7 Way universal bypass relay wiring diagram

Many modern cars have Multiplexed or Can bus wiring systems. Wiring systems such as Can bus were developed in order to be a simpler and more cost effective alternative to the traditional wiring harness.

This advancement in wiring has also led to the development of various fault finding and diagnostic systems. One fault finding system sends small test pulses to all of the lights in the system to identify faulty bulbs. If a faulty bulb is detected the diagnostic system will display a symbol on the dashboard, alerting you to the fault.

This system can cause problems when trying to fit a tow bar, LED lights, or even when towing a trailer that has been fitted with LED trailer lights. Unlike regular bulbs, LED lights are so sensitive they will pick up the small test pulses through Can bus and flash constantly. As well as constant flashing, LED lights do not have sufficient ‘Load’ enough to trigger the diagnostic system and dashboard warning symbol.

This can be avoided by fitting an expensive manufacturer approved plug-in wiring kit when your tow bar is installed. This should resolve any problems with the diagnostic or fault finding systems, but it won’t be cheap.

Fitting this Universal Bypass Relay to your car between the wiring system and trailer plug socket bypasses all of the fault finding and diagnostic systems created by Can bus. This basically leaves you with a standard 12v power supply. This allows LED lights to function properly, without flashing, at a the fraction of the cost of a dedicated plug-in kit.

This relay isolates the 12v power supply and bypasses any fault finding or diagnostic systems after the relay. A fuse should be fitted to the relay’s power supply so in the event of a fault or short circuit your car remains unaffected. Please be aware that no system on your car will alert you to faults with your trailer’s lights while using this relay.

7 way universal bypass relay

On the left hand side of the relay are 8 colored cables, a 12v power terminal, and another terminal marked TT. The 12v power terminal requires a dedicated feed (with fuse) directly from the cars battery, while the terminal marked TT is for an optional ‘tell tale’ dashboard light.

7 way universal bypass relay - left side

The 8 colored cables have the following functions:

Yellow – Left Indicator
Blue – Fog Light
White- Earth
Green – Right Indicator
Brown – Right Tail / Side Light
Red – Brake / Stop Lights
Black – Left Tail / Side Light
Gray – Reverse Light

On the right hand side are 7 terminals marked 1, 2, 4, 5, 6, 7 and ‘R’. There is no terminal 3 as this is an earth wire which must be secured to the vehicles bodywork.

7 way universal bypass relay - right side

These correspond to the pins in the trailer plug socket and are wired as shown below.

Click the image to enlarge it.

7 Way Smart Relay Wiring Diagram

To purchase trailer parts and accessories, including tow bar wiring components, please visit towingandtrailers.co.uk :


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43 comments on “7 Way universal bypass relay wiring diagram

  1. Peter Roberts on said:

    Does this relay have a cover please?

    I have recently had a scooter rack fitted to my motorhome and one of these bypasses has been fitted inside the chassis member. It seems as all of the wiring and terminals is/are open to the elements which of course may lead to a premature failure.

  2. Hello,

    This relay is not waterproof because it is intended to be fitted inside the vehicle, like in the boot of a car for example. If your vehicle does not have a boot, I would recommend finding a small, dry space under the bodywork where the relay can be protected from the elements.


  3. do i need this bypass relay for my suzki grand vitara

  4. Hi,

    It is recommended on all cars as it isolates the power supply to your trailer lights and can prevent fuses from blowing, although it is only necessary on cars with Can Bus or Multiplex wiring systems.


  5. do i need this relay for hyundai IX35

  6. Hello Rob,

    Please see my last comment on this topic.


  7. John Taylor on said:

    Hi, I have a motorhome and have had a smart relay and loom fitted for towing, I have only recently noticed that the relay appears to click (as if it is switching on / off) every 2 to 3 minutes when stationary. Is the NORMAL as I cannot remember it clicking when we had if fitted approx 10 months ago.

    Thanks for any information. John.

  8. Hello John,

    Try checking the power feed to the relay, maybe it has come loose or has a bad connection. Alternatively, this relay has an optional ‘tell tale’ buzzer for your indicators, which does make more of a clicking sound than a buzz. It could be possible that this ‘tell tale’ feature has been wired incorrectly and is clicking randomly.

