wiring a trailer brake relay
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Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=95000
Printed Date: July 05, 2025 at 2:07 PM
Topic: wiring a trailer brake relay
Posted By: justa4
Subject: wiring a trailer brake relay
Date Posted: June 21, 2007 at 6:08 PM
I need a relay that does the following. I am not sure how to hook it up if anyone can sketch me a diagram that would help.
12v System:
Lets say input (1) needs to allow 12v power thru the relay under normal conditions but when power is sent to input (2) the the final output is no power.
Usage:
I have a trailer I am pulling that has surge brakes. Under normal conditions the brakes are always on. Meaning when the car slows the trailer pushes forward and brakes are applied on the trailer. When you go in reverse and the reverse light kicks on it shoots power to the brakes and disables them to allow you to back up with out the brakes applying. The problem I am having is when driving down hill for long periods of time (i.e. in the mountains) the brakes hang up. I basically need constant power to the brakes to always disable them unless you hit the brake which shoots power to the relay and allows the brakes to kick on while actually braking.
This sounds complicated but I know that it is very simple but I dont have an idea on how to hook it up to a standard automotive 40amp or 50amp relay. Please help.
Replies:
Posted By: i am an idiot
Date Posted: June 21, 2007 at 8:52 PM
Would you need to be able to switch between the current setup and the desired new setup?
Posted By: INSTALLER_MSS
Date Posted: June 22, 2007 at 11:01 AM
if i'm understanding correctly, the brakes apply when they do not receive a 12volt signal. if so, you can wire a relay as such: 85 - ground 86 - in from brake wire of vehicle 30 - out to trailer brakes 87a - 12 volts 87 - not used ------------- "If a man made it, another can modify it...it just takes some thinking."
"If you ask questions, you're a fool for 5 minutes; if you don't, you're a fool for a lifetime."
Posted By: justa4
Date Posted: June 22, 2007 at 11:24 AM
Perfect!!! Thats exactly what I was looking for. I will try that out and see how it works. I really appreciate it!
Posted By: justa4
Date Posted: June 22, 2007 at 12:57 PM
Would this work better? Because a turn signal overrides a brake light on the truck. Only pulling power from one side would cause the trailer brakes to go on and off with the trun signal while slowing lol. This would allow it to get constant power from either brake light in the event of braking and turning. The only time the brakes would not work now is while the flashers are on. With this particular set up it would only function while the clearence lights are on because that is the source of constant power to disable the trailer brakes. https://www.blinktekdesign.com/photoalbum/trailer.gif
Posted By: INSTALLER_MSS
Date Posted: June 22, 2007 at 11:25 PM
actually the brake lights are dual filament bulbs, so really they do not "override" the brake light, they merely engage both elements in the bulb. if they were connected and the turn signal was directly linked to the brake wires, then every time you turned on your signals, both lights would come on. the way the lights in a vehicle work are simple. the headlights are on their on circuit independent from all other lights...even though the park lamps engage simultaneously, they are not tied in together. the brake lights are run by one wire only and engage when the brake pedal is depressed and activates the switched attached to it. the turn signals in fact are their own circuits consisting of a left and right controlled by two wires that are engaged by the turn signal switch and both when the hazard signal switch is engaged.
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"If a man made it, another can modify it...it just takes some thinking."
"If you ask questions, you're a fool for 5 minutes; if you don't, you're a fool for a lifetime."
Posted By: INSTALLER_MSS
Date Posted: June 22, 2007 at 11:37 PM
catch the wires inside the truck and not at the trailer plug and save yourself a lot of work ------------- "If a man made it, another can modify it...it just takes some thinking."
"If you ask questions, you're a fool for 5 minutes; if you don't, you're a fool for a lifetime."
Posted By: INSTALLER_MSS
Date Posted: June 22, 2007 at 11:38 PM
that setup would work if you caught all the wiring at the outside plug
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"If a man made it, another can modify it...it just takes some thinking."
"If you ask questions, you're a fool for 5 minutes; if you don't, you're a fool for a lifetime."
Posted By: justa4
Date Posted: June 23, 2007 at 1:16 AM
The point is to make the trailer be compatible in this way with every truck not just one. I pull the trailer with multiple trucks that I can not modify.
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