The first test we had to do was to test the signal coming out of each wheel speed sensor, this is a analogue signal which uses magnets on a toothed wheel which as the magnet passes by the pick-up or sensor voltage is induced. The ABS system needs wheel speed sensors so that the ABS module/ECU knows the speed of each wheel whilst under heavy braking. This is because if one of the wheels starts to lock up then you could lose the steerability of the car or even the stability or traction to the rear wheels of the car. If the ABS ECU sees that one of the wheels is spinning slower than the rest of the wheels then it determines that the wheel is locking up so the ECU will release brake pressure to that wheel to allow it to start spinning again. The ECU can determine wheel speed by the number of cross counts or peaks in the wave form the greater the amount of cross counts the greater the wheel speed.
^ABS Demonstrator wheel speed sensors^
^wheel speed sensor signal (LF)^
^Wheel speed sensor signal(RF)^
^Wheel speed sensor signal (LR)^
This signal has suck a large wave form because the sensor was much closer to the toothed wheel than the other sensors. This would cause a fault code in the system lighting up the ABS light, as this would read a signal that much stronger and it would look faster than the other wheels so the ABS ECU would think that wheel is spinning faster than the others which means the ABS would not work properly on that wheel under heavy braking and it would brake that wheel heavily, more than normal to try and slow it down as it thinks its spinning to fast, which could cause vehicle instability in the rear wheels.
^Wheel speed sensor signal(RR)^
Then there is holding pressure, this is when the inlet valve is closed, so that no more fluid pressure can enter and act on the brake callipers, and the outlet valve is closed so that no brake pressure can go back to the fluid reservoir or accumulator tank. This means that the brake fluid pressure is at a constant pressure acting on the brake callipers.
The last stage is release pressure, this is when the inlet valve is closed so that no brake fluid pressure can enter to the brake callipers to try and act on them. And the outlet valve is open so the fluid can be released back into the brake fluid reservoir or accumulator tank to be re-used again.
The above stages occur many times in split seconds whilst the ABS system is working, to ensure that the wheels do not lock up and allows the vehicle to stop without loosing control safetly.
A basic diagram of the ABS system can be seen below.
http://www.cvel.clemson.edu/auto/systems/images/Hydraulic_modulator.png
The next part to test for the ABS system was the relay powering up something in the ABS system and watch the reading using an oscilloscope, the wave for shows the coil and the switch, the top waveform shows the switch side of the relay when it is switched on and 12 volts is supplied to it. The waveform below shows the coil for the relay, the reason it has its peak and then drops off to zero is because when the coil is first provided power the ABS ECU has not yet provided ground or earth so voltage is at 12 volts on that wire, when the coil is grounded voltage goes to zero as now the coil is using the voltage to create the magnetic field and voltage has to be used up to get to earth, voltage remains at zero even when the coil is switched off as now there is no voltage being supplied to that coil.
The next waveform to measure using the oscilloscope was the ABS pump relay. The top waveform shows the ABS pump and the bottom waveform shows the relay. At point (A) the ignition is on, from point (A) to (B) there is a 12 volt supply voltage to the relay switch, and point (B) to (C) is when the coil/relay is grounded and powering the ABS pump. At point (C) to (D) there is 12 volts supplied to the relay but it is not grounded, at point (D) the ignition is off at point (D) to (E) the coil is grounded again, (E) to (F) is the 12 volts supply voltage. The top waveform shows the ABS pump operating, from point (G) to (H) the Relay is grounded so the ABS pump is operating, after point (H) the pump is off, which takes time for it to fully switch off even when there is no power being provided to it. Point (I) to (J) after point (J) to pump is off but once again it takes time to fully switch off.
What the above waveform is essentially showing is the ABS pump switching on for a set time when the ignition is on and when the key is switched off the ABS pump comes on again. The point in this is that if you you need to do an emergency stop and the engine stalls the ABS pump comes on so that high brake pressure is still maintained to the wheels. This is essentially a fail safe programme for your car so that high brake pressure can be maintained even when the engine is off.
The next part of the ABS testing system was to do an on car test, this involved finding and identifying the wheel speed sensor and then measuring the waveform output to the ABS ECU using an oscilloscope. The wheel speed sensor on the Daihatsu YRV that we used was an inductive or magnetic pick-up wheel speed sensor that produces an analogue signal, these sensors work by using a toothed wheel with magnets on it, as the magnet passes by sensor or pick-up which is a coil that has current and voltage induced into it as the magnet on the toothed wheel passes by, It then relays the information to the ABS ECU and the ECU can determine how fast the wheel is spinning by the number of cross-counts. The signal that the wheel speed sensor produces can be seen below.
These are some of the procedures that you can go through to test the ABS system on all cars, to see whether the ABS system is working correctly.
ELECTRONIC TRANSMISSIONS
The electronic transmission normally uses two shift solenoids to act on or redirect transmission fluid in the valve body, so that they can hold or change gears by having the transmission fluid flowing to the bands to act on the clutch packs which inturn acts on the planetary gear set to give the different gear ratios.
Here is a shift solenoid chart below of the different solenoids that where on, for a 2008 mitsubishi lancer station wagon.
Gear: shift solenoid 2 , L.R solenoid, Over drive solenoid, under drive solenoid
1st: on off on off
2nd: off on on off
3rd: on on off off
4th: off on off on
The next task was to measure when the torque converter clutch came on using a scan tool, the result that I got was when the car was at a constant cruise in third gear or higher the torque converter clutch came on. This is to lock the turbine and impeller in the torque converter together so that a 1:1 ratio is created and the maximum power and fuel efficiency can be created. The torque converter clutch(TCC) solenoid would switch off when the car is decelerating or the vehicles speed is constantly changing as this gives the driver smoother gear shifts. When the brake pedal is pressed the TCC solenoid switches off as it presumes that the driver wants to slow down so their is no need to deliver drive to the wheels any more.
These are the very basics on how a electronic transmission works and how the different solenoids switch on and off at different times to give the transmission its different gear ratios.
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