Honda multiplexing Board and CAN signal pattern

This is a low speed CAN multiplexing used for body control in Honda. It utilizes a 3-node control units for driver's door, driver's side and passenger side. Now the basic of electronics is always Input-Control Unit-Output. Talking about body control means window motors, lights switches & indicators, wipers etc... So on the board we also have parts that are used to simulate the car's body, this one particularly is door control.

The testing procedure to see whether if this CAN multiplexing system works properly on window motors or not, is divided into 3 level: Communication lines and nodes(mode1); inputs(mode2); mode3: must be output. Note that these are not exactly how we should diagnose a real car body control unit, but similar in concept.

To proceed into mode1, basic preliminary check must be done, using the provided wiring diagram, we must identified all the pins numbers and wire colors for the communication lines between the nodes, and between power supply to each modules and their earths.

When a fictional fault is created, we don't know the fault or having any clue so best is to check every buttons(inputs) to see if they are OK. When a fault is spotted that a rear window motor on the driver side only spins one way(window up) but not the reverse, test procedure 1 is now carried out.

The board is designed so that we have a fault light that doesn't light when an input or communication line is malfunction. All we need to do now is try switching the window down button that wouldn't work. If the test light comes on then we haven't found our source yet. But if it stays off, that gives us a clue to recheck the communication wires from  nodes, to the motor's relay, and that means the power & earth side as well. Of course, when checking the faulty window button, re-checking other input buttons is not a bad idea, other faults could also be found and it will even give us more clue where to look next.

Mode 2: this is the input check mode - that means if there is no continuity between door switches and the power side of the motor, that the switches are malfunction, then the check light should not light and there will be no BEEP!.....The switch beeps the system, so we had to check the relays and the motor's wiring.
Because each window uses 1 motor, so that motor will be wired so that the current will flow backwards when going up or down. Finally, we had to trace to the motor and reconnect the reverse power wire to ground. And we re-check the rear reverse motor, it works again.

The CAN signal pattern.

 What you see is a standard awake multiplex signal...with aliasing. As mentioned on the Land Rover CAN system earlier, aliasing is when the "ghost signal" overrides the "real signal". When observing, it is important to set your oscilloscope to a suitable time and Voltage division, to be able to spot aliasing as easy as possible. The ghost signal tends to flash over the real signal, which seems to stay more solid, and you have to observe over a period of time. So when you are certain which one is which, use your own visual processing to choose the right signal patterns you capture.

 Unfortunately, the CAN signal pattern is not as easy to obtain as any other diagnostic codes, because they communicate really fast. Still, we can identify the difference between an "idle"(not idling engine) awake signal pattern and the switched(communicating) pattern, well, by focusing.

 When switches(a brake light switch), the pattern will change a little bit. And of course, the high CAN and low CAN always mirrors each other.