Marker lights blowing Centech fuse

[Hopper]

The marker lights on my 74 were working fine until a couple days ago. Now as soon as I pull the headlight knob, the marker light fuse blows in the Centech harness. Obviously a short somewhere but wondering if anyone has dealt with this or can recommend a starting point. Everything else is working- headlights, brake lights, etc

 

[Broncobowsher]

If you can isolate parts of the harness, as unplug the part of the harness that goes to the rear. Start with that. If you stop blowing fuses after disconnecting something, it is in that thing you just disconnected.

Unplug tail lights, unplug front marker lights, side marker lights. Lights can short directly to the socket.

Two to approach this. Splitting the system. If you can disconnect half a vehicle in one shot, that gives quick filtering to which half of the vehicle the problem is in. One wire feeds the whole back half, disconnected and the problems remains, you know it is in the half you just disconnected. Or it remains, you can leave that part disconnected and treat it as a non-problem.
The other is small steps. Take one light apart at a time (disconnect from the harness if there is a plug). This is small steps so it is possible to isolate to a single part. Not taking all the lights off the front of the truck at one time and the problem stops, not knowing which one was the problem.

It is possible it is not in a light. The harness can be pinched and that will short out as well. I had that once. Routing looked good, but as I drove a couple parts moved a little bit and a wire got caught in there. Happen to be tail lights. I ended up snipping the wire to the rear tail lights, which was both tail lights and the trailer plug. That stopped the fuses blowing. Didn't find a bad socket. Followed the harness and found the pinch point.

 

[DirtDonk]

Dual exhaust is often the fiend behind a rear harness problem. But front running lights/turn signal sockets are not without guilt sometimes too. Just like the back sockets.
Luckily Centech does have that main connector over by the driver's side wheel well near the firewall that feeds all the rear wires other than the backup lamps I think.

Since Centech does not supply a wiring diagram, finding a fault on a circuit with multiple loads can be a problem sometimes. In your case you know it happens only when you turn on the running lights, so you've already narrowed it down enough to get things rolling.
And don't forget the license plate light in your hunt.

Hmm, speaking of "other" things on the circuit, do we know whether the same fuse protects the dash lights too? In a stock setup the dash lights are on their own fuse. Not sure about Centech.
So don't rule them out, but disconnect that rear harness first like was suggested to see if that gives you a starting point.

Good luck.

Paul

 

[Eoth]

I dealt with the same situation. Turned out that a PO spliced in additional plow lights at one time. When he removed the plow he just cut the wires. Just sitting there everything would be fine.. But when driving, the vibration and wind would shake the wires enough for contact and then "pop".