First we looked at the CAN systems. A ECU is like the brain of the car. It monitors and alters the way a car behaves. Every action taking place in the car for example the radio, the air conditioning, and interior lights are all monitored and done by the ECU. The ECU communicates with these systems using binary code. Binary code is a simple computer language. It uses sets of ones and zeros to communicate. The ECU does this by pulling up or down the voltage. These communications with the different systems in the car requires a lot of wiring. Too many wires in a car can cause the overall vehicle weight to increase. Also having too many wires going everywhere will make it very hard to diagnose a fault and will also be taking up too much space inside the car. To eliminate this problem we use a CAN system. A CAN system is like having mini brains all around the car monitoring and performing set task. This means that now we have a a lot less wiring in the car and fault finding is also less difficult. The ECU now instead of performing every single task now only needs to keep check of the CAN systems; make sure that they are working fine and that there aren't any problems. Think of it as owning a shop. If you owned and ran your shop by yourself you would be running around everywhere and wont be able to run your shop efficiently. Instead you hire workers to perform set task which leaves you to do your job which is to run your shop. The ECU also uses binary code to communicate with all the different CAN systems.
Honda Multiplexing Board Worksheet
In this worksheet we were presented with a multiplex system used by Honda to control body functions the year 1996 till 2002. We used this system to diagnose a fault within the CAN systems. This system had two nodes (control units). Our first task was to identify the pins and wire colours for the communications lines between the nodes using a wiring diagram. The Multiplex control unit to drivers door was Pin A15 - A2 and the wire colour was Brown. From the driver;s node to the passenger node was Pin B1 - B9 and wire colour was pink.
Our second task was to identify pins and wire colours for the earths and the voltage supply between the nodes. The voltage supply wire was pink and the pins were A1, A12 and A24. The ground wires and pins were: Drivers door, black wire pin A12 (G401). Passenger doors were black wire and pin A19 (G551 LD & G581RD).
Then we checked to see that everything was working and we got out tutor to come and create a fault in the system. We were then meant to diagnose this fault. After the fault was created we checked to see what was not working. We noticed that the unlock function on the central locking unit did not work. It would lock but wont unlock. We were asked to use the wiring diagram and analyze the fault. I noticed the switch we were using was a two was switch. I figured that there might not be any power going to the unlocking relay. To check my theory I placed my finger on the unlocking and locking relay and locked the car. I felt a very small buzz coming from the locking relay but when I unlocked the car nothing happened over the unlocking relay. This assured me of my diagnoses of no power going to the unlocking relay. After we analyzed the fault we put the system on diagnostic mode. The nodes had two modes of diagnosis. Mode one was for communications lines between the two nodes and mode two was used to check all the inputs.
When you put the nodes in mode one (communications test) it is meant to beep out a fault code to you. If there are no beeps it means that the systems are working fine. No fault codes were beeped at us so we moved on and put the system into mode two (inputs). Mode two works similarly to mode one, every time you perform a function like putting the window down the system will beep indicating that there is power (input). When we locked the car the system beeped, but when we tries to unlock the car there was no beep. This assured us that there was no power (input) on the unlocking side of the switch. This meant that I was right in my initial diagnosing of the fault (no power going to the unlocking relay). We then looked at which wire and pin number to check for power going to the relay. It was PinA16 and the wire colour was blue.
ABS (Anti-lock Brake System)The purpose of an ABS system is to provide maximum braking force while not allowing the wheels to lock up and also providing steerability while under heavy braking. The ABS system does this by monitoring the wheel speed using a wheel speed sensor and by monitoring how much braking is intended by the driver my monitoring the brake pedal movement. What the ABS does is as it senses the wheel is slowing down and is about to lock up, it releases the brake pressure. This then causes the wheel to spin again as no braking pressure is being applied. When the wheel starts to spin again the ABS system re-applies the braking force, slowing down the wheel. This process is repeated several times within one second. Its this that provides the driver with the ability to steer while under heavy braking. If the wheels lock up then the car would have no steering and no braking.
