3 Wire Trailer Light Converter to 4 Wire Vehicle

This is how to make your own 3 wire trailer light + Ground converter to a 4 wire + Ground vehicle. Are you confused about how these
epoxy filled electronic converter are made ? I am shedding a little light on the subject here ( no pun in 10 did ). I also made a utube
of this, but for those instructables fans, here is how I wired my 2010 Honda Civic to a Harbor Freight utility trailer.

Step 1: This Converter Will Require a Pair of Radio Shack Relays

This project requires the purchase of a pair of Single Throw Double Pole relays, I used the Radio Shack Mini Relays
as they can handle 10 amps and are 12 volts. If you have a lot of trailer lights you will have to size the wire and relay
load to your need. This instructable is more of a how I did rather than a how to. I could not find any information
on the high price trailer light converter kits on the web, so this is how I think they work.

I have built this system on my new (at the time 2004 Honda Civic) and it has served me with no problem, to date.

Step 2: Pinout of Relay

This is the pin out of the relay. I cemented two of them with RTV together so that the coils of each were located in the upper left hand corner and it could be stuffed in a pill bottle to keep anything from shorting the system out while it is in the trunk.

Step 3: Electrical Schematic

This is how I preceived that the system worked. The brake light on the trailer lights are in common feeding the two relays to the normally open contacts. The common for each side attaches to the brake lamp so that both are lighted when the brake is applied through the relay. When you want to signal a turn a wire from the vehicle (independently) turn indicator is applied to the normally open contact and the coil. Each time the coil is energized it sends a voltage to light the respective brake light through the common connection to it. This way you have both turn indicator and signal lamp at he same time. I used the vehicle's fuse to protect the
wiring as I only had one situation where a pinched wire blew the running light lamp.

Step 4: Connecting Wires to Brake Tail and Left Turn Signal

The ground lug for all the Civic's lights are connected to the left side of the rear of the trunk. You will notice a yellow and brown wire together from wiring I had left over from the original trailer kit from Harbor Freight .
The brown wire by itself has a red tape around it
to indicate it is the brake light source. Yellow is the code for left side signal light (think yellow center line on the drivers side) and ( Green grass for the passengers side) code for turn signal and Brown for the running lights.

Step 5: Connection of Right Turn Signal

Green signal feed is attached (in this picture) on the passenger side.

Step 6: Connecting the 4 Wire System to the Relays

Wiring the vehicle to the NO and NC and relay. I used a diode to suppress the colapse of the electromagnet field
when the relay is de-energized. The cathode (banded end) is attached to the the source voltage, and the other
end is attached to the relay common.(white wire that goes to the vehicle ground lug on left side of the Honda Civic)
Note if the diode shorts or is connected in reverse it will blow the fuse. At this point I will say it is optional.

Step 7: Connecting the Trailer Side to the Adaptor

Here is where you connect the trailer lights to the two pins, note the brown wire without the tape connects to the running light and not connected anywhere on the the relay. ( the running lights connect to the smaller filament of the trailer light bulb .) The board in the picture is a jig I made to hold wires while I solder them and to hold electronics parts, sort of the third hand and is not part of the project.

Step 8:

These wire are the trailer lights and the brown wire with the white tape on it is my trailer ground, then you have the brown wire for the running lights and the two signal lights yellow and green.

Step 9:

I connected the white ground wire to the vehicle for the 4 wire side and tywraped the wires and pill bottle out of the way under the trunk
flooring. The wiring, when not being used to connect to the trailer is stored inside the trunk and when the trailer is connected it
just has the trunk lid shut on it. This keeps the plug contacts from becoming weathered and just looks better than having a plug dangeling down.