Brake and Running Lights wires suddenly grounded

Question

I have a 1 year old BigTex utility trailer that was working fine but all of a sudden I have no running lights and an electric brake fault showing up on my truck dash.

If I plug in the trailer to the truck I immediately blow a 20A fuse in the truck for the running lights.
The trailer is a 2 Axle with the brake on the back tires.
Breakaway kit with the brake controller and small 12V battery.

When I was first trying to diagnose the problem I was looking underneath and found a wire near the brake had way too much slack and was apparently rubbing on something and had been cut through. I trimmed back the wire and put it back together again hoping that would fix the running lights. But no luck.
I removed the sheath on the 7 wire plug to see if I had some short occurring and everything looked clean.

When I test the wires at the plug using my meter the blue brake controller wire and the brown running lights wire are both showing a short to the white ground wire.

Any ideas? Is that caused by the brake controller? If it is how do I test the controller. I'm a fair mechanic but I'm new at electric brakes on trailers.
Thanks,

jkwilson · Answer

First thing I’d check is bulbs. They are the common item between the circuits.

enblethen · Answer

Agree with previous post, a shorted or wrong lamp! Doubt full as brake wiring does not go to lamps.Check junction box under the tongue.

matt_colie · Answer

Wgregb,

Should I start with a BTDT??
I'm the guy that gets called when nobody else can figure out what to do.

In your case, I would lay chips that the cable has been crushed someplace along its run. Sometimes this is not visible. I have used two different methods to locate the damage.

Method 1 - Buy a wire tacker. A cheap one will work. They put a signal on the line and you can follow it until the signal disappears and then hope that you can actually find the damage.

Method 2 - Get a small compass. Rig a light in series with the circuit. As it is shorted, the light should show. Now take the compass and follow the wire path as much as you can as long as you can and watch for the compass to change its reaction. Now you are near the short.

There is a third way to correct the issue, but it can be costly and time consuming. Rewire that part of the trailer. The marker lights might be a real issue, but most of the rest is sort of under the trailer.

Good Luck guy

Matt

bb_tx · Answer

Being two separate wires showing shorts to ground, I too would guess a crushed or crimped cable. Not likely simple rubbing would simultaneously ground two wires. I would crawl underneath and carefully check every inch of the wiring from hitch end to the termination points and look for anything abnormal along the wiring.

ca_traveler · Answer

A light in series is an excellent short detector. Have someone watch the light while you inspect and wiggle the wiring. If the light goes out you've found it. A likely place for shorts is loose wires in the connector plug, pull the covers and recheck and leave it open.

Isolate the problem to the lights by disconnecting the electric brake wires in the now open plug. Do you have a battery for those brakes and if so what condition is it in?

If all else fails cut the light wire near the rear and continue forward to find the problem. If that fails run temporary wiring on any convient route like over the roof and resolve later. I've done that because it was the quickest way to continue the trip.

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Brake and Running Lights wires suddenly grounded

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