By-pass relays may be used on the modern motor vehicle where a vehicle specific kit is not available. Also, some vehicles that predate the computerised canbus system may even have a bulb failure monitoring system which requires a by-pass relay system.
They are used because vehicle manufacturers have reduced the size of the wires to provide only enough capacity to run the vehicle requirements with no allowance for any additional lighting. They use one wire to do two separate jobs. It does this by sending rapid pulses at different frequencies, so for example your side lights would operate at a low frequency rate of pulses, and the brake lights at a higher frequency. This enables manufacturers to keep weight down by reducing the cable required in a vehicle. Bulb failure is also monitored in a similar way.
The modern computerised vehicle lighting wires do not have the capacity to supply the lighting for your caravan or trailer, hence the use of by-pass relays. These relays essentially take these small signals from the vehicle and translate them into a conventional voltage supply for the trailer or caravan.
It is worth bearing in mind that these small frequency pulse signals can be shut down by a modern canbus vehicle when a corresponding bulb fails. If, for example, your tow vehicle rear brake light failed, the canbus system will shut down that particular circuit and the by-pass relay will not receive a signal to activate the trailer or caravan lights!
The by-pass relays essentially isolate the towbar electrics from the canbus towing vehicle. This is the down side, your canbus vehicle will not know it is towing!
All the features which would have been available with a vehicle specific kit are by-passed, including important safety features, like TSP (Trailer Stability Program).
Where a vehicle specific kit is available this should be your first choice and is our recommendation.