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Old 05-26-2008, 07:46 PM   #1
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Troubleshooting electric brakes

I just put in a new axle and before I re-hooked up the wires to the brakes tried to check if I had juice coming into my new axle and brakes. While the leads to the brakes were unconected I took a 12v test light and connected to the leads from the trailer to a test light. I had a person in the TV step on the brakes and also try to apply the trailer brakes only with the brake controller. Either using the manual control or stepping on the brake pedal the indicator on the brake controller lights up. The brake lights on the trailer light, but the test light does not light. Any troubleshooting suggestions? How can I tell if my Brakeman brake controller is fried?
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Old 05-26-2008, 07:57 PM   #2
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Do you have electric at the pin for the trailer brakes on the tow vehicle when you apply the controller?
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Old 05-26-2008, 07:58 PM   #3
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Hook up the wires to the brakes and then hook up a meter and check for voltage at the wire connection
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Old 05-26-2008, 08:09 PM   #4
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The power for the brakes comes from the controller. The controllers needs a 12 volt source besides the brake light switch. They are typically 4 wire setups: ground, stoplight, power in, and power out (to the brakes).
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Old 05-26-2008, 09:50 PM   #5
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The manual swtich on the controller turns on the tail lights and also the pin for the trailer brakes is hot. I didn't yet check the trailer side of the umbilical or the connection box inside the trailer but will do that tomorrow.

If i hook up a test light to the 2 wires coming out of the belly pan which lead to one set of brakes when the brakes are applied I should get a light, right? or do the brakes need to be connected to get the light to light? or do I need to use a meter and not a test bulb?
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Old 05-26-2008, 10:52 PM   #6
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The controller should send voltage directly to the brake magnets. You'll have to work yur way downstream from the controller until you find where it goes missing. A light will work OK, but a meter is neater!
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Old 05-27-2008, 07:54 AM   #7
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Floating right along, I have power on the pin from the brake controller (along with tail lights when I either step on the brake or activate the trailer brakes manually at the controller) Next I'll check the trailer end of the umbilical and the7 way junction box. I was suspicious a month ago when I could not feel the trailer brakes apply. To ask I dumb question, the electric leads to both sides of one axle are now disconnected. Having one set disconnected would not create an open circuit for the other one would it? Are there any fuses or breakers, etc, between the 7 way connector and the brakes?
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Old 05-28-2008, 08:05 PM   #8
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another AS lesser mystery/ or dumb strikes again

OK. I have juice through my umbilical to the point where it enters the trailer. I assumed the 2 wires for each brake are positive and negative. When I clipped a test light between the black and white lead, nothing. But when I attached the litttle alligator to a frame ground and touched either the black or white lead they are both hot. How can this be right? Do I just not understand how the brake magnets work?

While I am playing with electricity, While checking my 12v power out I noticed the ground and charge pins seem to be reversed from my schematic. I didn't change anything and my Univolt has worked fine for the past couple of years and is not fried. Also the polarity in my 12v outlets matches my 12v appliance chargers. My guess is some BOZO (probably me ) reversed the wiring and reversed it again before it plugs into the Univolt so even though it looks bass ackwards it's not. So if it's not broke don't fix it! what do you think.
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Old 05-28-2008, 08:17 PM   #9
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I think you will find the wires to the brakes are in a "loop", that is, if you disconnect any wire, the circuit will be broken. I once had to remove one brake, backing plate and all, and drive with just one, being very careful of course. In order to get the remaining brake to work, I had to connect the wires that had gone to the removed brake together.

By "loop" that is like the very old Xmas tree lights. One goes out, the all go out. Continous circuit from the controller to one brake, out to the other brake, then to ground. That's why it doesn't make any difference which way you connect the wires at the brakes.
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Old 05-28-2008, 08:44 PM   #10
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That's called series connection.
One of the brake wires is hot, the other is ground, to each magnet. The magnets don't care which way the power goes through them. As long as one of the wires gets power to each magnet, and the other one is grounded, the brakes will work if power is applied.
I also think your "it works, don't mess with it" theory is a good one. Leave rewiring your coach for another time.
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Old 05-28-2008, 09:07 PM   #11
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Smile Hi

I wish I coud draw you a schematic. Here it goes.

+-------------- to magnet from Controler in TV Battery
_ ------------- to Magnet Chasis ground and TV ground. The controler modulates the 12volts .It alows only voltage directly proportional to the pressure on your brake pedal. There fore you can get anywhere from 6v to 13.5v or what ever your alternator or battery are putting out.
If you have 12v volts on both side if the pos. and negarive you have not disconected all your breaks or you have a cross over some where in your system. A gross over means that your ground is nor groundet and you are appying voltage to both sides. Check your plug at the TV and AS one or the other is wired wrong.
Regards from Russell In sunny and warm Tucson Az.
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Old 05-28-2008, 09:36 PM   #12
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At risk of muddying things up more...
The power out from the controller should go to each brake magnet and the other side of each magnet goes to ground so that each magnet "sees" the same output from the TV, i.e. a parallel circuit. The best wiring will split the controller wire into separate wires, one for each brake, and each brake will ground separately to the chassis/umbulical ground. If the brakes are wired so that power goes from one wheel to the next (still in parallel) a break at any wheel will shut the brakes downstream off.
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Old 05-28-2008, 10:12 PM   #13
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Here's a sort of circuit diagram
Attached Files
File Type: pdf brake wires-1.pdf (170.7 KB, 696 views)
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