Multiplex System

This particular multiplex system was used for body controlled functions on the 1996 to 2002 honda accords and 1996 onward honda preludes. It utilises three control units. The drivers door node, drivers side node and passengers side node.

This system works similar to CAN systems where the nodes communicate between each other in order to actuate the specified component.




Multiplex system: note the two white nodes are passengers side node and drivers side node. The drivers door node is at the bottom containing all the input switches


The testing procedure is divided in to three sections. Communication lines and nodes (mode 1), input (mode 2) and if the system passes these tests but is still faulty, the problem is most likely to be an output.

Using the wiring diagram that was provided I have identified the communication lines as; MCU(door) terminal A15 to MCU(drivers side) terminal A2 is a brown wire, MCU(drivers side) terminal B1 to MCU(passengers side) terminal B9 is a pink wire.

I have identified the earth and voltage supply lines as follows;

From the under-hood fuse/relay box (No. 54) up the yellow wireto the passengers under-dash fuse relay box (No.13). Pink wires connect this to the MCU(door) at terminal A1, the MCU(drivers side) terminal A12 and MCU(passengers side) terminal A24. They are then each earthed out black wires through MCU(door) terminals A12 and A19, MCU(drivers side) terminals B11 and A14, MCU(passengers side) terminals B22 and A8.

After a fault had been created via simulated fault switches, diagnosis went ahead;

All the windows work as normal. Drivers side windows operate normally, yet the drivers side passenger interior door light switch is not working/illuminating. The connection from the MCU(drivers side) terminal A22 through the drivers side passenger door light switch to earth is posibly shorting before the switch. The switch is normally open (when door is closed), yet it is behaving as though the switch is closed (thinks the door is open).

When testing further the system is put into mode 1. No codes are given. So communication lines and nodes are fine. They are communicating between each other, this is because the short/fault is occuring after the module. I was expecting this.

Voltage is tested to ensure proper communication is happening. MCU(passengers) at terminal B9 = 7.59v and MCU(drivers side) at terminal B1 = 7.59v. MCU(drivers side) at terminal A2 = 7.32v and MCU(door) at terminal A15 = 7.32v. This concludes that all communication is good.

Further testing brings mode 2 into play. And as presumed the fault is isolated as 'drivers rear door light switch'. So the final conclusion is that the fault is preventing signal from the input door switch to the drivers side module. Because the drivers door light switch is operating normally, this means the output (light) is not the problem.

The MCU(drivers side) terminal A22 green/yellow wire shall be tested and replaced accordingly.

Sleep mode on this system allows it to stay live for a few moments after the car has been turned off completely.