Trailer Brake Voltage
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Junior User
Joined: Aug 2013
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Trailer Brake Voltage
Hey All. I am having an issue with my trailer brakes. The truck had a brake controller installed when new, back in 1999. It was not using the factory wiring, the previous owner ran separate wires to the controller. I don't think the truck towed much, so keeping the trailer brakes working was never a priority. I replaced the controller with a POD and the harness. The controller can be mounted any orientation, according to the directions. When I measure voltage at the 7 way, and the blue wire leaving the controller, no brakes applied, my reading is 2.7vdc. When I hit the brakes(pedal and manual control), it jumps to roughly 7 vdc then slowly backs down to approximately 5vdc. I let off the brakes, voltage goes back to 2.7vdc, never down to 0, and never up to 12 vdc with brakes applied. The ground seems good, continuity on the harness, everything else on the 7way works as normal. I may hook up the old controller again and see if it acts the same way. Could my new controller be faulty? Bad ground somewhere? I did some research, found one other post that had the same problem, but no conclusion was made on it. Thanks for your help.
Hotshot
Joined: Jan 2010
Posts: 10,060
Likes: 491
From: Bristol, TN.
Are you measuring voltage with a trailer seven pin plugged in or just at the seven pin on the truck? Why did you change the controller? Was there a problem previously? Controllers are seldom the problem.
Steve
Steve
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I probably replaced the controller prematurely, to be honest. The old one had been lighting up green with no trailer connected, then turning red when I hit the brakes. I turned the **** and the lights flickered then never returned, and with the truck being in a very dusty environment for over 20 years, I figured something went wrong. I have not tested the controller with a trailer attached, I didn't know if that would make a difference. I can hook it up to a neighbors RV, see how it does. I measured voltage at the 7way plug and at the new harness between the controller and factory plug, same results from both. I have read that until a truck is moving a certain speed, the trailer brakes will not work properly, unsure if that is true or if it applies in this situation on these trucks. Could I be losing voltage to ground on the factory wiring to the back? I was thinking of testing the voltage out of the controller, with just the blue wire disconnected(cut?) and see if the readings are the same. Anything else I should try?
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Junior User
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Here is the link to the brake controller I bought. It was inexpensive, no doubt, but simple.
https://vi.raptor.ebaydesc.com/ws/eB...=2715620807023
https://vi.raptor.ebaydesc.com/ws/eB...=2715620807023
Hotshot
Joined: Jan 2010
Posts: 10,060
Likes: 491
From: Bristol, TN.
No problem with your choice. Accelerometer action so voltage output is proportional to the rate your truck is deaccelerating. I am betting it is working. You are measuring the search voltage the controller puts out. It can not be accurately tested by what you are doing. Hook up a trailer with working brakes and try it.
steve
steve
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Ok cool, I will hook up a trailer in the next few days and get back and post my results. Thank you for your help!
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Hotshot
Joined: Jan 2010
Posts: 10,060
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From: Bristol, TN.
That controller should have come with instructions as to how to set the output to the brakes so as you brake it should work in proportion to how hard you are braking up to the limit you set automatically. Test it out in a parking lot or similar rather than the open road.
Steve
Steve
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FTE Legend
Joined: Jul 2005
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From: Millersville, MD
...and you will not be able to test that variable voltage... just follow the directions for adjusting the stopping power and attack (how fast it sends the voltage to the trailer). The voltages you are seeing now is showing to me it is working fine. If you have problems now look to the trailer for problems.
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Junior User
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I hooked up to a friends RV this weekend, the controller seems to be working fine. I did not actually pull it anywhere, but the voltage readings look normal. It would go to build up to 12volts at full power/gain, then return to 0 volts when brakes released. We could hear the brakes activate with the brake pedal and manual switch on the controller. Thanks for your help.
Lead Driver
Joined: Jun 2005
Posts: 8,475
Likes: 728
From: Calgary Canada
The "smart" controllers are kind of hard to diagnose, but they are so reliable, it's almost never an issue with the controllers themselves. Wiring, at the connection or on the trailer is the usual culprit.
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Junior User
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Good to know that they are reliable, my experience with trailer brake controllers is limited. I am used to big rig 7-way plugs, several obvious differences between the two. I just need to run an auxiliary wire, then it is all complete.
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