Trailer Light Frustration

[hriehl1]

1999 Honda Odyssey with an OEM 4-pin extension plugged into the car's built-in pre-wired trailer-light takeoff (behind right rear trim panel near taillight). A 4-Pin test device and a circuit tester both indicate all 3 functions (brake, tail and turnsignal lights) work correctly at the plug where the trailer connects.

I have tried 4 different trailers whose lights I know are good and on every one ONLY the tailights work... no turn signals and no brake lights. Yet the tests on the carside plug indicates all works OK.

I have rewired, I have emory-clothed all connections to their shiniest... all to no avail.

I'm a ticket waiting to happen. Can anyone suggest what may be the cause of the problem?

Many Thanks

[BillHerrick]

No way am I any kind of electrical authority, but how's the ground on the trailer? If that connection is bad it will raise havoc. Oh wait, you tried 4 trailers and all worked the same . . . Ground on the car?

[jpclowes]

I'm no expert either. But most of the trailer lighting problems I have faced over the years have eventually been traced to a grounding problem. How are these trailers grounded? Often trailer lights use the trailer itself as a ground, which may or may not be adaquate. I actually ended up putting a ground line from the trailer lights to the ground on the tow vehicle, and my lights are much brighter and easier to see in bright daylight.

[adam aunins]

Something else you might try, take one or two trailer lights and wire them straight to a plug with no more than say tree feet of wire (that way you can put it up in your back window to test the lights by yourself). Then hook it up to your van and see what happens, that way if the lights don't work you can sit and trouble shoot it with out having to run around a trailer and worry about wiring problems in the trailer. Once you have those lights working right then you can start hooking up trailers and seeing what happens.

PS. I once had all but my brake lights go out on a trailer, it turned out that they had seperate fuses for the trailer lights and some had burned out but of course the fuses for my trucks lights were still good so the trucks lights were still working fine. Took me a few hours to find that one out.

[hriehl1]

Well... I'm stumped, but it sounds like a car-side issue and probably a ground.

Could there be enough of a ground to light a single LED on a trailer light test-plug, and flip a needle on a circuit-tester.... but NOT be a good enough ground to light a lamp? Does it make sense that taillights work, but brake lights and turn signals do not?

I will wire up a test-light with alligator clips and try it on all three circuits right at the car and see what happens to eliminate all trailer-side possibilities.

I'm probably 5 total hours into this and still stumpified.

Thanks for your suggestions.

[adam aunins]

I'm sure that what ever it is is real simple, the hard part is finding out what that real simple thing is. Best of luck to you and when you do figure it out let us know so maybe we will not have to fight it in the future.

P.S. good clean connections(bulb in socket, wires etc), good grounds (wired not just tongue on ball) and dielectric grease, all of these will be your best friend.

[hriehl1]

Well... I got it. Of course, in retrospect, it was obvious.

I had bought a combination dongle from some internet trailer place that plugged-in to the Honda's proprietary-connector trailer-light take-off behind the rear trim panel.... that OEM take-off was 5-wire and the dongle also (was supposed to) convert the setup to 4 wire.

I put a volt meter on the dongle output and got 11+ volts for the taillights, but only 3 or 4 for the brake and turnsignal lights. I replaced the faulty 5-wire to 4-wire black-box converter portion and all is OK now.

Morals of story.

1. Those little LED testers will light up with 3 volts when a trailer lamp won't. Don't be misled.

2. When putting a voltmeter on the circuits, don't just casually look to see you're getting some volts... look closely at how many volts.

Thanks all for your help.

[algonquin]

Sounds like you have solved the problem.

Bad grounds are usually the culprit. When ever I have had trouble with the trailer lights I run a ground wire from the vehicle and splice it to the ground wire on the trailer harness. In most cases the lights then work without any problems. Brad

[foredek]

The new autos use such small wire sizes that problems develop when even a small load is attached. I solved my trailer light problem by going to all LEDs and thereby reducing the current draw. I had this my problem's on an '07 Ford Escape, eventually solved by the switch to LEDs. Maybe this wasn't the most elegant of solutions but it "got 'er done".
As mentioned in previous posts, a good ground is the most important part of the whole thing. It's the same with the 6 volt system on my '50 Chrysler only with larger wire sizes with a few shorts thrown in.