Bypass Relay Checklist

The following checks will offer some guidance

  1. In the case of an older vehicle, if you think the vehicle may not require a bypass relay, check the fuses AND wire sizes in the vehicle loom to see whether additional current can safely be carried to supply the trailer road lamps directly. (Determine wire capacities by counting the strands, not by looking at the apparent wire thickness. Thinwall cables may look thinner but have higher capacities than previous "conventional" cables. ( See cable chart.) If you are in any doubt, you are probably wise to fit a bypass relay.

Once you have identified which circuits need bypass relays, use the checklist below.

Reversing Lamps: Capacity is built into the seven way relay TF22128/7H

Vehicle Circuit 


Stop or fog lights, etc. (Single) 


Tail lights only (Double) 


Flashers only (Bypass Relay) 


Stop lamps and Tail lamps (3 lamp) 


SmartCAN multi-compatible CANbus module (Pre - 2019 Vehicles)


All lamps (Universal Smart LOGIC Bypass) 


If you decide to cut out the vehicle's foglamp when towing a caravan or trailer


If you need to externally cut off Parking sensors when towing