Trailer wiring on a 77 K5? How do I do that?

So I need to install one of those trailer light adapters on a 77 K5.

Is there any easy way to do that? Do I have to cut into the wires?


Thanks.

To put a 7 way trailer plug on that is going to take some minor wiring. I don’t think they ever offered a factory tow package with 7 pin trailer plugs.

If the taillight harness is intact, cutting it would be a last resort. And if you do cut it, DO NOT USE CHEAP BUTT CONNECTORS!! Use good proper wiring splice methods. Use solder to ensure a good connection, use heat shrink tubing with the glue to make sure the connection stays electrically sound.

Ideally, get one of the aftermarket trailer plugs adapter harnesses that goes inline with the factory taillight harness. Plug it into the truck, then cut the 4 wire trailer plug off and use that to connect to the 7 pin.

That still leaves the electric brake, reverse light, and battery power (assuming you want those connections). On my truck, I ran a 10 gauge wire to the junction block under the hood, protected by a 25 amp circuit breaker. I ran a 14 gauge wire to the trailer brake controller for the electric brakes, and carefully spliced a jumper to the reverse lights. Since my trailer has a battery that powers the electric trailer jack and winch, the truck keeps the battery charged. I also have some small led lights for reverse, so wiring the truck to trigger those lights comes in handy.

fast 99 said:

https://www.curtmfg.com/media/catalog/product/5/5/55311_1024x768_a.jpg?width=1024&height=1024&store=default&image-type=image

Click to expand...

Crap, I took a screen shot of that exact part, but forgot to share it.

That solves part of the no cutting trailer harness issue, the OP wants a 7 pin plug.

You need to use the Curt adapter in post #4 above to connect the wiring for a 4 pin adapter, which should have these colors.
White - ground (will be grounded to the truck body)
Yellow - left turn/brake
Green - right turn/brake
Brown - Running/tail
Then buy one of the trailer plug adapters that is similar to the one I linked above, and plug it into the first adapter. And then you have three more wires to connect.


The SAE standard for the other three wires is: (However, these adapters seem to use a different RV standard. Not sure why.)

- 12 volt ignition-on power for interior lights and to charge the trailer battery. Usually the red wire.
Best way is to run a wire all the way up to the engine compartment for power, and then use an ignition-on 30 amp relay and 25 or 30 amp fuse to connect it to power.
- 12 volts for the backup lights. Usually the black wire. Tap into the backup wire in the back of the truck if you have low amp draw for lights on the trailer. If you have a high amp draw, then use a relay and 25 amp fuse to connect it battery power, and activate the relay with the backup light wiring.
- 12 volts to activate the trailer brakes. Usually the blue wire. Runs up to the activation wire on the brake controller. The controller will be connected with a relay to ignition-on power with a 25 or 30 amp fuse.

75gmck25 said:

(However, these adapters seem to use a different RV standard. Not sure why.)

Click to expand...

you mean where the center pin and one of the outer pins seem to trade roles?
I feel like I remember coming across that , a lot of the 7 blades have more than one possible pinout, and its seemingly arbitrary iirc. The circle pin one is different by virtue of shape being different

This diagram shows the difference between the wiring standards. So you need to know what the trailer has. No wonder things get wired wrong.