Sunday, 13 November 2011

Multiplexing Worksheet

                                                    C.A.N Waveform on Oscilloscope

The task to be done was to test and check the C.A.N (controller area network) system on a 2001 Range Rover. This was done by using an oscilloscope to measure the voltage waveforms and a scan tool to check for codes in the controller units or nodes. The purpose of the C.A.N system is reduce the amount of wires going through the car, instead of each actuator having wires going to it now theres only two wires going between nodes (Control units) and then wires going to each actuator which means that wire length is largely reduced since the node is placed in a central location.

The first task to do was to find the twisted wire pair which is the indication to where the C.A.N communication lines are located. The twisted pair is located next to the front left strut tower in front of the A.B.S(anti-lock brake system) module. Using an oscilloscope the voltage wave form was captured to see what code is being sent, as the C.A.N system uses binary code to talk to other nodes, this means that the voltage increases like a digital signal so each node gets codes in the form of 0's and 1's for example to low voltage which is 1.3 volts would a 0 and the high or on voltage of 2.4volts would be a 1. So the code could look something like this 1100010101000111, and the node would decipher this code into something that it can use for example this theoretical code could tell the A.B.S module to activate as heavy braking is being applied.

When setting the oscilloscope to read to the code it had to be made sure that time wasn't to long that aliasing is created, this is when a signal is jumbled as it is happening to quickly to make any sense. So the oscilloscope time per division was 10us (micro seconds) this means that this is a high speed C.A.N system so this is for systems that need to act quickly and are priority like A.B.S systems or traction control and other stability control programmes. Then there is a low speed C.A.N system that has a much slower signal rate that is used for low priority items such as windscreen wipers or interior lights, items that dont need to be fast acting.
The first C.A.N wire in the twisted pair we tested was a yellow wire with a brown stripe, this wire increased its voltage from 1.3 volts to 2.4 volts, this means that the voltage increases to talk this is also known as C.A.N high. Then there is C.A.N low which was a yellow wire with a black stripe with C.A.N low the voltage decreases from 3.7 volts to 2.6 volts to talk or send code to other nodes.

http://www.picoauto.com/automotivetopics/images/canbus_waveform_2.jpg


The reason that the C.A.N wires are twisted together is so that voltage is not induced from other wires that could interfere with the codes being sent down the communication wires which could mean that incorrect information could be sent or information that does not make sense meaning that if you want the windows to open nothing may happen.

C.A.N systems can be scanned for error codes using scan tools just like the ECU, with the Range Rover there are three main areas that the C.A.N system can be scanned there is the drive, the chassis, and the body. The scan tool can be used to read codes and clear codes but it can also be used to remotely control different functions in the C.A.N system for example you could control the electric mirrors and adjust them using the scan tool or you could open windows or switch on the windscreen wipers all using the scan tool. The reason this is done is so that all the actuators in the C.A.N system can be checked to make sure everything is in good working order. The scan tool we used was an Auto Boss, as it had so many more functions than the other scan tools. The next part of the check for the C.A.N system was to list all the different systems that are controlled by C.A.N this was done under the chassis section of the scan tool so this included,

ABS/ASC/DSC (Dynamic stability control) C.A.N High Speed
LEW/LWS (steering angle sensor) C.A.N High Speed
EDC(electronic damper control) C.A.N High Speed
RDC (tyre pressure check/RDW) C.A.N Low Speed
MFL (Multi Function steering wheel) C.A.N  Low Speed


The next part of the test was to put the system into sleep mode however we had trouble putting the system into sleep mode as we probably hadn't shut everything off properly so the C.A.N system remained active however the purpose of sleep mode is to reduce parasitic draw, parasitic draw is when battery voltage is being drained to power different things when the engine or ignition is off. Not all of the modules or nodes are connected to the ignition so there needs to be a signal sent to the nodes to tell them to sleep from other nodes. Any of the nodes can send a signal to tell other nodes to wake up or come into active mode. The CAN low system base voltage goes to a steady level like 5 volts and remains at a constant voltage when the node goes into sleep mode, then when its active the voltage will drop its base voltage to 3.5 volts. CAN high reduces its base voltage when it goes into sleep mode to around 0.5 volts, and when its active the voltage will increase to around 2 volts. These figures do vary for different manufacturers, and for the honda multiplexing board it took around 10 seconds for the board to go into sleep mode however it could take up to two minutes for the C.A.N system to fully go into sleep mode. 

HONDA MULTIPLEXING BOARD WORKSHEET

The multiplexing board we used was C.A.N parts from a Honda Accord that was used from 1996-2002, the board uses two nodes (control units) from the Honda cabin/interior C.A.N system and is used to help us learn C.A.N diagnostics. 

There are two modes that the multiplexing unit can be put in to aid with C.A.N diagnostics, mode 1 is used for diagnostics for communication lines between nodes. Mode 2 is used for inputs, if everything passes these tests this would mean that the fault is an output.

The first part of the task was to identify the plugs/pins and wire colours for the communication lines between the two nodes on the board. 
Multiplex Control unit driver door - brown wire, pin: A15-A2 
Driver node to passenger node - pink wire, pin: B1-B9

The next part was to Identify the plugs/pins and wire colours for the earths and power supply lines for the two nodes.
Ground wires and terminals: Drivers door Multiplex control unit - black wire, Pin: A12-G401, A14-G551 left door and right door nodes, G581

Passenger Multiplex control unit - Black wire, Pin: B22-G501, A8-G401.

12 Volt supplies from battery/alternator - pink wire, Pin: A1 ,A12 ,A24

Once all the pins had been identified we got the tutor to place a fault in the board and see how it affected the operation of the system. The fault we got was with the rear passenger door lock actuator, it would lock (if it was connected to a door) but it would not unlock, meaning that the rear passenger door would never unlock.

Now using a wiring diagram we had to determine what the problem might be and what is causing it to stay locked. After I looked over the wiring diagram I thought that the fault might be no power/voltage going to the unlocking relay of the door lock actuator.  Then we had to put the system into diagnostic mode so we put it into mode 1 and listened to see whether there where any fault codes, fault codes in the C.A.N system are much like check engine flash codes except instead of flashing a code the system will beep the code to you. However there where no codes.

The next test was to check whether there was continuity for the communication lines between the nodes. If there was no continuity between the communication wires then the nodes can not communicate with each other and this could mean that certain things cant work as there is no signal telling the actuator to work or operate. There was continuity and voltage available between the communication lines and there was continuity to earth or ground, this leaves a problem with the input side of the unlocking door actuator. This can be tested by putting the multiplex system into diagnostic mode 2, this is done by switching on mode 1, when mode 1 beeps to indicate that it is in diagnostic mode, switch it off for around 7 seconds and switch it back on again, it will indicate that it is in diagnostic mode 2 by doing one long beep and two short beeps. If all the inputs are working then the system will beep when you switch something on, the system did not beep when it came time to unlock the door lock actuator indicating that there was in-fact an input problem to the unlocking side of the actuator.

This means that my diagnosis of there being no power going to the unlocking relay was correct, as there was no voltage going to the unlocking relay. The tests that could have been carried using the wiring diagram was to check whether there was available voltage for the drivers door key cylinder switch, which is a blue wire coming from the pin: A16 to Pin 1(drivers door key cylinder switch) this means that there was no signal coming from pin A16 to the unlocking relay.

These are some of the tests that can be done to test and diagnose the C.A.N system on most cars from the mid to late 90's and onwards.

1 comment:

  1. Very good Richard, everything looks good here so far

    ReplyDelete