Anti-Lock Brake System (ABS) & CAN

AYOB AGHAZI TTEC 4848 

11 /11/12

Anti-Lock Brake System (ABS) & CAN

ANTI-LOCK BREAKING SYSTEM (ABS)

 

Theory:

Anti-lock Braking system (ABS) is a very important feature in modern vehicles. During sudden braking it allows for better control, steer-ability, and stability while stopping vehicle in the shortest distance and time. (Note ABS only kicks in when brakes are fully applied)

Types of ABS: There are 3 types

• 1 Channel
• 3 Channel
• 4 Channel

There are two main types of wheel sensors on modern vehicles and they are

1. Magnetic inductive pick up type (Analogue signal)
2. Hall Effect which sends an On/Off digital signal

On the graph below is an example of a hall effect type sensor that switches 5V every 2 seconds.

Below is an example of an analogue signal with a frequency of 0.5Hz and a maximum of 3volts

How ABS brakes work

The ABS system prevents your wheels from locking up when you apply extreme pressure to your brake pedal. This is important because wheels which have become locked are essentially useless. When your wheels are locked, your tires will not rotate which means you’re skidding.
The main components of an Anti-lock Breaking system:
1. Brake Disks
2. Wheel Speed Sensor
3. Toothed Gear
4. Hydraulic Control Unit (Hydraulic Modulator)
5. Master Cylinder
6. Brake Calipers
7. Brake booster

Wiring Diagram and wire colours:
Using the wiring diadram I identified the wheel speed sensors and listed their wire colours.
Front Right: Black and white Front Left: Green and Red
Rear Left: Blue Rear Right: Yellow and Brown

1. The reason for the braided wires is to reduce electro-magnetic interference which could affect the operation of the ABS.

2. Fuses that are used in the ABS circuit are listed below:

• F14 fuse box 50A
• FL main 3,0W
• Gauge 10A
• Dome fuse 20A
• Stop fuse 15A
• ECU IG 15A

3. Identify the grounds for the ABS control unit and ABS motor their wire colours and pin numbers

ABS motor grounds
ABS Motor Pin number is 1A on the ABS actuator, colour is white and black

ABS control unit grounds
ECU control unit Pin 7A on ABS ECU, colour red and black and Pin 20A also ABS ECU, red and blue or Pins 4B, red-black and 1A, red-blue and are both on the ABS relays.

4. On the wiring diagram for the ABS actuator, identify which solenoids control which cylinder and note wire colours and pin numbers.

Front Right Wheel:
Pin numbers: 2B and 6B Wire Colours: Red/White and Red/Green respectively

Front Left Wheel:
Pin numbers: 3B and 7B, Wire Colours: Blue/Red and Blue/White respectively

Rear Left Wheel: 1B and 5B, Wire Colours: Bronw/White and Brown/Red respectively
Rear Right Wheel: 4B and 8B, Wire Colours: Green/Black and Green/Yellow respectively

5. Using the diagram below provided as a guide we have identified the correct condition of the inlet and outlet solenoid valves

• When the ABS is operating under normal braking the inlet valve is open and outlet valve is closed
• When the ABS is operating to reduce wheel brake pressure the inlet valve is closed and outlet valve is open
• When the ABS is operating to hold pressure the inlet valve is closed and outlet valve is closed
• When the ABS is operating to increase wheel brake pressure the inlet valve is open and outlet valve is closed

In the 4 senarios described above the ABS motor will be working only be working when
- The the inlet valve is open and outlet valve is closed (ABS is operating to increase wheel brake) pressure.
- The inlet valve is closed and outlet valve is open (ABS is operating to reduce wheel brake pressure)


Graph of a digital signal wheel speed sensor that switches 5 Volts every 2 seconds

Below is a graph of a wheel speed sensor (Analogue signal) of 0.5 Hz and a maximum of +3V showing volts on the vertical axis and time on the horizontal axis.

ABS Demonstrators:
The wheel speed sensor pin out connections to the ECU on the wiring diagram and the demonstrator are listed below:
Left front: ECU Pin# 4 and 5 Right front ECU Pin# 11 and 21
Left rear ECU Pin # 7 and 9 Right rear ECU Pin# 24 and 26

ABS Relays:
K39: 4 pin relay (the relays or switch that powers up the ABS ECU)
K100: 4 pin relay (the relays or switch that powers up the ABS pump)
K38: 5 pin relay (the relays or switch that powers up the ABS HCU)

• ECU pin# for the wire that brings in the power to the ECU: pin 1 (black/red)
• ECU pin# for the wire that controls the relay for the ABS ECU: 15 (Ignition switch/Ignition On)
• The pin for the wire that brings in the power to the ABS pump: 13 (from actuator)
• The pin # for the wire that controls the relay for the ABS pump: 28 (from ECU)

Inductive low speed wheel speed sensor

Inductive high speed wheel speed sensor (frequency and magnitude increased compared to low speed one above)

Wave form of a speed sensor captured on a oscilloscope (AC voltage)

With the wheep speen sensor spinning we measured in AC voltage with a multimter the volatge readings of each sensor:

Left front: 3.2V Right front: 2.79V

Left Rear: 4.09V Right Rear: 2.87V

The volatage differences can be due to differnt gap size, dirt build up on sensor causing to read incorrect;

Magnetic Wheel Speed Sensor showing the toothed gear

Pic. showing two channl pattern of sensor waveform being actuated by the relay and switching on of the power to the ABS pump

Speed Sensor Gap:
Front right: 0.406 mm
Front left: 0.66 mm
Rear right: 0.406 mm
Rear left: 0.558 mm
All the gap fall within specifications. normal gap should be within 1mm

Reflection:

ABS or Anti-lock Breaking Sysytem allows for better breaking (shorter breaking distance) while still maintaining control and steerability.

The ABS signal coming from the wheel speed sensor is a sine waveform and when wheel speed the signal's frequency and magnitude increases and decreases when speed is reduced.

There are certain things that can go wrong with it. when ECU the detetects a problem it triggers or illuminates the engine check light to indicate fault.

Many things can affect signal and trigger check light like if work is done on wheel bearing, dirt covers wheel speed sensor, broken gear tooth etc. In things like this the code has to be cleared to turn check light off.

Controlled Are Network (CAN) SYSTEMS

Modern vehicles are equiped with communication network system whereby various modules communicate with each other by way data bus system to monitor and control vehivle functions. All the communications is done in real time.

CAN systen is a more efficient and less expensive (less wires) and has cut down the number of wiring looms used in modern vehicles.

Waveform showing CAN High & CAN Low signal

The two wave forms are mirror images of each other and their base voltages are 2.1V for CAN Low and CAN High 2.6V

CAN High Yellow colour

Can Low Blue colour

CAN High speed (high priority) actuates such things as:

• ABS
• TPS
• ECT
• Brake switch light
• Traction control

CAN Low speed (low priority) actuates things such as:

• Wiper motor
• Alarm
• Central locking
• power windows
• De-fog switch

refrence.
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