Trailer & Bike Rack – The Bypass Relay

What is a Bypass Relay?

Since about 1990, most cars have been controlled by some form of electronics. You used to be able to connect a trailer electrics directly into the wiring loom, but with modern cars, this is no longer the case. This is where the bypass relay comes in…

A bypass relay is a little plastic box containing some electronics. The electronics operate a switch or a bank of switches (relays). These switches (relays) are used to switch on or off the trailer or bike rack lights.

The bypass relay acts as an isolator between the tow bar electrics and the vehicle’s electronics. The bypass relay prevents the vehicle from thinking there’s something wrong, (the trailer or bike rack wiring placing additional load on the vehicles electronic system) and getting upset about it.

Vehicle Trailer & Bike Rack – The Bypass Relay
Vehicle Trailer & Bike Rack – The Bypass Relay

How does the Bypass Relay Work?

The bypass relay is connected to the vehicles wiring loom. The connection(s) operate the relays (ways) inside the bypass relay. These relays then operate the trailer or bike rack lights. The trailer or bike rack lights are completely isolated from the vehicle lights and ‘talk’ to the car via the bypass relay. This is so:

  • You don’t interfere with the vehicle’s electronics (the electronics can be sensitive to any changes).
  • You don’t place an additional electrical load on the vehicle electronics (there is an additional load, but it is very small – in milliamps).
  • Any electrical faults on the trailer or bike rack, don’t cause further issues with the vehicle.

The bypass relay has to be fed by its own 12 volt supply from the vehicle’s fuse panel. This supply feeds the lights on the trailer or bike rack.

As an example – When a vehicle light is switched on, the bypass relay senses this, activating the appropriate switch (relay). This relay then supplies 12 volts to the corresponding light on the trailer or bike rack.

The Bypass Relay - How it Works (1 Way)
The Bypass Relay – How it Works (1 Way)

Which Bypass Relay Should I buy?

A bypass relay comes in a variety of shapes and sizes:

The main criteria for buying a bypass relay is the number of ways (or individual relays, switches) it has. The number of ways will be dependant on what you want to control. A typical trailer or bicycle rack will need a 7-way bypass relay (see the table below).

No. of Ways It controls the…
1 way A single light, or set of lights.
7 Way Rear lights (pair).
Rear brake lights (pair).
Rear left indicator (single).
Rear right indicator (single).
Reversing light(s).
Fog light(s).
Number plate light(s).
Table illustrating the number of ways required.

Hints & Tips:

  • Check if the bypass relay comes with a connection cable. This can save time and money, as the cable supplied should be of the correct size and specification.

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