Troubleshooting Trailer Lights - Ground Short/Fault

Hooked the trailer up to the excursion, plug in the 7-pin pug. Do the usual light check. Turning the parking lights on pops the truck fuse for the trailer parking lights (that fuse is specifically for the trailer parking lights). Figure there’s a ground short somewhere.

Break out the Klein multimeter. Check truck-side plug, no continuity. Check trailer-side plug, it beeps to indicate continuity.

There’s been one light which has always given me grief (fender light). Locate the wire where it taps into the brown parking light wire. Test it and no continuity, so no ground fault there. Do the same for three more wires that branch off to marker lights. All three beep to indicate continuity. Think, how could all three go bad at the same time? Pull those three lights from their sockets. Retest those interior wires again, now there’s no continuity. Hmm. Decide to test the three unplugged marker lights, and all three beep to indicate continuity. Double hmm. I unplug the fender lights (on the wire tested earlier) and test. No continuity. Wtf?

I then pull all the marker lights off the trailer, including the two brake/parking lights. Test the trailer-side 7-pin-plug again, no continuity. So this should mean there’s no ground fault in the wiring in the trailer, right?

Test each light that was unplugged. I note that the only ones with no continuity are the LED replacement lights that were installed to replace the few standard ones that burnt out at various times. All the standard bulb lights tested beep indicating continuity. I’m thinking, how could they all have gone bad at once?

How do I test which one(s) is/are bad and are tripping the excursion’s fuse? Do I recheck them all again and note the resistance reading for each one, and the one(s) that are zero or out of wack with the others are bad? What am I missing besides and a degree in electrical engineering? I’ll probably just replace all the standard bulb lights with LED’s, but I’d like to know if I’m missing anything.

Thanks in advance.
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Pull all the bulbs out on the parking light circuit.

Put in a new fuse.

Install bulbs until the fuse blows.

Troubleshoot that lampholder

 

This is how I’d do it.

 

WHAAAT?? Good, clear advise in the first post? No smartalek comments like 'draw a pentagram, light a candle and worship Satan for the all-knowing answer'? Not even a !?


Where am I right now, I thought this was the Beeb? Thats IT! This place sucks. I'm outta here..

 

You need to pee on the wires and look for the little bubbles.
That's where the magic smoke is escaping that makes the lights work.

 

Thanks for the non-smartalek reply, Egg. I was hopeful that I could bypass the “test each light” option. If I may, one followup question regarding “troubleshooting the lampholder.” I did pull all the marker lights off the trailer and then test the trailer-side 7-pin-plug again. It indicated no continuity. So no problems. So, doesn’t this mean that there are no faults/problems with the wiring or any of the lampholders on the trailer?

<<Edit>> And brex saves the day for the smartaleks!
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Some lampholders have a spring loaded contact plate to maintain contact with the terminals on the lamp. I’ve seen them ground out with the lamp installed.

Hence why this is my troubleshooting flowchart now.

 

Buy a case of beer and invite the eggman to come over
with his meter. Pull up a lawn chair and drink the beer while he finds the problem.

 

I had the same problem so I pulled a Broome and just sold the trailer.

 

Gotcha, wasn’t aware of the spring loaded contact plate thingy. This is the style of the marker lamp-holders on the trailer:




Are these the same spring loaded contact plate lamp-holders that you were dealing with?

Also, out of curiosity, is the reason why the led marker lights indicate no continuity because the lamp assembly has at least a series resistor, and probably an active current-regulator circuit for the diodes?
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Was thinking about this overnight. The marker lamp-holders (bracket mounts) for the marker lights on this trailer are about 22 years old. Even if I find the offending lamp-holder(s), its only a matter of a short time until I'll be doing this again, and again, and again as they each fail. While I don't want to channel Broome at SuddenB's level (and sell the trailer), I may just replace all of the marker light bracket mounts on the trailer (and replace the non-LED lights with LED's as well).

That said, are there any particular style of the marker lamp-holders that don't have a spring loaded contact plate thingy? Or, since these lasted 22 years, I'll probably be dead and gone by the time I'd need to worry about the next replacements.
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Usually it’s the sockets that take an 1156 or 1157 lamp, but any lampholder that you “twist lock” the lamp in are the most common culprits.

LED lamps are open between the terminals because it’s a diode. Applying voltage closes the circuit in the diode (at least that’s my understanding of how they work…I just bring power to ‘em)

 

I'd look for a chaffed wire that's touching the trailer metal structure. On my trailer the mfr runs the wire thru a crudely drilled hole with no protective grommet so over time (22 years) it would be possible to wear thru the insulation. A bad ground in a light will just result in an open circuit and would not pop your fuse but result in that light not being lit. As an aside on this style light base if you run into that you can loosen and re-tighten the mounting screw where the brass tab connects to re-establish the ground.

 

Yup, that's what I was initially thinking. Which is why I pulled all the marker lights off the trailer, including the two brake/parking lights. Then re-tested the trailer-side 7-pin-plug again, and there was no continuity. So no continuity should mean there’s no ground fault in the wiring in the trailer. I plug just one light back in (in a few different lamp-holders), test, and I get continuity.
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You’ll get continuity through the filament in an incandescent lamp.

If your meter is zeroing on the Ohm scale, that’s where your problem is. You’ll have a resistance level through the lamp greater than zero.

 

Yep..checks the complete circuit.

 

Understand. I figured that out early on.

That is why I pulled ALL the marker lights off the trailer and retested. There was no continuity. Doesn't this test (with no lamps plugged in) tell me that the wiring of the trailer is fine ... and that my diagnostic focus should be on the lamps and the lamp-holders?

Further, I also tested each lamp that has a filament. Each lamp gave me a resistance reading of between 1.9 - 2.3. There was only one lamp that tested at 1.9. I'm not sure whether that would cause a fuse to pop, but I doubt it.

Thus, I am thinking the problem lies somewhere in one of the fifteen lamp-holders.
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Yup. 100% spot on.

Isolate systems and test components is generally the most effective troubleshooting tactic.