Jump to content

Trailer Electric Brake Circuit


Recommended Posts

Without the brake depressed, the reading should be 0V and with it depressed but nothing connected to the plug it will read about 12V, even though the voltage applied to the brake solenoids varies. The reason is that in any open circuit, the voltage read will be whatever the supply voltage is. If you were to measure the voltage across the brake solenoids while connected and with the brake applied, only then would you see what is actually applied to the solenoids. 

With most modern brake controllers, there is also a motion sensor that lowers the voltage to the brake solenoids when motion stops, so when sitting still you might still see 0V. 

Edited by Kirk W
Link to comment
Share on other sites

1 hour ago, edgreenberg said:

If placing a voltmeter on the output of the seven pin trailer connector, is the brake circuit a steady 12 volts, or is it a variable voltage in proportion to braking desired?

Good question, while a more precise answer subject to what's going on with the tow vehicle braking DEPENDS on the type of controller, my basic answer to your question agrees with Kirks good advice in that:

 If your tow vehicles pedal isn't depressed,,,, and the tow vehicle is stationary (no braking inertia) and you're NOT operating any manual slider or brake controller function,, I WOULDNT EXPECT ANY VOLTAGE ON THE SEVEN PIN BRAKE TERMINAL.  If any were there it would send current to the electric brake magnets which you don't need or want when just sitting there right?? If the pedal were depressed then I would expect some voltage, the amount being subject to the controller, its design and what if any load current was being drawn..  

 Now, the voltage I would expect at the seven pin when the tow vehicle is braking is of course a different thing. Some controllers may sense merely when the brake pedal is depressed via a simple switch while others may also and/or use an inertia based controller so the harder you're braking/slowing the more voltage is sent to the trailer brakes.

 I have (with some basic brake controllers) had a person sit in the tow vehicle and depress the brake pedal and if I listened carefully near the trailer wheels I heard a high pitch squeal I KNEW THE MAGNETS WERE GETTING POWER    This is NOT accurate or precise or scientific lol ONLY something I've done.

 The most common problem I've encountered with elec brakes IS A POOR OR LOOSE OR RESISTIVE GROUND be it in the wiring or connections or pigtails or at the magnets. While the trailer frame can serve as a conductive ground return path I preferred hard wiring a dedicated ground wire to each and every magnet as well as an oversized Ground wire from the tow vehicle to the trailer..

Nuff said

John T

 

Link to comment
Share on other sites

Thanks for both answers.  My problem is that I can apply the brakes with the manual handle, but I don't think I'm getting any braking from applying the truck brakes.  I suspect one of two things:

  • The brake (light) switch voltage is not reaching the brake controller
  • The brake controller's pendulum is not indicating that I'm slowing down. 

I believe that the connection from the controller to the seven pin connector is good, since I can manually brake, and I think that you've set me on a good troubleshooting path. 

Thanks

 

Link to comment
Share on other sites

An easy way to do a very superficial check would be to raise on side of the trailer so that the wheels do not touch, then get the wheel to spinning and apply the brake to see if the wheel stops turning. 

Edited by Kirk W
Link to comment
Share on other sites

Those are indeed good suspects. It shouldn't be that hard to depress the brake pedal and see if voltage appears on the pin (still doesn't prove if its switch to controller or controller to pin DRATS) using either a DC voltmeter or even a test light.  Are the tow vehicle brake lights working ??? I have seen some pretty simple mechanical push brake light switches as well as brake line hydraulic pressure switches and more modern sophisticated units. 

The inertia mechanism control may be harder to test since there has to be de acceleration for the pendulum to swing. Even if all else is working the magnets or their grounds can still be at fault. Hard to fix over the net and not much easier even if we were there lol Again so often its a ground problem grrrrrrrrrr

 John T  

Link to comment
Share on other sites

    Just a quick thought. I do believe that the max voltage for braking would be about 6 vdc.

   If you pull the safety switch out, then you should see full voltage.

   With one wheel jacked up off the ground, pull the safety plug and see if the wheel will still rotate. If not then that brake is adjusted properly.

  This is a quick simple test.   One wheel at a time, Jack it up and see if al wheels are functioning.

   Can you adjust the unit for more aggressive braking.

 

  If your controller has a pendulum controlling it. That makes things harder to figure out.  Is there a readout on your controller.

Link to comment
Share on other sites

Each wheel magnet will pull approx. 3-4 amperes @ 12-13 volts when the controller is at max. setting. The brake wire/terminal in the plug only shows 12V when the controller is activated. The controller gets its signal from the same brake light switch as your tow vehicle brake lights.

The most common mistake when wiring electric brakes is inadequate wire size. A 4-wheel electric brake system can draw 12-16 amps when activated. Nearly all trailers with electric brakes have the feed wire ran down one side of the frame. This means the electric brakes on the opposite side receive slightly less amperage the side with the feed wire, especially with under-sized wire between feed and magnet wiring.

The ideal trailer wiring is with the feed wire ran down the center of the frame, then  Wyed off to both sides brake magnets.

etrailer.com has an entire series of Q&A on trailer brakes and how to test.

Edited by Ray,IN
Link to comment
Share on other sites

Unfortunately, the end result is that I gave it to a local RV shop that does brake controllers regularly. I was unable to get under the dash to look for the stoplight switch to trace the wires. As I said in an earlier post, I'm pretty sure that it's either a wiring problem to the brake light switch or it needs a brake controller. We'll see what the mechanic finds. Thank you all for your suggestions and information. I understand a lot more now. That still doesn't help me get under the dash :-)

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
×
  • Create New...