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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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Comment Pages

There are 55 Comments to "7 Way universal bypass relay wiring diagram"

  • Alistair Penman says:

    I require the wiring diagram for a C4 Picasso on a 2014 plate ie the newest model out
    I got a mobile Towbar fitter to fit it and the caravan indicators etc only work when the tail gate is open it is a twin socket type I have fitted

  • Pete says:

    again we are not a towbar fitter, however we have seen this problem before. The fitter has not wired correctly, they have looked for power in the boot to make it easy to fit, the nearest place is the switch for the boot (although this is not a suitable power supply) and the fitter has wired the wire to the wrong side of the switch ( the switched side) this only gives power when the boot is open and will have worked when the fitter checked it with the boot open, but as soon as the boot it closed the contact brakes. This would work if the wire is moved to the other side of the switch but I would not recommend this as it is usually not on a good enough supply for the power required.
    I only recommend wiring from a fuse box or direct from the battery with a minimum of 2.5mm cable and a fuse of 15amp fitted.
    I personally would not use a mobile fitter on a modern car due to the coding equipment and specialist tools that are required to fit a towbar. In my experience most mobile fitters do not even carry torque wrenches, and tend to cause bumper damage. The main thing I would point out is they have been working on your wiring system not even knowing how to wire a power supply, interfering with wires. If you were to have a wiring problem and take your car back to Citroen I would expect the work they have done to invalidate the warranty in respect of the wiring system.

    If you are fitting a towbar on a vehicle I would recommend that you only use NTTA QSA (Quality Secured Accredited Companies) and pick a towbar centre these can be found at the following webpage


    Please note there are companies for trailers and companies for towbars on this web page, we are in there for trailers as we specialise in trailers, although I have vast experience of working on Towbars and wiring systems. I would recommend selecting a company that specialises in the job you want doing,

  • Micko says:

    I had a towbar & bypass relay fitted on my Peugeot 207 (2012 model), it buzz’s every time I open a door or use the key fob to lock the car, should it buzz like this?

  • Pete says:

    No this should not buz when you open a door or use the key fob to lock or unlock I have not come across this problem so I can not advise.

  • Ian b says:

    Brilliant info thanks, here goes on fitting a bypass relay…….

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