    The most important thing is that your lights are still working. If all of your lights are working correctly, I would not worry about it too much.

    Thanks, Kris.

  9. Hi,

    I am wondering what happens to the reverse light as not part of the 7 pin plug. Does it go direct into the s type plug or does it need another relay?



  10. Hello Simon,

    The reverse light is not part of the 7 pin ‘N’ type plug/socket as it is not necessary on smaller trailers here in the UK. If you are towing a caravan on the other hand I would recommend wiring your reverse light into pin 1 of your ‘S’ type plug/socket. Though the best solution would be to install a 13 pin Euro plug/socket as that would be able to do everything in one.


  11. Sam Tait on said:

    thank you for this page its a great help just trying to fit one to y 1997 Volvo 940 which im guessing needs one as has the bulb fault circuit

    wish me luck!

  12. Derek Nixon on said:

    I have just bought an Elddis Motorhome 2009, I would like to fit a tow bar and socket along with a reversing sensor. Can I tap the reversing sensor feed wire into the reversing light terminal on the back of the towing socket on the motorhome. I will not be towing often but need the sensors for insurance purposes.

  13. yes the Reverse light feed for the towing socket is fine to use on most towbars if fitted with a bypass relay, or wired direct into lights but some Dedicated wiring kits that can be fitted to vehicles do not turn on until a load is detected these systems will not work.

  14. Hi i have a toyota auris on a 58 plate does this have multi plex or canbus? or can i use a simple audio buzzer and solder my 12n wires to the correct lights in the vehicle? as the specfic wiring kit is £130!,or is it just wurth buying one of these for piece of mind?

  15. with the age of this car I would not recommend just fixing a buzzer as Modern cars are too complex for this and the wiring is not designed to take the additional load. you can fit the vehicle specific kit this will integrate with some systems on your car. However you may encounter problems if your trailer runs LED lights. if you would like to tow anything with LED lights you can get a bypass relay this bypasses all the systems that work with a vehicle specific kit but works fine with LED lights.

  16. Hi I have a vauxhall Astra cdti 57 plate I’ve fitted a bossal tow bar and have the vauxhall wiring kit but have no fuse box in the boot to plug it in . Is there any way I can get around this ? Don’t want to hard wire it up incase it upsets the canbus any ideas ?

  17. frank craven on said:

    i have just fitted a tow bar to a volvo xc70 2006 will a 7 way [ teb7as ] relay be ok for this car ; volvo dealers want £ 140 for fitting modems and software thank you

  18. Kevin on said:

    Will I need a bypass relay or just an audible warning when fitting a tow bar to my 2005 Ford Transit Connect van?

  19. I have one of these relays fitted in my audi a4 and the warning light for my right brake keeps showing up in my dashboard. Any ideas how i can fix this? Thanks

  20. Mark Smith on said:

    i have just had a late 11 plate passat estate. i will be fitting a tow bar shortly, will i need one of these 7 pin relays? My current caravan does not have led lights.

  21. Richard on said:

    Hi, I want to tow a Caravan,2009 13 pin, with a VW T5 Autosleeper Motorhome.57 reg.

    Can I fit a split charge relay to charge the Caravan Battery while towing? as well as the one fitted in the van, or will they conflict with each other. or would I be better off just wiring up for road lights only?

    Thanks Richard

  22. A split charge relay should be fitted in the tow vehicle this should do what you require the charger in the caravan should operate from the 240 supply.

  23. Pete on said:

    due to the Systems on this vehicle I would recommend either a bypass relay or a dedicated wiring kit you can not wire straight in to the lights. on the Passat the rear lights (stop/ tail) are led this only has 1 wire feeding it you will need to connect the red on the relay to this and the brown and black will not need connecting. the red will detect the voltage going through the cable and light the side lights and brake lights accordingly.

  24. Pete on said:

    you did not mention the year and the type of lights fitted to the car.This is not a problem I have seen before but if it is anything like the passat which is from the same group the side lights do not want wiring up you just wire the brake lights. this is because the brake and side lights are led and run from 1 wire.

