B-ManFX4

For everyone that thinks there is some kind of mysterious magic that allows the truck to determine if a trailer is present, the truck simply detects the trailer being present by current flowing in the lighting circuits.

Fun fact - place a 100 Ohm resistor across the left brake lighting + and ground, a 100 Ohm resistor across the right brake lighting + and ground and a 100 Ohm resistor across the running lights + and ground and the truck will tell you there is a trailer connected. A higher value resistance might work - I just grabbed some 100 Ohm units because I figured it would look like a LED light to the truck and they won't get hot.

Note - If you do this exercise with the truck running (like I did) remember that you have to get in the truck and put it in Drive before it will detect the trailer.

I made a 7 pin connector with the resistors inside so that the truck would enable the +12V aux output and I could use it to power my portable compressor, aux lighting, etc.

Works like a champ.

abideejay

What wattage resistor did you use?

babock

The ones in the picture look like 1/8W but if you stepped on the brakes or turned on the lights, those resistors will pop in a second! They need to be 2W resistors if you use 100 ohm

BlueSpark

Does it detect a tailer if you draw a current across reverse only?

Sawdust102

So how does it detect a trailer when plugged in when no lights are on?

Note on the 2018 no need to drive or press the brake pedal. Plug in and it shows connected.

rliles1

The trailer connector does not have a reverse wire. The 7 wires are (note that numbers 1 to 4 are the 4 pin wires):
1. right turn signal/brake
2. left turn signal/brake
3. tail lights/marker lights
4. ground
5. electric brakes
6. 12v+
7. auxiliary (can be used for reverse lights, or anything else.)

I don't know the answer, but can venture a guess. It has a continuity test between one (or more) of the light circuits and ground. A continuity of less than infinity indicates a trailer is connected. Kind of like a GFCI outlet testing for a ground/neutral bond even if the appliance is not turned on.

MarkBinSC

The center pin on the 7 pin is hot only when in reverse. It does not set off the trailer alert on my 2016. That is where I am getting the current to run my auxiliary back up lights.

B-ManFX4

I used 1/4 watt because I was just demonstrating the concept for the forum. If you wanted to actually build the plug adapter like I did, you would need to use much larger wattage resistors. I used 5 watt units in my plug adapter.

Not true - the center pin is hot ONLY when the truck is in reverse and always goes hot when the truck is in reverse, with a trailer connected or not.

babock

You are correct. Many vehicles have this pin as reverse.

rliles1

Ford may do this. Many vehicles may do this, but it is by no means a standard.