7 pin to 7 blade trailer plug

Needing to pull a utility(car hauler/uhaul) style trailer behind a class 8 straight truck, that has the semi trailer plug wiring. This truck normally pulls a pup trailer equipped with air brakes. This utility trailer has electric brakes, which won’t be used. Do the wire colors match up, other than the blue wire which won’t be used.TIA

Can take an extra wiring cable(the semi coiled type) and put a 7 blade receptacle on the end of that. Instead of tapping the truck wire harness? I extra of those cables.

7 pin class 8 truck wiring has a red wire for brake lights and then green and yellow for turn. If you put 7 blade socket on and hook into trailer socket on truck you will have marker and turn lights but no brake lights

Lol I know what you mean. If a guy had time you can make a pretty nice harness that would be plug and play and would replace all the cobbed mess from previous repairs over the years. I've done it before just need access to some weather pack connecters and terminals.

I suggest you check the operation of the signal lights on your class 8 truck and the round plug.  I know many tractors, and  some straight trucks, are wired so the brake lights go through the signal switch, as well as the stop wire in the socket/plug.  Most tractors only have one tail light, each side, on the tractor for bobtailing; so the brake lights work on the same filament as the signal just like cars and pickups (same for straight trucks).  If a semi trailer only has one parking/signal light each side on rear, the stop light wire is not used.  The separate stop light wire in the trailer plug comes into play mainly on trailers that run two, or more tail lights per side.   That gives them a signal light and multiple stop lights.  If that is the case you will only need the two turn signal wires, the ground and the parking light circuit to make a jumper with a 7 round pin male plug on the truck end and a 7 flat blade female socket on the trailer end to have lights on your car hauler.  I also suggest you go by pin/blade position as far as function not colors.  There are plenty of pictures available on line of the SAE standards for both the round and flat blade connectors.  The 7 flat blade connector blade positions are the same the SAE and RV standards, but they use some different color wires for the functions, Hence I say use blade/pin positions not colors.  Just my experience over the years, others may vary. 

Edited by Jim.ME - 30 Oct 2021 at 9:14am

Sorry if I wasn't clear on what I was trying to convey.  The function positions of the blades of the two 7 blade connectors are the same, the wire colors may vary.  The function positions and wire colors of the round pin connectors are different than the flat blade ones.  One needs to match the function to the correct standard pin/blade position of the type connector they are using.  One can jumper from round to flat so as to have just the lights, as Lars asked.   He will only need  wire the  ground, parking light, left turn and right turn from the round pin plug to the proper blades on the flat blade socket to make a jumper for lights, no others should be used to avoid shorting anything out.  If one wants full 7 blade SAE or RV function (electric brakes, aux power, and reverse lights) separate wiring would be required to a 7 blade socket,  on a truck with the 7 pin socket currently.

I don't disagree there are a lot of odd balls out there, and I have found screwed up 7 pin DOT round ones.  I use the standards as a starting point and use a test light to confirm what is where, as I expect you do.  That's also one of the reasons I say not to go by wire colors, but to check the function.  Nothing like chasing a 7 pole trailer wired with all one color and gauge wire, or the ones that are one color at the front and you find totally different colors at the lights, when you start working backwards from a dead light.

Thanks for everybody’s input, brings flashbacks of when on the farm dealing with IHC truck wiring, all wires were black, with silver numbers on them, if the numbers weren’t worn off.

BPM75 wrote: I agree with not always going by wire colors, especially in a situation like this. As far as some tractors having brake light power on turn light posts I'm not saying some don't but I've never seen it and would consider the possibility unlikely. Best bet is check the 7 pin connector on back of truck and verify, if it does work that way by chance would be simple to make an adapter for the utility trailer.

Some are, some aren't.  If a trailer has amber signals those are supposed to be signal only, no brake or parking function, so the signal and stop lights have to be separate.  If the lamps are all red on the rear the stop and signal can be combined on at least one lamp on each side for turn and stop (generally the outer one of the two).  They can be wired either way, so checking the socket pins is the only certain way to tell if the tow vehicle signal wires are also carrying brake light power on a given unit.  Cycle the signals on and off with the brakes applied, with air up and engine running.   With the brakes applied if the light goes steady (bright) when the signal is turned off, they are combined through the signal switch.

DMiller, that is 100% the best answer, especially if anyone is going to tow the trailer more than one time.  An electric brake controller can be easily added as they no longer have to hook into a brake line like the old ones.  Then there is no question.  Did that when I rewired the tractor we have this summer as we are going to set it up to haul goosenecks.