Trailer Wiring, Kenworth

[buzzinghalfdozen24]

Ok folks. I'm only writing this here because there's a lot of intelligent people that also do some trucking. I'm trying to do some trailer wiring. We've got a Kenworth T300 dump truck that has a tag trailer pulled behind it all the time. It has a standard 7 pin rv style trailer connector, with standard wiring inside. Ex. brake lights are powered through the turn signal wires. The trailer is air brakes. We are trying to make this hook up to our Kenworth T800 semi tractor. This T800 has ABS brake signal through the typical for semi's, 7 pin round. With air brakes, the brake light signal is sent seperate of the turn signals.

What do I need to do to make these match each other? I can't split the abs brake signal wire and join it to the signals because then both sides would be connected. Are there any converters or adapters that do this?

Thanks!

 

[mwilson]

I'm doing inventory prep but will help as I can.

I think that I would put a common commercial truck style light sockets on both your T300 and T800, and the commercial truck style plug in on the tag along trailer. That truck style set up will work with the shared brake/turn and separate brake/turn light feeds. No adapters or such needed.

When hooked to the T300 the outside tail lights will function as combo stop/turn lights. When hooked behind the T800 outside tail lights will be turn and inside tail lights will be stop.

Does that make sense to you??

 

[buzzinghalfdozen24]

It makes great sense. That should work. So put the factory intended 7 pin round back on everything, rather than a 7 pin flat(RV). That's a good idea, wish I would've thought of it like that

 

[mwilson]

Do you have the commercial wiring diagrams available to you???

Here is a pretty good one that I just found showing both the commercial and RV for a 7 wire set up.

Scroll about halfway down the page in the link and you will see the 7 wire set ups. Nice reference as it shows wire color, front and rear veiws of each connector too.

https://www.etrailer.com/faq-wiring.aspx

 

[HPSimpson]

Thanks Mike this really helps.

 

[buzzinghalfdozen24]

Okay, so after getting distracted for 2 months, I'm finally back trying to fix this truck to trailer wiring again . Your answer made sense a few weeks ago, but not now. I'm confused at how just switching the connector over to a7pin round vs a 7pin rv will make a difference. The connector is different, but not the job of each signal wire. Or am I missing something?

 

[mwilson]

No, other than it standardizes and makes the trailer work with both trucks. I am assuming that you are going to keep both power units.

Then you have not disrupted the standard commercial truck plug in case you want to hook the T-800 to another trailer in the future.

 

[mwilson]

I am also assuming that the trailer has four tail lights. Inside (2) are brake / tail and outside (2) are turn / tail.

 

[buzzinghalfdozen24]

Yes, planning to keep everything. And hopefully want to hook up more trailers on the future. Which would mean you're correct. If I went to pick up another trailer with this tractor having a 7flat it wouldn't connect leaving us stuck without any electrical at all. Good point.

I guess what I forgot to mention is our fleet consists of 4 other single axle freightliners, all with 7 rv. So to make things right were looking at a massive change in connectors, haha.

I may for now just check the trailer plug and see if I can wire it the same as a 7round, but have an rv plug on it. So basically just matching the pins.

 

[mwilson]

Ahhh, the rest of the story. In case of having so many trucks I would take a male truck connector, a foot of 7 wire cable and a female RV connector to make an adapter that you can carry and use for the T-800 at this time. That would solve the problem, right??

 

[mwilson]

Here's one all ready made from Pollack......http://www.etrailer.com/Wiring/Pollak/PK12-724.html

 

[buzzinghalfdozen24]

I have this lil guy..

http://www.etrailer.com/Wiring/Hopkins/HM47425.html

The t800 has one of those 7round connectors on the back of the semi cab. I'm assuming this is so the drivers can replace the pigtail once they accidentally pull away with wiring connected. Haha.

I tried plugging that adapter in right there and using my 7rv test light in the end. It obviously didn't switch up the brake and signals. But i guess that's not the goal then, correct? Next bad thing was that adapter wouldn't go in all the way. Couldn't tell any difference between that adapter, and the real semi trailer wiring connected. Must be something.

 

[mwilson]

There are not two sockets on the back of the cab, right??

 

[buzzinghalfdozen24]

Nope. Should there be?

 

[mwilson]

I'll see if I can explain this....There are two different turn signal feeds on a commercial tractor. One set feeds the trailer cord and they are turn only. Brake lights are on their own feed. Hence the reason that there are always (4) tail lights on a semi trailer, two inside are brake and two outside are turn.

However on the trucks tail lights they function as a car....shared turn and brake lights...

The primary difference being that the truck brake light circuit runs through the turn signal switch to allow the brake light to blink when the turn signal switch is activated.

If your KW was here I could outwit it and make your lights work. However with some distance between us that aint gonna' happen today at least.

 

[mwilson]

"Nope. Should there be?"

Just making sure that you did not have a second socket for ABS which many trucks do nowadays. If someone wants extra wiring for lift axles and stuff there are not wires enough to go around so many times the ABS in in a second trailer cord. Had to make sure you were not playing with one of those.

 

[mwilson]

Trying to think of an easy way for you to get where you want to be....

Maybe you should install a 7 way RV plug somewhere and feed it from the tail lights on the truck. Avoid the cab mounted plug altogether where you have air brakes on the trailer anyways. Just pick up your left turn, right turn and tail from the tail light feed along the frame. May even be a junction box on the firewall but then you will need to run cable back under the cab.

 

[mwilson]

You could also use an empty terminal in the RV 7 way that is designed for another function for the brake light feed....and then make sure on the trailer that it matches and connects to the inside brake lights of the lowbed. The only trouble with that is making sure your other tow vehicles don't use that terminal for something.......

 

[buzzinghalfdozen24]

Ueah, I'm digging the tractor tee in...I was just about to say the same thing. Just reach down and grab my wire feed from the tractor bumper lights. Just have to do a little differentiation between lh and rh, and be good to go. Hopefully

 

[mwilson]

That should work, just tap into the truck's tail light wiring.