Antilock Braking Systems Lab Workbook and notes (4825)
Off-car Exercises
Some things to think about ABS;
Misconceptions, The ABS system should be simple and robust, with the Module/ECU/Controller almost never giving any trouble.
Below are causes for damaging an ECU;
Spikes by careless welding, MIG welding, without disconnecting the battery this is a debated issue and as a safe working practice always disconnect the battery regardless of what may or may not happen
Enclosure seal damaged and with obvious sign of water ingress this is because water can easily damage components and cause shorts.
Obvious signs of mechanical damage to the enclosure this could damage small sensitive components and cause leaks.
Often the ECU will be misdiagnosed as faulty, usually because the technician is inexperienced in faultfinding.
It’s true that faults are much more likely to be with connections or sensors.
These ECU issues could be incorporated into diagnostic practices to test an ABS
System (if not please advice via blog comments???):
LOOK at the ECU to see if there are obvious signs of mechanical damage or of water ingress,
ASK a more experienced technician to check the ECU for faults such as connections and sensors.
Ordinary scan tools for reading engine fault codes are usually no help when it comes to reading fault codes or doing other diagnostics on antilock brake systems. But Factory scan tools and high end professional scan tools have bi-directional capability that allows them to run various ABS systems self-tests to help detect faults. The scan tool may also cycle the ABS solenoids to make bleeding the brakes easier. Most entry level ABS scan tools have little or no bi-directional capability and cannot run these tests.
Point by point resistance and voltage checks can help you isolate problems with or without codes, but such tests can be very time-consuming and may not always reveal a problem. Dynamic tests are also impossible with this approach because you have no means of exercising or testing the operation of the ABS solenoids and valves. Such components may show the correct voltage and resistance readings, but may not be functioning correctly. So some means of testing their function may be necessary for an accurate diagnosis. That is where a bi-directional scan tool or dedicated ABS tester can make a big difference.
ABS Wiring and Operation
Let’s review the ABS system to remind ourselves what components are included and how they should work.
1 Disc Brake driven by the engine, slowed by the brake
2 Wheel Speed Sensors supply wheel speed data to the electronic control unit
3 Toothed ring/rotor/ pulse ring – spins in sync with the wheel
4 Electronic control unit – operates the hydraulic unit which adjust the pressure to the brakes
5 Master Cylinder provides brake fluid to the hydraulic control unit or just straight Thur to the brake calliper
6 Disc Brake Calliper- applies hydraulic force to the disc to convert kinetic energy into heat energy
7 Brake booster assist the driver in apply pressure to the brake
Wiring Diagram
There are a number of components in the ABS systems that are also used for fuel injection. These are mostly speed sensors e.g. wheel speed and engine speed.
Wire colours for each sensor;
Front right – B W
Front left – R G
Rear left – L P
Rear right – Y BR
The reason for the braided wire on the ABS wheel sensor is to reduce noise and shield the conducting wires.
Fuses that are used by the ABS circuit; Gauge 10A, Dome 20A, Stop 15A, ECU 15A, FL Main 3.0W, F14 fuse box 50A ABS
The earths for the ABS control unit and ABS motor are; E2 EB & EA, BI BJ BL pins 10 & 7 - W B (all white and black) pin3 parking brake - B
On the wiring diagram for the ABS actuator, solenoids control the wheel cylinders.
The wire colours and pin numbers are;
Front Right Wheel
Pin numbers 2, 6
Wire colours R-W R-G
Front Left Wheel
Pin numbers 3, 7
Wire colours L-R, L-W
Rear Left Wheel
Pin numbers 1, 5
Wire colours BR-W, BR-R
Rear Right Wheel
Pin numbers 4, 8
Wire colours G-B, G-Y
WIRE COLOUR KEY
B= BLACK L= BLUE R= RED
BR= BROWN LG= LIGHT GREEN V= VIOLET
G= GREEN O= ORANGE W= WHITE
GR=GRAY P= PINK Y= YELLOW
The correct condition of the inlet and outlet solenoids valves under normal braking are Inlet valve Open Outlet valve Open.
The correct condition of the inlet and outlet solenoids valves when the ABS is operating to reduce wheel brake pressure is Inlet valve Closed Outlet valve Open.
The correct condition of the inlet and outlet solenoids valves when the ABS is operating to hold brake pressure is Inlet valve Closed Outlet valve Closed.
The correct condition of the inlet and outlet solenoids valves when the ABS is operating to increase wheel brake pressure is Inlet valve Open Outlet valve Closed.
In the four cases above the ABS motor will be working only when the pressure needs reducing and increasing.
There are three main types of wheel sensors on modern vehicles. One sends an analogue sign using and inductive pick up the others send a digital signal using a Hall Effect or magneto resistant encoder.
The graph below shows a digital signal that switches 5 volts every 2 seconds.
The graph below shows an analogue signal with a frequency of 0.5 Hz and a maximum of +3 volts.
ABS Demonstrators
The wiring diagram for my demonstrator vehicle – ‘Opel Vectra A 2.0’ is pictured right. The ABS wheel speed sensor, pin out connections to the ECU on the wiring diagram and the demonstrator recorded below are the ECU wires which go to wheel speed sensors:
Left front ECU Pin # 4 and 5
Left rear ECU Pin # 7 and 9
Right front ECU Pin # 11 and 21
Right rear ECU Pin # 24 and 26
By looking at the wiring diagram, the type of speed sensor is an Inductive type, also called Magnetic, or Pulse Generator.
