VW POLO CANBUS

If you own a Volkswagen Polo, you more likely that not already know that a Controller Area Network (CAN) bus is an automotive wire network loosely referred to as a bus. The word "bus" comes from the electrical power distribution sector where bus-bars were considered a metalic strip made of copper, brass or even aluminium that served as a source of electric power to the load. 

CAN BUS

However CAN Bus is more akin to Ethernet than a bus-bar. Ethernet is a computer networking technology using Unshielded Twisted Pair cable (UTP) either CAT5 or CAT6 which is now commonly used in local area networks capable of sending  IPv4 / IPV6 packet across its networks at speeds ranging from as slow as 10Mb/s to as fast as 1000 Gb/s, hardware dependent of course. 

Twister pair electrical wires with various colour tracers.

Likewise CAN is a network technology commonly used in automotive networks capable of sending CAN-frames across its network at various speeds, again application dependent. It is essentially a  very reliable multi-master arbitration free serial bus, connecting numerous Electronic Control Units (ECUs) aka nodes together.

CAN BUS vs ETHERNET

The big difference between the two, is that Ethernet is an 8-wire bus comprising of 4 unshielded twisted pairs of wire, each with a specific colour coding, whereas CAN has only a single unshielded twisted pair of wires also with a specific colour coding. The CAT5 protocol insists on 2 twists per centimeter and CAT6 with more twists per centimeter whereas the CAN protocol insists on a 1 turn per centimeter. The lay of these wires are very specific and necessary to reduce or cancel interfering signals picked up from the environment by them, which is more commonly referred to as "crosstalk".

CAN BUS HIGH & LOW

Bearing in mind CAN comes in two varieties used for different functions, viz CAN-High (CAN-H) and CAN-Low (CAN-L).  CAN-H is used for the Powertrain, the Convenience and Infotainment buses. 

Whereas CAN-L is used for the rest of the bus wiring.  Both CAN-High and CAN-Low uses different colour wires for different makes of vehicle. For example:-

Manufacturer        CAN High        CAN Low

Mercedes                  Brown/red         Brown

Volvo                        White                Green

Vauxhall                   Green                White 

BMW 1 & 3             Green/orange     Green 

BMW 5 & 6             Black                 Yellow 

Porsche                     Yellow              Black

The big difference between the three buses for VW,SEAT, Skoda and Audi is that:-

1) The Powertrain bus wires interconnected to all the powertrain modules / nodes are coded Orange & Black CAN-H 

2) The Convenience bus wires interconnected to all the convenience modules / nodes are coded Orange & Green CAN-H 

3) The Infotainment bus wires  interconnected  to the infotainment modules / nodes are coded Orange & Violet /Purple CAN-H 

4) CAN-L bus wires to all the interconnected convenience modules / nodes are coded Orange & Brown. (Electronics colour code 31)

REPAIRING WIRES

CAN wires are typically multi-strand 0.35mm to 0.5mm square with 120 ohm termination impedance, capable of transmitting information using two complementary signals which makes them even less prone to crosstalk. But thin wires are prone to break and if and when they do, it is recommended  that when repairing these CAN Bus wires, that both wires must always remain the same length and of equal thickness. 

Implying CAN BUS is extremely unforgiving. So, when wire 1 of the pair is broken, wire 2 should also be cut and the piece of wire added in-between must be exactly the same length; and that the lay length of 1 turn per centimeter must be observed. 

ELECTRICAL INTERFERENCE

Failure to do so, may created a discrepancy in the wire length of the one wire in the twisted pair as well as in their differential voltages, hence result in network errors — ground noise, electrical interference, hum, buzz,  spark plug spikes — cannot and will not be appropriately cancelled. 

Whenever repairs are made to any CAN Bus wiring, it is highly recommended that all CAN Bus wire repairs are covered and highlighted with yellow insulation tape to signify to anyone doing successive work, that a previous repair was carried out.