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.
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.
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.
These correspond to the pins in the trailer plug socket and are wired as shown below.
Click the image to enlarge it.
To purchase trailer parts and accessories, including tow bar wiring components, please visit towingandtrailers.co.uk :
There are 15 Comments to "7 Way universal bypass relay wiring diagram"
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.
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.
Thanks.
do i need this bypass relay for my suzki grand vitara
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.
Thanks.
do i need this relay for hyundai IX35
Hello Rob,
Please see my last comment on this topic.
Thanks.
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.
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.
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?
Thanks
Simon
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.
Thanks.
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!
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.
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.
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?
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.