  25. A buzzer would work with the old shape transit but I would always recomend a bypass relay as it would protect the wiring on your vehicle and not overload any of the fuses in the vehicle if you ever have a short.

  26. This should be fine for this car.

  27. This is what the bypass relay is designed for. when fitting I always wire direct from the battery. this can take some time. but you know that you are not disturbing any systems on the car.

  28. Alan Henderson on said:

    My car is a 2006 Merc E class estate with led stop/tail, can this bypass relay be connected direct with the red wire to the single light feed or does it have to be wired to the light switch.

  29. Alan Henderson on said:

    I’ve checked the car handbook and the stop/tail lights are described as HiP which i believe are gas pressurised, is this the same as led and should they be connected to the teb7as as per the instructions for single wire stop/ tail lights

  30. Pete on said:

    I have not heard of HIP bulbs there are HID (High intensity Discharge) these are usually used in headlamps due to there brightness. the TEB7AS should be used Stop only if lights are powered from 1 wire or if it is powered on 2 wires then wire 2 both wires in.

  31. Pete on said:

    the stop wire only (red) should be connected to the single light feed

  32. STUART on said:

    What size of cable should i run for live feed from battery to the TEB7AS ?

  33. you require a cable that will take at least 15 amps I use a 2mm cable

  34. hello
    i have fitted a 7 way bypass relay to an 04 vectra
    fog/indicaters/reverse are as i expect and work as they should
    however the tail/brake has me scratching my head
    seems i have 2 wires at each of the lamp assys that emit a lower voltage on ‘tail’ and the same 2 wires emit a higher voltage on ‘stop’
    which of these wires require conecting and in what fashion to make the relay send a simple 12v signal to each of the tail stop circuit when required
    many thanks

  35. Hi Bryan

    on newer cars with LED lights you get one power for stop and tail this is 2 different voltages.
    the way it needs to be wired in these circumstances is not to connect the Side lights (black / Brown) and the Brake light will detect the voltage differences and light the side lights with the low voltage supply.

  36. Can someone please advise me what 7way bypass relay and splitter charge relay is suitable for my 2008 Skoda Octavia Vrs …. as I have had my installation done by some muppet purporting to be a towbar specialist. I have a new caravan 2013 model with a 13pin connector but since he wired it up my o/s side lights will not work or the rear fog lamp on the caravan, also the caravan fog lamp comes on when connected then goes off and on again almost like it’s pulsating without switching the car lights on , all bizarre really o0 …….Many thanks

  37. Hi
    You can fit most bypass relays or split charge relays.
    when you mention pulsing it sounds like the caravan has LED lights but you do not mention this. this could be a standard plug in kit fitted to he car that is trying to check the lights and the LED lights pick the pulsing up. this has been a common fault. but over the last year or so most manufactures of wiring kits for tow bars have managed to sort the problem out.
    you should be able to use a bypass relay (cheap option ) and remove the plug in wiring kit this will bypass a lot of systems used on tow car such as TSP fault checking the lights and any other systems your car has.
    or the more expensive fix is to use a plug in wiring kit that is compatible with LED Lights.

    We can supply either of these solutions
    Tel 01909473749

  38. jean paul lacan on said:

    I connected the SMART WAY MULTIPLEX 7, everything works on the trailer peise
    the buzzer beeps continuously that the housing is supplied,
    in flashing mode, the buzzer works well
    why continuous beep (schema wiring meets)

  39. Hi I am assuming that the buzzer is just continuously sounding remove all wires except the power supply then see if it still sounds. if so there is a fault with the relay I have known this. if it does not sound then start to add wires 1 at a time. when you find the wire that starts the sound then investigate the circuit on the car or trailer.
    Hope this is of some help.

  40. Hi,
    Does this unit have an internal buzzer? As I only want a dash light supply.
    Thanks J

  41. Yes it does unfortunately.

  42. Neil on said:

    I have a ford transit 2008 and want to use a bypass relay, do you need to break into the rear tail light loom, or does it plug in, and on which side is it best fitted on.

  43. Pete on said:

    These are a universal relay and they do need to be cut in to the wires.

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