Antilock Braking Systems (TTEC 4825)

Off-car Exercises

Anti-lock Braking Systems

Below are some possible causes for damaging an ECU.

Spiked by careless welding: (True)

Enclosure seal damaged: (True)

Obvious signs of mechanical damage: (True)

Faults are much more likely to be with connections or sensors. (True)

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To check faults, first of all check wiring for poor signal or corrosion, bad connections and sensors condition. Used proper testing tool like equipment such as scan tool and possible multi-meter.

ABS Wiring and Operation



No:	ABS Main Components
1	Brake rotor
2	Wheel speed Sensor
3	Toothed rotor (Tone Wheel)
4	ABS Modulator
5	master cylinder (reservoir)
6	Brake caliper
7	Brake booster

ABS SYSTEM (RHD)

(Magnifier this Diagram)

Used the workshop manuals to identify the wheel sensors and colours

Wheel Speed Sensor	Wire/Colour
Front right:	Green and Red.
Front left:	White and Black.
Rear left:	Pink and Blue.
Rear right:	Yellow and Brown.

braded wire

The braded wire is purpose to stop frequency from interrupting the signals and from ABS wheel sensors

 ABS Fusible link

-ABS main fuse (50 amps)
-Gauge fuse (10 amps)
-Stop light fuse (15 amps)
-ECU fuse (15 amps)
-Dome fuse (20 amps)

Identify the earths for the ABS control unit and ABS motor their wire colours what pins numbers

ABS Control Unit/ ABS Motor Pump	PIN NUMBER	COLOUR
ABS control unit	Pin #10 and #7	white-black
ABS motor/pump	Pin #1	white-black

On the wiring diagram for the ABS actuator, identify which solenoids control which wheel cylinder.

WHEELS	1PIN	COLOUR	2 PIN	COLOUR
Front right wheel	#2	red-white	#6	red-green
Front left wheel	#3	blue-red	#7	blue-white
Rear left wheel	#1	brown-white	#5	brown-white
Rear right wheel	#4	green-black	#8	green-yellow

ABS Pressure Modulator Circuit Unit

(Magnifier this Diagram)

Under normal braking, the fluid pressure goes right through the hydraulic valves and the ABS doesn't interfer. (Inlet valve open and Outlet valve closed).

When the wheels start to lock-up the ABS operate to reduce brake pressure (Inlet valve closed and Outlet valve open).

When ABS is operating to hold brake pressure both Inlet and outlet valve closed.

When ABS is operating to increase wheel brake pressure Inlet valve open and Outlet valve closed.

(A) - The driver took a heavy brake and the wheels start to lock up at holding position point this would give the driver the time to maintain control of the braking to respond.

(B) - Indicate the tires is at 20% slip, turning ABS ON and reducing brake pressure to prevent the wheels from locking. When this happen the tires still have some brakes but lots of slip with the road. It also allowed the driver to take some control of the vehicle (inlet valve closed/outlet valves open) for short time.

(C) - This shows that the vehicle is now holding pressure by closing both valve to prevent brake fluid pressure increased.

 (D) - As the vehicle continues to lock, and tires still up to about 80% of braking force, the Hydraulic Control Unit release pressure by opening a valve that drains of brake fluid pressure and often stores it for later. On the graph figure above the system change dramatically, the inlet valve is now in-charge of adding and holding constant pressure so the vehicle can take some control. For this reason the driver have the advantage of maintain steerbility and stability and also help decrease stopping distance.

(E)-  As the system allows  adding and holding brake fluid when it was needed according to division (D), this drawn too much of pressure and therefore the system again start to reduces fluid pressure to prevent the wheels from locking.(inlet valve closed/ outlet valve open). 

(F) - Holding pressure at this point and the vehicle is maintain stability control.

In the above cases the ABS motor will still be working when the inlet value is closed and the outlet value is open to reduce pressure.

There are three main types of wheel sensors on modern vehicles. One sends an analogue signal using and inductive pick up, the others send a digital signal using ether hall effect or magneto resistant encoder.

Digital signal     

Voltage: 1v/div – (On the left)

Time: 1ms/div – (On the bottom)

Analogue signal

Voltage: 1v/div – (On the left)

Time (Hz): 0.5Hz/div – (Down the bottom)

ABS Demonstrators

Inductive Type Sensor

The oscilloscope we using is (Tektronix TDS 1001B/digital), and these are inductive type sensor. As the rotor tooth approach to pick up, the voltage becomes more positive until it peaks when the tooth and pick up are at their closest. This is when the magnetic field is at its strongest and the voltages highest range. When the tooth starts to break away the voltage becomes more negative (as the magnetic field has been collapsed) until the next tooth comes and repeat the process.

LEFT FRONT 

Left Front: Multimeter Test: AC voltage: 04.7V
                  ECU Pin # 4 and 5

Voltage: 200mV/div

Time (Hz): 1.64598 kHz

LEFT REAR

Left Rear: Multimeter Test: AC Voltage 0.33V

                ECU Pin # 7 and 9

Voltage: 200mV/div

Time (Hz): 10Hz/div

RIGHT FRONT

Right front: Multimeter Test AC Voltage 15.1V

                   ECU Pin # 11 and 21

Voltage: 200mV/div

Time (Hz): 10Hz/div

RIGHT REAR

AC Voltage: 05.2V

Right rear ECU Pin # 24 and 26

Voltage: 200mV/div

Time (Hz): 10Hz/div



The differences between sensors waveform signal

The waveforms can be varied by different problems such as air gap could be too big which would give a low voltage. The magnet might have magnetic dust on it which would give a distorted reading. Some vehicle had different size wheels this would affect the wheel speed reading due to a larger wheel can spin slower than smaller wheel which on the steering wheel to the front.

Can a multi-meter be as accurate in finding problems with the wheel speed sensors as an oscilloscope?

The Multi meter cannot be as accurate as an oscilloscope, as an oscilloscope will provide more information due to frequency which is too fast for the meter to detect, the duty circle, shows the entire operation, and pick voltage.