How it works:
• A toothed ring (tone ring or rotor), containing iron teeth, rotates in relation to the speed of the wheel.
• A magnetic pickup has a magnet which changes reluctance as the magnet is near or far from the iron in the teeth of the ring. Iron makes the magnet “feel” stronger; distance from iron makes the magnet “feel” weaker.
• As the magnet gets stronger and weaker, the moving magnet field flux lines generate an AC voltage in wires wound around the magnet.
• The AC signal is sent to the ECU with wires. These are called passive, because the ECU does not have to put out a voltage to make the sensor work.
Using fully operational DIGITECH Dual-Cannel oscilloscope 20 MHz, QC1922 the following recorded waveforms for each wheel speed sensor in the images below were obtained.
The voltage and time per division is 5V/Div and 0.2ms
This is wheel Front Right
This is wheel Rear Right
This is wheel front Left
This is wheel Rear Left
All the waveforms are not exactly the same the waveforms have: no missing waves- no missing teeth, same hight - so same air gap,same frequency so same speed.
With the wheel speed sensors spinning, the AC voltage is measured with a Volt-meter; also the resistance of the wheel speed sensor is measured while they are stationary.
Left front ECU Pin # 4 and 5: 5.5V 0.864K
Left rear ECU Pin # 7 and 9: 4.22V 0.722K
Right front ECU Pin # 11 and 21: 16.96V 0.878K
Right rear ECU Pin # 24 and 26: 6.25V 0.714K
A multi-meter cannot be as accurate in finding problems with the wheel speed sensors as an
Oscilloscope
Because the oscilloscope displays a sine wave that could identify any noise, a chipped tooth and
Discuss what that the multi-meter cannot find and why:
ABS Relays
The name of the relay that powers up the ABS ECU is K38
The name of the relay that powers up the ABS pump is K100
The name of the relay that sends power to the ABS HCU solenoids is K39
Relay wire identification:
The ECU pin number for the wire that brings in the power to the ABS ECU is pin 1
The ECU or other number, for the wire that controls the relay for the ABS ECU is 15 via 86
The pin number for the wire that brings in the power to the ABS Pump is 13
The pin number, or other number, for the wire that controls the relay for the ABS Pump is pin 11
Relay waveform: The following is a captured waveform that shows both the control circuit change when it turns on the relay, and the power switching on to power the pump motor in the ABS system. To do this an oscilloscope with two channels is used to capture this pattern.
What is happened in the above wave form is; the power increased to 12V later relay switches on to 12V then down to 0V over 1.6 second
And when turning the switch off the relay increases to 12V then down to 0V over 2seconds the power reduces from 12V down to 0V.
An interesting fact is that the rate of change is 7.5V/s on and 6v/s off
ABS Pump Relay waveform: The captured waveform shows both the control circuit change when it turns on the relay, and the power switching on to power the ABS Pump. (An oscilloscope with two channels was used to capture this pattern.)
The control relay takes 0.8 seconds to turn the power on.
And it takes 2.0 seconds to turn the power off.
During the ABS Self Test when you first turn the key on. The warning
Lights flash on and power tested with the oscilloscope shows the
ABS system during the Self Test the power is turned on, off, on then off.
This is because each time the vehicle is driven, as soon as vehicle speed reaches approximately 20 km/h, the anti-lock brake control module turns on the pump motor for approximately one-half second. At this time, a mechanical noise may be heard. This is a normal function of the self-check by the anti-lock brake control module.
When the vehicle speed goes below 20 km/h, the ABS turns off.
Most malfunctions of the anti-lock brake system and traction control system, if equipped, will cause the yellow ABS warning indicator to be illuminated.
A fault is created in the system by slowing down a wheel and affecting a speed sensor while you are applying the brakes. As the brakes are applied, the ABS pumps turns on, solenoids turn on, the hydraulic pressure changes in the brake lines and shows up on the pressure gauge as large fluctuations up to 180psi this shows the ABS is allowing fully pressure from the master cylinder to the wheel that is not skidding. The other wheel pressure gauge shows a drop in pressure down to 60psi to prevent skidding on the wheel that has lost traction.
An Oscilloscope gave a noisy pattern when an ABS solenoid was actuated. The pin number is 5 colour green or 7 yellow of the solenoid. To do this test paper clips are connected to the correct pins or into the HCU pin 7 or 5 for the left wheel speed sensor, to an oscilloscope, while the brake pedal is held down and the wheel speed sensor slowed down to represent a skid or loss of traction on the road surface. As a method of getting a more reliable wave form a digital scope meter ‘Senintek S2800’ was used. This gave the picture below, which clearly shows the abs in action as it quickly turns the solenoid off and on to create the pumping effect that prevents the wheels from locking up, this can be felt through the brake pedal as a series of vibrations.
This pattern is produced by the solenoid switching on and off and has a spike of back EMF of 24V
This pattern is produced by the solenoid switching on and off and has a spike of back EMF of 24V
