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 

Relay

Stop or fog lights, etc. (Single) 

TF1011X

Tail lights only (Double) 

TF1013

Flashers only (Bypass Relay) 

TF110RX

Stop lamps and Tail lamps (3 lamp) 

TF1013

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

TF2302

All lamps (Universal Smart LOGIC Bypass) 

TF2218/7H

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

TFC2

If you need to externally cut off Parking sensors when towing

TFR1