The ABS worksheet given to us was based mostly around the wheel speed sensor waveforms. These waveforms were obtained using an oscilloscope. The class room had an ABS demonstrator board. We were first meant to identify all the wires that went from the ECU to the wheel speed sensors using a wiring diagram. Each wheel speed sensor had two wires going to it from the ECU. Also we were meant to identify the type of wheel speed sensor that was being used.
Left front ECU Pin # 4 (Brown wire) and 5 (Black wire)
Left rear ECU Pin # 7 (Black wire) and 9 (Brown wire)
Right front ECU Pin # 11 (Black wire) and 21 (Brown wire)
Right rear ECU Pin # 24 (Black wire) and 26 (Brown wire)
The wheel speed sensor being used was a magnetic pick up sensor. The magnetic pick up sensor uses a toothed rotor that spins in front of the sensor. Each tooth of this rotor is magnetized. As the rotor spins and the tooth comes closer to the stationary sensor, the sensor picks up the magnetic field and sends the voltage to the ABS module. The waveform looks like the one pictured below.
We then using the oscilloscope had to capture the waveform for each of the wheel speed sensors.
This is a picture from the rear left wheel speed sensor
We noticed that not all the waveforms were exactly the same. The voltages were different from all the wheel speed sensors but the right front wheel speed sensor had a very high voltage output. This is not a good result as it tells us that not all the wheels are spinning at the same speed. The right front wheel speed sensor send out a voltage of 16.48V (AC voltage) while the other sensors were sending out a voltage between 4 to 6V AC. The reason for this was clear. The right front sensor was mounted significantly closer to the rotor compared to the other sensors. This close mounting position caused the magnetic field between the rotor tooth and the sensor to be higher, hence creating a higher voltage output.
After we used a multimeter to measure the AC volts coming from each of the wheel speed sensors.
Left front 5.44V
Left rear 4.30V
Right front 16.48V
Right rear 6.29V
The multimeter can show us the different voltage being sent out from the sensors but it is not as accurate as the oscilloscope as it cannot show any signs of damage to a teeth on the toothed rotor. On a oscilloscope you get the waveform shown to you and if you have a damaged tooth then you will be able to notice a chance in the waveform however quick it might be. Where as a multimeter wont be able to do this as it only gives you the final out come and it doesn't show you the whole picture.
We then looked at the ABS pump relay waveform and explained what is happening.
Wave A is the ignition power wave (supply voltage) and wave B is the ABS pump relay power wave. When the ignition is turned ON we have supply voltage (Point A). Point A to B is the supply voltage. When the ABS pump relay is grounded by the ECU we have a drop in the power wave (Point B) and as you can see we have power to our relay (Point C). Points C to D indicate the ABS pump on time and so do the points B to F. Point D is when the relay is ungrounded by the ECU hence turning the ABS pump off. This is also shown by point F as the supply voltage is back up. It then takes time for the voltage to reach zero (Point E).
On car ABS
We were given the opportunity to look at the ABS systems on cars. We were asked to point out and identify the different components of the ABS system. This was done under tutor supervision and we were marked off by our tutors once we identified all the components correctly. We were to then identify whether the wheel speed sensor used analogue or digital signals. The car we were doing this on had drum brakes on the rear wheels. The wheel speed sensor was located inside the drum. To find out what type of wheel sensor we had we took out the frontal casing of the drum. I noticed that there was a toothed wheel attached to the frontal casing of the drum. This straight away told me that this wheel speed sensor used analogue signals.
We were then to identify one pair of wheel speed sensor wires and back probe them to get a wave form. To get this waveform we had to put the drum casing and the wheel back on and spin the wheel so that we get a signal. We used an oscilloscope to get this waveform. The waveform we got is shown down below. We weren't able to do scan tool ABS live data test as we were not allowed to drive the cars as they are not registered. And when the cars were stationary all the live data showed was 0KPH.
