Truck's Air to Electric Trailer Brake Problem

Hoping you can help me on this one. We are trying to hook up my tag trailer's electric brakes to my air brake equipped dump truck through a large HD 7 way round pin connector. We have the controller's brake wire is connected to the center pin on the connector and the power and ground wires are connected directly to the batteries. With or without the controller in the mix all lights and functions work but the trailer brakes are locked up until the trailer is unplugged. We installed a Hopkins digital brake controller which functions properly and applies the trailer brakes with the pedal when the key is turned off. When the key is placed in the on or run position, the controller's display disappears, the "connected to trailer" light turns off and the trailer brakes are locked up. In the on or run position the test light still shows the brake connection at the controller's red "activate" wire functioning properly when the brakes are applied or released. It's like I'm visiting backwards world...Any ideas?

is all the wire from the controller to the center pin new
Do you have a hot auxiliary wire in the trailer connector there could be a strand of wire reaching over and touching the center pin
Disconnect the trailer and check with a test light or better yet a headlamp draws juice closer to trailer brakes isolate it to the truck
I'm thinking that power is getting put to the blue with via the ignition circuit if you put the controller wire to a junction box might have missed that there was already power on the center pin circuit

Thanks Jonas, that would make sense. Another thing that dawned on me is that the guy I bought the trailer from mostly runs big RV's and horse trailers probably all with 7 way flat blade connectors. I was reading that there are a couple of different ways to wire the center and 1 o'clock pin functions on the HD plug depending on application. He may have made an adapter to run this trailer and set it up the opposite way I was thinking. I'll pull the trailer plug apart and check. Maybe there's not just a stray strand of wire but I'm actually sending 12 volt constant directly to the brakes. Working on wiring is my least favorite thing to do by far.

7 pin round and 7 pin flat each get wired up in a different pattern. I would say you will need to move a couple wires around in the 7 way plug on the trailer.

This will be how the trailer 7 way flat is wired:
https://www.etrailer.com/question-363.html

This is how the 7 way round is wired:
https://www.etrailer.com/question-27196.html

7 pin round is not only a different pattern the wires do different things. 7 round has a discrete turn/brake lights whereas 7 flat has combined turn/brake and electric brake.

What they said. I would wire up two separate plugs as they both are unique in their function and application. That way you're setup to work either style correctly. Center pin of 7 way, if wired correctly, is blue and key on constant 12 volts. There actually aren't enough wires in a semi 7 way to accomplish all light functions and electric brake application. I suppose you could combine tail and marker light functions to open a spot but if you ever wanted to use it on a legit class 8 trailer you'd have to swap back.

Just my .02 having dealt with this since the days of the 6 pin round before the RV style got so popular.

Oh, and if you do an RV style you'll need to tap into the turn/tail harness for the lights on the truck NOT through the 7 way trailer wiring as it still won't be right due to separate turn and stop.

Clear as mud? Lol

It rained today so we didn't mess with it but yeah, sounds like if I continue down this road I may end up with a cluster. The truck is setup for a pup trailer as is and the plan is to get one soon.

Whoa Junkyard, you just gave me a good idea. The truck does have a small 6 pin round plug. Think I'll change that out for an RV plug and roll with that on the tag trailer. Both of my flatbeds already have the RV style and I'd like to be able to tow this trailer with either in a pinch.

JNB said:

It rained today so we didn't mess with it but yeah, sounds like if I continue down this road I may end up with a cluster. The truck is setup for a pup trailer as is and the plan is to get one soon.

Whoa Junkyard, you just gave me a good idea. The truck does have a small 6 pin round plug. Think I'll change that out for an RV plug and roll with that on the tag trailer. Both of my flatbeds already have the RV style and I'd like to be able to tow this trailer with either in a pinch.

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If it's functioning that would make it pretty easy! Every once in awhile I come up with something good but don't tell anybody. They'll expect it all the time!

No worries...your secret is safe with me. lol

Assuming everything works, only thing you are missing with a 6 pin wiring is backup lights, and most trailers aren't wired for that anyway. Switching the 6 pin to 7 flat would be best.

That is what I was thinking, too. Use the 7 pin round semi-style for air braked trailers and the 7 flat RV-style for electric braked trailers. Or use the 6 round.

Round pins give better connection under dirt and corrosion than the flat RV style pins, though. It used to be that equipment trailers around here had the 6 round and RVs had the 7 flat. But 7 flat seems to be getting more popular for everything. Is that what you guys see?

Also, another problem with 6 round is there is disagreement as to which 2 pins are used for trailer brakes and hot at all times battery charge. If hooked up opposite it causes the trailer brakes to lock on full and the battery to not charge. Maybe that's why the 7 flat caught on, because there is no disagreement on what those pins are used for.

Hvy - Yeah it makes more sense. Sometimes I get entrenched in my task and need the HEF brain trust to remove the blinders lol.

Birken - The 2 pin deal on the 6 way round is a real problem around here, even on rental trailers. Depends on who wired it up. In fact when I bought my wife's GN horse trailer I had to switch the wires on the spot just to get it home. It's wired with an RV plug now.

As an aside...When I lived in Cali I had my truck and car trailer wired up by a place that did a bunch of studio equipment. The studios use the 7 pin semi style plugs with their own wiring pattern. Keeps folks from borrowing trailers.

Seventeen years later and I still have that truck. I happened to be in it when I found the tag trailer for sale on the side of the road and brought it home. I tried using it's 7 pin semi plug but the trailer's plug is wired differently.

JNB said:

Birken - The 2 pin deal on the 6 way round is a real problem around here, even on rental trailers. Depends on who wired it up. In fact when I bought my wife's GN horse trailer I had to switch the wires on the spot just to get it home. It's wired with an RV plug now.

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At one point I had contemplated adding a switch to the truck to flop the two wires as necessary but never quite got around to it, haven't missed it much now.

The RV to 6 pin round adapter plugs are made to be opened up so the two wires you speak of can be swapped without too much fuss.

Birken, I hardly see a 6 pin round anymore. Lots of RV. I don't like them, very few feel like things are really making contact as well as feeling like the plug is securely retained physically.

Might as well cover all of them while you're wiring, 7 way semi, RV, 6 round, 4 round, 4 flat, 5 flat..... or carry a box full of adapters. Or just do what the lawn crews around here do....drag the plug down the road with the safety chains!

Adapters...Reminds me of my '66 VW with a 8 track to cassette adapter. Used a stick to hold it up which required constant adjustment for "easy listening."

My boy is out with a 6 pin and both 7 pin schematics wiring things up as I type. Thankfully I have many many important customers to call haha!

Have a car hauler 6 wire plug equipped, a gooseneck 7wire flat terminal and have worked the truck industry with seven wire connectors all of which have multiple options depending on the customer's requests. Seven pin semi was easiest, uses DOT code, 7wire RV is RV code and none of it makes sense as neither does the six wire so I wire that to a DOT ethic for color code for my stuff. God help anyone that pulls my trailers!!