I’ve owned a few cars in the past, and all of them except my current one used relay boards and wires that ran through the car. I’ve recently repaired the driver’s door loom and patching cables is a bind, so what happens now?

Relay control

In older cars, when you pressed the demister button, it would apply power to a relay which would close a much larger circuit and the demister would turn on.

It doesn’t take a lot of power to control a relay, so the wires to the switch could be very thin.

Unfortunately, if a relay failed then that part of the car would stop working.


The CAN BUS (Controller Area Network) was designed by Robert Bosch and removed the need for 50kg of wiring in the average car. The first car with CAN BUS was the BMW 850 coupé in 1986, and in 20 years, 70% of the cars on the road had CAN BUS. Just 2 years later, the SAE (Society of Automotive Engineers) required all cars to be fitted with CAN BUS.

Each device communicates across two twisted wires that connect every component of the car together, operating at different speeds depending on whether they control the engine (the fastest speed); body functions and diagnostics (the lowest speed)

When you press a button in a modern car, it sends a signal to the BCM (Body Control Module) which passes an instruction code down the twisted pairs to it’s destination, where a module converts it to an action.

CAN BUS versatility

CAN BUS is a robust system, as well as being simple. Everywhere you imagine information that needs to be passed around, CAN BUS is used.

Think of aviation; agricultural vehicles; industrial automation; military vehicles; marine and even in space. There are open standards which are used for hobbyists and robotics projects.

CAN BUS Security

CAN BUS is a low level protocol, which relies on physical security of the environment it works in. It has no security protocols built in.

Cars have been hijacked when the manufacturer has hooked their ICE (In Car Entertainment) system into the CAN BUS and allowed the entertainment system full access to the CAN BUS.

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