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How do you fool a vehicle in to thinking it has a trailer with electric brakes attached? Answered

I am working on a vehicle and trailer testing box. I would like to have a circuit built in to it that makes a truck think there is a trailer attached and will send out voltage on the wire for the electric brakes. This is so I can have a volt meter measuring how much voltage the truck is sending and make sure it is linear. Would hooking up two 6ohm load resistors in parallel to the blue (brake) and white (ground) wires off of a pigtail plugged in to the 7way plug on the truck make it think there is a trailer connected and therefor send voltage, or does the truck sense a trailer hook up a different way?


Jack A Lopez

Best Answer 1 year ago

That sounds like a good guess to me. I mean, this idea of just putting a resistor in the place where electromagnet would normally go.

You know, the resistor will draw current in proportion to the voltage placed across it.

Or time averaged voltage. I am naively guessing that's what your voltmeter will read. I mean even if it is some kind of PWM signal, that your voltmeter will just low-pass filter that, and the number it displays, will be, time averaged voltage.

I was kind of wondering the amount of current these electromagnets are expected to draw, and I asked a search engine this question, "electric trailer brake magnet current amps",


and the answer seems to be something in the range from 2 to 3 amperes per brake, maximum.

Which corresponds to a magnet with DC resistance of 6 to 4 ohms, since 12/6 =2, and 12/4=3.

Also consider the maximum amount of power these resistors have to dissipate; i.e. P=V^2/R

In the case of a 6 ohm resistor, with 12 volts across it, this is:

P = (12V)*(12V)/(6 ohm) = 24 W [max, at 12 volts]

As far as "fooling" the brake controller into thinking some electric brakes are present... Well I am not quite picturing that, or I am not sure to what extent the controller is "thinking".

It might be the case that PWM signal is the same, no matter how many brake magnets are connected, be it 4, or 2, or zero.

Or is there an indicator light, or something, that turns on when the brake magnets get connected?

By the way, I have never actually seen one of these gizmos, up close, in real life. So all I know about them is what I can glean from the web, or Wikipedia,



cerberustugowarJack A Lopez

Answer 1 year ago

Had a reply to you earlier and then mobile wouldn't let me post it and then instructables gave me a 404, so I got mad haha.

The resistors I have on hand are the cheap gold coloured 6ohm 50W, so I thought they would do alright for the amount of time they would be on for. Was going to attach them to a chunk of aluminum anyways just to draw out a bit of the concentrated heat they generate as this will all be contained in a sealed plastic box..

Newer (15 years and newer?) Fords will say "trailer connected", "trailer unconnected" when you plug in and unplug a trailer with brakes in the 7way connector. The dash will also say "no trailer" if you manually activate the trailer brakes and don't have one connected. I believe Fords within the past few years even have an option that pops up when you have a trailer connected that you can choose which one you are towing so you can better maintain your trailer. This is all if you have a factory installed trailer brake controller. I am unsure about the aftermarket controllers, but I did try hooking the 2 resistors in parallel and plugging them in to a truck with aftermarket TBC and was told that it says when a trailer is connected, but it didn't with my resistor set up :\ Maybe all this is a question for the dealer or an aftermarket installer.

For not having seen one of these, your info and best guess is quite good and has gone along the same lines as mine. I never thought about a PWM signal but is plausible with trucks having CAN Bus and such. I will do a bit more digging. Thanks!!!

Jack A Lopezcerberustugowar

Answer 1 year ago

Maybe your trailer simulator needs wiring for the loads besides just the brake magnets.

I mean, maybe the way the truck senses a trailer is connected, is by sensing one of the other electrical loads it expects to be present in a trailer, like the tail lights, brake lights, or turn signal lights.

cerberustugowarJack A Lopez

Answer 1 year ago

This is an answer I got from a dealer that sells trailers and everything for them including the trailer brake controllers.

"For a brake controller to sense a trailer connection it needs to sense a small amperage draw on the circuit as that's what happens when the circuit is connected to brake magnets. Resistors like what you mentioned should work well. Or you could wire in a brake magnet like the part # AKBRKR-M7 on the circuit and that would do the trick too."

If some do sense if a different way, I can only hope that my other voltage meters/indicating lights will be enough to make it recognise it. Otherwise I guess I could hook those resistors in parallel on the ground circuit or maybe one of them to the 12v charge circuit. I suppose it might be a bit of trial and error if it's not straight forward. Thank you again.

Jack A Lopezcerberustugowar

Reply 1 year ago

I am hopeful you will figure out something that works, and I glad I could help in some small way. Thanks for the Best Answer.

cerberustugowarJack A Lopez

Reply 1 year ago

Just so you know, I have been able to try the resistors on one truck that has an after market brake controller and it didn't recognize a trailer being connected. I did some more digging and found someone that said that some controllers don't sense a straight resistive load but will on a magnetic load. I'm guessing it's CEMF (counter electromotive force) that does it? So I will have to do some more testing and figure out a component that will simulate the load. A friend came up with the idea of using a starter relay (it's smaller than a starter solenoid) or maybe in the end I will just have to use magnets? :/

Jack A Lopezcerberustugowar

Reply 1 year ago

Well, I think at this point, you, and the people on these other forums you mention, are closer to cracking this thing than I am, since I do not have any of these electromagnets in my possession, nor any brand of controller gizmo made to sense them and drive them.

The only thing I have to add, is this property of electromagnets you call CEMF (counter electromotive force), might be more understandable as "self-inductance", often called just "inductance".

The voltage seen on an inductor, due to its own changing current, is

V_L(t) = L*(dI(t)/dt)

where V_L(t) is the voltage on the inductor and dI(t)/dt is the time derivative of the current I(t).

I claim this formula is actually coming from Faraday's Law:

V_L(t) = N*(dPhi(t)/dt)

where N is the number of turns and dPhi(t)/dt is the time derivative of Phi(t), which is the magnetic flux through those turns. The magnetic flux is there due to the current I(t), and the formula connecting these together is:

Phi(t) = (L/N)*I(t)

Anyway, inductance is a real physical property,




and inductance can be measured. If I had one of those trailer brake magnets, I would put my LCR meter on it,


and tell you what it reads.

Since this thought may have already occurred to someone in possession of both a trailer brake magnet and an LCR meter, I tried searching for that kind of chatter with queries like, "inductance of trailer brake magnet", "trailer brake magnet inductance henrys", and similar, but so far I have not found anyone writing about their trailer brake magnets in this manner.