Vexing trailer lights

Moderator: GreenLake

Vexing trailer lights

Postby GreenLake » Fri Sep 15, 2017 7:45 pm

I've always suspected trailer wiring of being irrationally complex ;)

Here's my conundrum: there are two identical (same model / type / manuf) LED lights. They are connected as shown. One will light and the other will show only a single LED in the center. Car end is toward the left.

Black is a duplicate ground that I run in addition to the actual ground. At each fixture, both types of ground are interconnected. Hard to believe that either fixture gets a different ground level from the other (Port will light even if connected only to the frame).

Red is the connection from the car (in real world color schemes, the brown cable). The diagram shows the port light tapping into the connection to the SB light. Should give same voltage to both.

But Port absolutely won't light fully when SB does.

tail light wiring - tail end
TrailerWiring.JPG (26.84 KiB) Viewed 72 times

Anything I should try?
~ green ~ lake ~ ~
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Re: Vexing trailer lights

Postby Alan » Fri Sep 15, 2017 11:21 pm

My experience with LED taillights is that a single LED in the center is for the taillight (or two LEDs in the center, depending on total number of LEDS), and the remainder are for the brake/turn signal light. They shouldn't all light at the same time unless the brake pedal is pressed (both sides) or the turn signals are in use (one side at a time).

It could be that your right taillight is getting constant power to the brake/turn signal light as well as to the taillight, which is why all of the LEDs illuminate at the same time. The left taillight, with only a single illuminated LED, is probably performing as it should.

Do you have brake lights and turn signals, or just taillights?
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Re: Vexing trailer lights

Postby TIM WEBB » Sat Sep 16, 2017 12:17 am

+1 on Alan's observations GL. Whereas grounding issues are almost always the root cause of trailer light problems, it sounds to me like you have a brake/signal short on the right side. Your schematic only shows one positive 12V lead back to the lights, when in fact there are usually three: lights to both sides, brake/turn to right, and brake/turn to left. Can you take a 12V battery and some jumpers and isolate each fixture, checking for proper tail light only/brake/turn light only/both? Then just work your way forward until you discover the short. TRW's old trailer had no lights at all when she got me, and I installed LEDs on her. Had the same issue, and it turned out to be a defective plug up at the hitch/vehicle. It was fairly easy to diagnose since I had dedicated/isolated ground wires running back, i.e. did not use the trailer frame for ground. Is this a new install you are doing, or an existing system that was working and is now not working?
Tim Webb
1979 DS2 10099 The Red Witch
(I used to be Her "staff", in the way dogs have owners and cats have staff, but alas no longer ... <pout>)
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Re: Vexing trailer lights

Postby GreenLake » Sat Sep 16, 2017 5:34 pm

The single LED I saw when I wrote my post did not at all look sufficient as a tail light. While I looked at it in daylight and not totally from behind, its appearance was rather anemic compared to what the SB side put out (SB had been on the trailer for a while and appeared to work fine, while P (bottom in the diagram) is new).

The single LED looked weaker than the two LEDs that made up the side light.

After driving for a stretch last night I saw the single LED in the center still a bit brighter but all the other ones lit as well for tail light. All of them light bright for turn / brake light. That matches the behavior on the starboard side.

I had an actual short in the brake light circuit on the P side earlier, and the seller graciously sent me a replacement unit. (The short was in the unit, or as I found out later, the cable part that came with it).

Then I mounted the replacement and all looked fine according to my expectations which are that all LEDs should light at half setting for tail light and at full setting for brake light. Then I decided to test all combinations, including tail plus turn signal. (I normally just test with the all-round flashers).

Suddenly things reverted to the single LED in the center. And nothing I did changed that behavior. Even the replaced unit showed that pattern (tail light part in that one had worked before).

I checked this morning, and am still getting what I expect.

There are several different designs or LED trailer lights on the market. Some have a single central LED, some have a center LED and a concentric ring. Mine has a 3x3 rectangular pattern.

At the moment, things appear to b e fine. Will add more here, if symptoms return, but for now boat and trailer are in off-season storage, so it will be a while.
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Re: Vexing trailer lights

Postby BaronDaniels » Thu Oct 12, 2017 5:01 am

Wow! I didn't know that it could get this complicated. I want to know the outcome of this.
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Re: Vexing trailer lights

Postby GreenLake » Fri Oct 13, 2017 5:58 pm

The "outcome" so far is that the new (left) light works as expected (after driving the car with trailer attached for a bit). It refused to work for a while after the 'test' I described.

Now, my problem with that trailer had always been that the light on the left would apparently fail (even when it still was incandescent) but after I drove home with it anyway, I would find it working properly. (Finally, this season that failed, and when I took the fixture off I could see why: the mounting brackets for the bulb had rusted off nearly completely - the one for the side light had disintegrated. Hence the desire to replace it with an LED light).
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