94 S10 Blazer,,,,have totally replaced all wiring on trailer, all new lights, wires and connectors. Problem now is I plug trailer to existing wiring on truck and I have a feedback whenever I turn taillights on, everything is fine. If I actuate the left turn signal, I get a slow flash, momentarily and then the 20 amp fuse for the tail and markers and instrument cluster lights blows. If I turn the tail lights off, the left turn signal works normally. I originally hooked up the truck plug lights years back, and they've always worked properly. Question, does anyone make a tow package wiring loom to tap the truck's electrical system? Am I missing something on the new lights on the trailer? I'm stumped as to where to go from here. All lights on the trailer work fine when tested with a battery. All systems on the Blazer work fine on the truck when the trailer isn't plugged into it. All systems test normally. What am I missing?

 

A few thoughts come to mind:

Have you checked the output from the truck's trailer wire connector to confirm that it is operating as you would expect? (In addition to a light-bulb circuit tester, also use your voltmeter to check the polarity of the voltages to be absolutely certain.)

Have you a dedicated ground wire to each of the lights on the trailer? Without such a wire strange things can happen. Also don't rely on the hitch and ball to be the ground connection to the truck. Use a connector/plug with a ground wire to connect ground from the truck to the negative terminal of each bulb.

Are you using LED lights? Could there be a polarity problem going on causing a one-way circuit you did not expect to have?

 

Standard lights (not LED), dedicated ground wire on truck and trailer. Grounds for lights are through the mounting bolts to the frame, (used star type lock washers on everything to insure good electrical contact. Trailer lights all checked out from plug through system. ( used an older style battery charger that I've used before for trailer light testing). Am wondering if my cheap NAPA light kit could have a bulb in one of the tail lights that has shorted across. Will check this AM. n the past when experiencing similar problems, it's always led me back to a ground issue That's why I always use a ground from the vehicle to the trailer through the connecting plug.

 

Yes, there could be an issue with the bulbs themselves. Good thought. As I write this, it occurs to me that perhaps the sockets into which the bulbs go could have a problem too?

As for grounding, I learned the hard-way that I needed to run a ground wire to each bulb as using the body of the trailer to act as the ground wire in the circuit was not a durable solution. I got years out of doing it your way (star washers, clean metal surfaces, threads chased with taps, dielectric grease) but eventually, the resistance at all of the fittings and attachment points got large enough to mess with the operation of the lights. I then ran dedicated ground wires from the trailer wiring plug to each light bulb. Even if it doesn't turn out to be a grounding issue, I highly recommend dedicated ground wires from the trailer electrical connector to each bulb.

The thing about trailer lights is that we tend to assume it is all about voltage only but the resistance in the circuit matters too. Whereas a battery charger can output quite a bit of current at ~12V, your truck might not be able to source as much with a commensurate drop in voltage at the trailer electrical connector when connected to the load that is the trailer electrical system, especially if there is excess resistance in the circuit in the trailer because of not-so-good electrical connections of the ground wires to the frame. Hence my recommendation to anyone to only use electrical wires to supply the ground to each bulb and to avoid connecting to the trailer frame.

Good luck

 

UPDATE!!!! Yesterday driving Blazer to town to check the mail and to the part's house to purchase a small 20 Amp. auto reset circuit breaker to replace tail light circuit fuse until I can solve the problem. Trailer disconnected and staying at home. Always use headlights when driving the secondary roads here (No Freeways for 70+ miles) noticed when I parked, and turned off ignition, reached up to turn off headlights, and it dawned on me,,,,,no headlight alarm bell. Got out and checked truck. No license plate tail or marker lights on Blazer. Hmm Blown fuse. So now I'm going to go into the truck, and rewire everything that I wired at least 10 years ago. I'm thinking one of my Scotchloks is shorting out. I'll follow up when I find the problem. Thanks for your help and suggestions.

 

I think you've identified a good starting point for trying to diagnose your electrical issue.


[RANT] My recommendation would be to immediately remove and repair all wires where you used scotchloks. I hate scotchloks. When I worked in the automotive electronics industry I fixed a ton of problems in vehicles caused by people using scotchloks. Saw a couple fires caused by them too. Some people will say they're fine, but of the hundreds of them that I removed, there wasn't a single one that hadn't cut at least half the strands on the harness wire being tapped. [/RANT]


Sorry for the rant and if you're a big fan of scotchloks, more power to you but you'll never convince me that they're an acceptable method for making a proper electrical connection in a vehicle.

 

I only do emergency repairs with them.

After that I strip the wire, push a length of shrink hose over the wire, solder the joint, slide shrink hose over the fix and shrink it. All stuff outside the weatherproof cab, I tape it over.

Could be a short or a bad ground that pulls too much current. May have to check a good part of the wiring harness. Any auxiliary power outlets? HiFi system wired in? A good point to start looking.

 

Over the years, I've used scotchloks and back when 3M first introduced them, I was a big fan. I wired everything with 14 ga. wire, and never had a problem. Used them to wire up tow vehicles for RV trailers, and wired pickup campers to trucks. Then when every Tom, Dick, and Harry got into the game and started copying them, the size of the blade in them became smaller, and like it was said, they started cutting conductor strands of wire. Like a lot of the things that I've discovered over the years, the spinoffs were never as good as the originals. It's just like the replacement tail lights that I installed on this utility trailer. It came with mounting nuts, but of courses they are metric. Now I have to go to my local hardware store, to find extra nuts to connect ground wires to them. I have an enormous supply of nuts and bolts in standard sizes, but not so much in metric. Oh well, I guess it's progress.

 

In a pinch, I like the heat-shrink crimp connectors but soldering with heat-shrink is the best.

I also have become a big fan of the self-fusing silicone tape, often sold as "Rescue Tape" I use it to bundle wires together, for strain relief, and to add an added layer of protection from the elements to my wires.

 

OK,,,,checked my local marine supply store, and an Ace Hardware. Where do I buy this tape?