85 K5 Trailer wiring

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by RSAP, May 15, 2019.

  1. RSAP

    RSAP Registered Member Premium Member

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    Today I found where the old trailer wiring was spliced in... I see 4 wires and judging by the old splices and also looking at a wiring diagram. It looks like they had running lights, left and right signals... but the reverse was not spliced...

    I tow a boat (not yet with the K5) which has a 5 flat, which I believe the blue connects to reverse tail light, which is required so trailer brakes are disabled when reversing... does that make sense?

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  2. RSAP

    RSAP Registered Member Premium Member

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    Found the answer to my question on a trailering forum.. sharing here in case someone searches on the same topic:

    https://www.etrailer.com/question-25802.html

    What are the Wire Functions for the 5-Flat Trailer End Wiring Harness, # 20017

    Question:

    trailor wire s blue green brown yellow white new wire harnes has yellow and brown together white green and brown together and blue wie my problem is i cant get left turn signal to work cant figure out wire diag no diagram was in box please help me thanks

    asked by: Al

    Expert Reply:

    The 5-Flat Trailer End Wiring Harness, # 20017, is what is a called a wishbone harness. The brown wires contain the running light signal. It is split at the plug and ran to each side of the trailer so you do not have to have a jumper wire between the two tail lights. So the wire that is yellow and brown goes to the driver side for running lights, and the green/brown wire goes to the passenger side for running lights.

    The all yellow wire contains left turn and brake lights. The all green wire contains right turn and brake lights. The white wire is the ground and needs to be connected to a clean bare metal surface on the trailer frame.

    The blue wire can be for a couple different functions. If the trailer has a surge brake coupler with electric reverse lockout OR reverse lights, the blue wire would be ran to one of those. The corresponding pin on the vehicle would then be connected to the reverse light circuit.

    If the trailer has electric brakes then the blue wire could run to them. The corresponding pin on the towing vehicle would need to be connected to a brake controller to engage the trailer brakes. If you have everything wired as explained above and the left turn still is not working there is probably a bad ground at the tail light itself and you will need to clean the attachment point or move the ground to a better location.

    If towing with a motorcycle, the bike could have a different wiring configuration that is not converted/ does not match the trailer. You would need to test the wiring harness on the bike using a circuit tester like # ALL640595, to make sure the 20017 harness is compatible or if a converter is needed. On many bikes, the brake signal is separate from the turn signals.
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    Suggestion, to standardize, it makes sense to convert over to a 7 pin round pattern. It's what all the newer rigs and trailers seem to come with. You can get the socket side pretty inexpensively, with a bracket as well. You can also get the 7 round AND 4 flat in one fixture, but I don't see the point, it's easy enough to wire up the 7 round. Plus, as I've planned with my little boat if ever completed, I can use one of the pins to charge the boat battery while I'm towing.

    My trailer had four flat, but when someone else wanted to use it, or I wanted to tow someone elses trailer, we'd have to have the adapter (which always get lost apparently, part of the function of them? lol), hope they were wired right, etc. By going with the 7 round on the truck and trailer, I've eliminated that problem.

    With wiring like your pic, I'd strongly suggest completely redoing it. The four wire weatherpack to flat four trailering adapters are easy and cheap to find, cut the four flat off and run it to the 7 round socket. Unfortunately, the wiring jackets are damaged, those scotchlocks are garbage (IMO), and at some point, that is going to cause issues. At worst cut the wires, install shrink tube, and splice the wires back together to try and seal it all up, or cut it all off, and wire in a new pigtail. Just a suggestion. Hacked trailer wiring is the reason for many trailer/truck lights not working right, and is a major cause of standalone truck lighting issues.
     
  4. RSAP

    RSAP Registered Member Premium Member

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    Right after my post, I ended up wiring a 5 flat, splicing into the same location, while adding the splice for reverse. Will need to come back at some point and cut it out and add pigtails. Was in a pinch for time, but need to come back to it.
     

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