DIY Towbar Wiring Harness - Alfa Romeo Forum
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Status: Do it wrong, do it twice. Do it right, do it once.
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Alfa 156 2.0 JTS

Boxing DIY Towbar Wiring Harness

I have decided to attempt to describe how to make an Alfa Romeo safe trailer wiring harness in a way that hopefully most readers will be able to follow. If you wish to make one you must know how to use a soldering iron and should probably have a basic understanding of electronics.

As always proceed at your own risk. But test and seek advice to mitigate this risk.

Background Reading

This link: may be helpful in providing some background knowledge of relays.

The only relays you need to know about are SPST (single pole, single throw) and SPDT (single pole, double throw). Single pole means that there is a single input or switch as it were. Throw refers to the output; in our case you only need a single output for a single input so that as a light in the car goes on the corresponding one in the trailer turns on. The outputs are specified as NO (normally open) or NC (normally closed) for light signals we want NO which means that the trailer lights will turn on when the car’s turn on if NC was chosen the opposite would happen. A NO single throw output is ideal but a double throw can effectively be turned into one if the output is correctly chosen.

Some reference electrical symbols can be found at this link: Electrical Symbols | Electronic Symbols | Schematic symbols

Trailer wiring conventions can be found at this link: trailer wiring diagram

There are also only two types of relay you want to consider. Firstly there is the silent Solid State Relay; these are good for the indicators as they will not make a ticking noise however they are a lot more expensive. Secondly there are the traditional automotive coil relays which do make a low tick when triggered but are a lot cheaper for any given power rating; these are great for driving lights and good enough for the brake lights.

To work out the maximum power rating (wattage [W]) of a given relay multiply its rated DC voltage [VDC] by its rated current (amps [A]).

Choosing the relays

Depending on the number of lights on your trailer and their wattage you can effectively work out the specifications of your relays. Have a look at what bulbs you are running as they should state there wattage. Your relays should be rated 2 to 3 times higher than the combined wattage of all the bulbs you are running as filament bulbs have more watts running through them when they are first switched on; put it this way if you want to be really safe you could go 5 to 10 times higher.

Only use DC relays rated to the voltage of your battery.

You will need four relays, one for the left indicator, one for the right indicator, one for the brakes and one for the driving lights.

I have used a dual relay in my example which is effectively two relays in one unit. My left and right indicator relays are rated to 2A at 12VDC and my brake and diving light relays are rated to 20A at 12VDC.

WARNING for solid state relays observe the polarity stated on the relay itself or corresponding spec sheet before wiring.

Making the Wiring Harness/Interface

You are going to want a sealed box to put the components in, some trailer wire containing 5 coloured cores, a wire long enough to reach a 12v supply (16 gauge thickness would probably be sufficient), two cable glands to prevent the wires being pulled out of the box and of course the trailer connector itself.

Cut the trailer wire into two pieces, one leading to the car (signal loom) and one leading to the trailer (trailer loom).

Wire one pin of each of the input poles on each relay to a ground wire which is to be grounded to the car (choose the white wire in signal loom).

Wire the common pin of each of the output poles on each relay to a single positive wire which is to be connected to a power supply with an appropriate inline fuse e.g. lighter socket, behind an unused fuse holder e.t.c.

All the wires in the trailer loom with exception of the white wire are to be individually wired to the NO (normally open) output pin on their assigned relay. The white wire from the trailer loom should be linked with the white of the signal loom and to the relays as described before.

All the remaining wires in the signal loom should be wired to the other input pole on their assigned relay.

The easiest way to find the correct wires to connect the signal loom to in the car is to work from the tail light bulbs back down the wires until you find favourable point at which they may be cut.

Wiring Diagram

"See first attachment"

Installation Schematic

"See Second attachment"


It may pay to test your newly made wiring harness using your car battery and a spare bulb. If you understand how it works and have made it testing should be straight forward. Just remember to observe the polarity on the solid state relays otherwise you may burn them out.

New Zealand based Materials/Components

Sealed box: Sealed ABS Enclosure - 115 x 65 x 40mm - Jaycar Electronics

12V, 2A DC Solid State relay: PCB Mount Solid State Relay 12V - Jaycar Electronics

12V, 20A Dual Coil relay: Dual 20A SPDT PCB Relay 12V DC Coil | Global PC or two 12V, 20A Single Coil relays: 20A 12V SPST PCB Mount Relay - Jaycar Electronics

Power Cable: Red 25 Amp DC Auto Power Cable - Jaycar Electronics

Two Cable Glands: 5-10mm DIA IP68 Waterproof Cable Glands - Pk.2 - Jaycar Electronics

Fuse Holder: Jaycar Electronics - Search results sz2040

Fuse: 20amp blade fuse should do

Total Cost: around $100nz

I am open to all advice and criticism on this one, but I can tell you I have done this to my car and it works well
Attached Images
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(Mechanical Engineer + Mechanic - Alfa)*time = ln(Bad News)

Last edited by MGG; 27-01-12 at 08:08.
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