Finding a SHORT

But,but...but, I don't get it.
YOU are the one we go to for electrical advice!
OH NO!

When you say you pulled four of the clearance lights, do you mean you pulled them off and checked where they tied in? If so, probably fixed. In that case you probably had a loose connection.

Once you traced the problem to the wire feeding the upper clearance lights, did you test each clearance light? I think that would be as close as you could get.

Trailer had a dead short. It melted the insulation off the test wire when connected to the battery. (at tongue) Cringing at the thought of a wayward screw or staple, I started by disconnecting Tail/Marker wires in the front J box. No short. Back J box next, once TM was disconnected, no short. On to the tail lights: both are wired/wing nutted inside the lens so it is easy to troubleshoot.

At the R tail, after separating the clearance wire to upper lights, everything worked with no short. NOTE: I was using a circuit breaker to prevent hot wires during testing. You can hear them "click" off. So, the culprit wire is isolated. Great, with those 7 lights up there... across the top and around each rear corner.



I disconnected the two upper side lights (again, wing nuts accessed behind lens) and also pulled the bases to check. The two outside rear lights were also loosened and checked. As said above, the ONLY flaw I found was 1 tiny copper hair out of place in one light. Might have been that? Middle three lights were not disturbed. At testing, all now work.

I've always tested trailer wiring separately (never connected to the vehicle), just eliminates several more feet of wiring and flaws. I suspected the trailer had a problem upon discovering the blown fuse in the Blazer.

Short Finders are the closest tool yet. (Thanks, Bigbill!) The problem is energizing the wire. Short finders are designed to replace a fuse, which doesn't exist in the trailer. I've figured out I can connect the vehicle, bypass at the fuse block and power the trailer wire that way. If the Short Finder would work at that distance... They aren't super expensive, might invest in one. At least results could be posted here.

Yes, I could fuse each live circuit at the trailer (TM, RT, LT) but that wouldn't really help, since the vehicle is already fused for trailer lights. Older cars ran trailers of the vehicle tail and turn circuits, in that case a trailer fuse would protect the car and those tails would still work if a wire shorted.

Thanks for all the ideas and thoughts! I take photos of wire locates and try my best not to screw thru the wiring. Use red masking tape to remind me NOT to pierce at that point. Still, I screwed thru a tail light wire in the Aladdin and now have done "something" to this Red Dale. Hopefully, it was the copper hair.

He's going for a trip around the neighborhood to see if the wiring holds. If it ain't broke...

The "hair" might well have done it.

Could it just be an overload rather than a short? What size is the fuse that's blowing? Each regular 1156 bulb draws 2 amps, and I've never been impressed with the wire size in tow harnesses. Maybe something like hitting the brakes pushed it over the edge.

Vikx,

As we all know, the resistance of a wire is directly proportional to it's length and I've used this in the past to find shorts, but it was in the specialized case that I knew it was two conductors in a multi-conductor wire shorting together. Given the likelihood that the wire is shorted to the frame or body, I don't think this can help you in general here.

Paul has a good idea, essentially adding a load resistor to the shorted wire, which would then allow you to use an inductive ammeter to trace the path of current flow -- assuming you have access to the wire. Problem is you usually don't.

If I were looking for a short, the first thing I would do is to find the circuit by testing resistance to ground with the trailer disconnected. Then I would pull the bulbs from that circuit to cut it into pieces, and test the resistance of each piece to isolate a single wire. At that point I would have to manually check the wire (or replace it). So, to me it seems like an ohm meter is about all that's needed.

Shorts sure suck

B

I'm thinking along the same lines as Cow. I recall reading somewhere recently that you can overload the lighting circuits of modern vehicles when adding trailer lighting to it. I'd think a Trailblazer would have a robust enough system to handle the extra load but you never know. If the fuse pops again, this is where I would start.