin#7 and Pin#9
Right front ECU Pin#11 and Pin#21
Right rear ECU Pin#24 and Pin#26
Record a waveform for each wheel speed sensor.
Left front wheel speed sensor
Volt/division: 5V
Time/division: 500uS
Left rear wheel speed sensor
Volt/division: 5V
Time/division: 500uS
Volt/division: 5V
Time/division: 500uS
Volt/division: 20V
Time/division: 500uS
Right rear wheel speed sensor
Volt/division: 5V
Time/division: 500uS
Discuss what are the differences and what can causes these differences between the waveforms.
As shown above patterns, the pattern of the right front wheel speed sensor has high voltage amplitude compared to other patterns. There is more big gap between the right front wheel speed sensor and a toothed ring.
With the wheel speed sensors spinning, measure AC volts with a multi-meter and record here:
Left front: 5.77V
Left rear: 3.97V
Right front: 15.80V
Right rear: 5.00V
Can a multi-meter be as accurate in finding problems with the wheel speed sensors as an oscilloscope? No
ABS Relays
Record the name of the relay or switch that powers up the ABS ECU: K39
Record the name of the relay or switch that powers up the ABS pump: K100
Record the name of the relay or switch that sends power to the ABS HCU solenoids: K38
What is the ECU pin number for the wire that brings in the power to the ABS ECU?
ECU pin#29, Brown/Yellow
What is the ECU pin number or other number, for the wire that controls the relay for the ABS ECU?
ECU pin# 1, Black/Red
What is the pin number for the wire that brings in the power to the ABS Pump?
ECU pin#14, Red/Yellow
What is the pin number or other number, for the wire that controls the relay for the ABS Pump?
ECU pin#28, Grey/Green
Relay waveform
Capture a waveform that shows both the control circuit change when it turns on the relay and the power switching on to power something in the ABS system.
ABS Pump Relay waveform
Capture a waveform that shows both the control circuit change when it turns on the relay, and the power switching on to power the ABS Pump.
Channel A -
Volt/division: 5V
Time/division: 1SChannel B -
Volt/division: 10V
Time/division: 1SPoint A: When the relay switch is closed, 12V applied
Point B: When the relay switch is opened, the voltage drops to 0V.
Point C: There is a voltage spike because of magnetic field collaping
Point E: There are some oscillations when the ABS pump stops.
Observe what happens during the ABS Self Test when you first turn the key on. Watch the warning lights and observe power at the wires with the oscilloscope. Then discuss what is happening in the ABS system during Self Test.
The ECU performs a self-test after the ignition is switched on. If there is a failure during the self-test, an ABS warning light will be on and it will disconnect the ABS system. If the self-test is normal, an ABS warning light will go off after about 5 seconds. The main purpose of the self-test is to monitor the input(e.g wheel speed sensor) and the output(e.g operation of valves).
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