Forum Discussion

CVD's avatar
CVD
Explorer
Dec 28, 2017

Another tow wiring conundrum

I’ve had a couple posts in the last week, as I solve one electrical mystery, another pops up. Now, when testing my towed vehicle lights, I found a myriad of things that didn’t make sense (e.g. left signal activated nothing, right signal activated both lights in alternating cycles, running light activated dim on one side and bright on the other side). Obviously, I had signals going in places they shouldn’t . I eliminated the toad, eliminated the recently installed tow plug at the RV bumper and tested the wires where they would enter that plug at the RV bumper. BTW - All seems OK on the RV; Left, Right, Stop, Running light all work as expected.

Issue: Running light circuit activates the left and right turn signal wires. With running lights on and a test light, I checked 1) ground to running light wire – on (as expected), 2) ground to left and right turn wires – both on (not expected).

I then started pulling bulbs to narrow down the problem. Pulling the 1156 single filament running light bulbs had no impact. However, pulling the 1157 dual filament bulbs on both sides allowed the wires to test properly (turning on running lights only activated the running light tow wire). If I left either side bulb in, I had the problem (running lights activated the turn signal tow wires).

Given I have to remove both bulbs to “fix” the problem, it looks to me like it’s not a bulb or housing problem. I also don’t know if it’s the running filament or brake/signal filament circuit in that dual filament bulb that is causing the problem

Suggestions?
Class A
  • DrewE's avatar
    DrewE
    Explorer II
    Dec 28, 2017
    My guess would be a bad ground somewhere. Usually strange light operation such as this is due to bad grounds, and consequently the current finding sneak paths through other lights or filaments to ground.
  • DiskDoctr's avatar
    DiskDoctr
    Explorer
    Dec 28, 2017
    DrewE wrote:
    My guess would be a bad ground somewhere. Usually strange light operation such as this is due to bad grounds, and consequently the current finding sneak paths through other lights or filaments to ground.


    ^^THIS^^

    Use a tester light on the bulb that doesn't light properly. Put the alligator clamp on a GOOD ground spot and use the probe to touch a GROUND spot (usually metal mount or outside of bulb) and see if it lights up properly then. The test light will likely blink, too.

    I've used about an inch or so of stranded wire, removed all the insulation and wrapped around the removed light mounting screw (used for ground). Leave a little wire exposed from under the head and then put it back in the hole.

    It should "repair" your light's grounding point for another year or three ;)

    I now use only LED lights when working on trailer lights and run a separate ground wire to all lights. Once I work on the lights, I'll never have to work on them again- even if the trailer sits a year or longer ;)

    Hope this helps!
  • CVD's avatar
    CVD
    Explorer
    Dec 28, 2017
    enblethen wrote:
    Does your MH have amber turn signals?
    If so you need a 3-2 combiner to operate common reds on your toad.
    Roadmaster wiring diagram


    Thanks, but I've narrowed the problem to the current in the wires at bumper of the towing vehicle, nothing to do with the toad (regardless, RV has no amber lights).
  • CVD's avatar
    CVD
    Explorer
    Dec 28, 2017
    Figured a pic might help. I’m at the back bumper of the RV (no toad), have taken apart the tow plug and taped the wires to a piece of wood. With running light switch on, the pic shows current going from the running light wire to ground wire – Expected. However when I touch the running light wire and either of the turn signal wires, I also show current flowing (indicator light on) – Not Expected.

    The kicker is if I pull both (left and right) coach turn signal light bulbs (combo dual filament 1157 running & signal light bulbs), then current only flows between ground and running light wires – i.e. fixes the problem (but I would have no back turn signal bulbs)

    I’ve put in new bulbs. Also reworked the white ground wires coming out the coach bulb housing creating it’s own ground, right at the housing (just to bypass the ground wire that I can’t see inside the coach walls.).

    Still scratching my head.

  • enblethen's avatar
    enblethen
    Nomad
    Dec 28, 2017
    Did you verify that the green is actually the ground?
    Try connecting to a metal portion of the rig and retest.
  • DSDP_Don's avatar
    DSDP_Don
    Explorer
    Dec 29, 2017
    On some coaches, they add a combiner to the coach to convert the coaches lights back down to something that the toad can use in four wires. Look for a combiner box under the rear of the coach. If you have one, it may have failed.

    I also think you need to use the frame as your ground when testing. How do you know what wires are what....they don't follow the typical code...
  • carringb's avatar
    carringb
    Explorer
    Dec 29, 2017
    CVD wrote:
    Figured a pic might help. I’m at the back bumper of the RV (no toad), have taken apart the tow plug and taped the wires to a piece of wood. With running light switch on, the pic shows current going from the running light wire to ground wire – Expected. However when I touch the running light wire and either of the turn signal wires, I also show current flowing (indicator light on) – Not Expected.

    The kicker is if I pull both (left and right) coach turn signal light bulbs (combo dual filament 1157 running & signal light bulbs), then current only flows between ground and running light wires – i.e. fixes the problem (but I would have no back turn signal bulbs)


    All of your symptoms still point to an open ground. Either the green wire is such a poor ground it can only power the test light, or it's not even a ground and you'd inadvertently powering something else (brake controller or backup lights?) when you complete the circuit with the green wire.
  • Executive45's avatar
    Executive45
    Explorer III
    Dec 30, 2017
    CVD, your picture does not show as photobucket no longer hosts. Go HEREand drag your picture to the screen. You simply cut and paste the link into your post and the picture shows up. Be sure to CTRL+D and save the above posted link for future reference....Dennis

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