trailer power loss at plug

markdol1 · 08-08-2022, 06:15 PM

Hi folks. need help,power to brake lead on truck male plug drops out or is intermittent, was fine until son replaced the plug because the old one was pretty shabby. he is also a good mechanic so i let him give it a go. the previous owner had a blue wire from that lead going into the fuse box .i am guessing he bypassed the factory harness. we removed it and went back to the stock harness lead. plug tester shows all other leads fine. Is it possible for a corroded brake wire ? truck used to back a boat down a ramp into water. is this line supposed to be hot all the time or just when trailer is connected? also what should be normal voltage at terminal lead when brake pedal is applied and not, can i check voltage output lead at the back of the ram 3500 factory brake controller which seems to function normally.

**chuckster57** · 08-08-2022, 06:39 PM

Blue wire is the signal wire to the brakes. It should carry voltage based on the brake pedal or brake control. It should NOT be hot if brakes are not being applied.

At the junction box the umbilical blue should be connected to whatever color goes back to the axles, and one lead if the break away switch. The other end of the break away should go to battery positive (usually black).

I haven’t had to use a meter on the 7way plug in years so I can’t say that the voltage will increase when using the brake control.

markdol1 · 08-09-2022, 08:09 AM

a blue wire that was not part of the stock harness was run from the brake lead at the truck plug into the fusebox somewhere. the stock harness pin shows a flickering test light.can this be a bad ground somewhere along the line? power loss and goes.

bsmith0404 · 08-09-2022, 08:30 AM

Sounds like the previous owner had problems with the stock harness as well and bypassed it. I don’t like bypassing factory stuff unless I absolutely have to. I’d prefer to trace that factory wire back and find the problem. On newer trucks there seems to be several harnesses and harness connections where they plug into each other. This makes it easy to remove/replace body parts and harnesses. My guess is there is another harness connection up stream from the 7 pin that is causing your problems.

**JRTJH** · 08-09-2022, 08:48 AM

I'd recommend doing some "research into how RAM proportional brake controllers work"...

On the Ford system, you won't read 12 volts on the brake connector, even if you "mash the pedal to the floor or squeeze the brake control lever with all your might"... The computer system incorporates not only the brake control input, but the vehicle accellerometers, antiskid system, ABS system, trailer antisway system and vehicle speed/braking effort to come up with an "ideal braking effort applicable to to the voltage sent to the pin....

In short, with my Ford in park, engine running, I still get "0 volts" on the brake control pin when measured with a voltmeter..... Your RAM may also be set up to only apply "specific voltage applicable only to specific conditions"... In other words, if your truck "ain't moving" the computer knows you don't need brake voltage to stop it.....

markdol1 · 08-09-2022, 09:42 AM

Yeah, brake controller and braking behavior is normal at times i think something is rotten in the brake lead upstream from plug too, i didnt want to unwrap a whole harness UGH to find a bad spot. i just thought maybe my kid did not make a good connection when replacing the new truck 7 pin plug. Thanks guys.

**flybouy** · 08-09-2022, 10:44 AM

Quote:

Originally Posted by markdol1

Hi folks. need help,power to brake lead on truck male plug drops out or is intermittent, was fine until son replaced the plug because the old one was pretty shabby. he is also a good mechanic so i let him give it a go. the previous owner had a blue wire from that lead going into the fuse box .i am guessing he bypassed the factory harness. we removed it and went back to the stock harness lead. plug tester shows all other leads fine. Is it possible for a corroded brake wire ? truck used to back a boat down a ramp into water. is this line supposed to be hot all the time or just when trailer is connected? also what should be normal voltage at terminal lead when brake pedal is applied and not, can i check voltage output lead at the back of the ram 3500 factory brake controller which seems to function normally.

If it worked fine before it was worked on and now doesn't would lead me to think that whatever was done is where the issue is at. Go over the work ( or have someone else do it) and start over, don't assume that because it was worked on/replaced that it's good. Anyone can make a mistake and it's not uncommon for new parts to be defective these days.

markdol1 · 08-09-2022, 12:22 PM

i agree im gonna pull the plug apart and see whats what ill get back to you.

markdol1 · 09-02-2022, 04:58 PM

Problem solved! After pulling front right wheel because of rattling noise i found the adjuster barrel, star wheel and threaded section were tumbling around inside. Drum was worn so badly at shoe face and magnet was worn down too. I decided to replace the whole brake /backing plate assy and drum assy cause bearings were about due to chuck. Out of curiosity i took resistance reading of new magnet it was showing 3.5 ohms and then the wiped out magnet it showed .002 ohms Bingo! So i decided to replace all 4 brake and drum assy. $800 for dexter parts down the street at trailer shop and cut 2 wires 4 nuts on backing plate and i am HAPPY,HAPPY,HAPPY. I can only guess what rv shop would have charged.

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