trailer light problem

[Grawolf]

Hi, I picked up my 2004 used. It came with hitch and flat 4 connecter. My utility trailer has all new wiring and lights. Everything works fine when the headlights are off. The front turn signal and the corner light blink in unison with the headlights off. With the headlights on they blink alternately. The trailer lights (turn signals) blink on both sides when the headlights are on. So right now I can't use the trailer at night. Any ideas?

[HenryJ]

Sounds like a bad ground.

These trucks did not come with a factory tow package, so someone has added the trailer wiring. Look to see if it uses the ScotchLock connectors. Those are a poor connection waiting to happen. Solder and seal those connections if that is what you find.
If it is the Tee connector plugged into the factory harness. Unplug it and make sure it is clean. Use some dielectric silicone to promote a good connection and prevent any oxidation forming.

Lastly check the four pole connectors and make sure the contacts are clean. The dielectric silicone is a good idea on there too.

Welcome aboard!

[Grawolf]

Bad ground at the trailer or in the truck? Know of anybody that puts out a decent repair manual for this truck?

[Grawolf]

The wiring harness for the flat 4 is the the t connector type, not spliced in. No skotch locks. I'll take a look at the connectors to check for corrosion. Thanks for the tip.

[HenryJ]

Good to hear you have the good wiring connector.

Bad ground at the trailer or in the truck? Know of anybody that puts out a decent repair manual for this truck?

Check both. "If in doubt ground it out" is a good rule. The hitch is not a good ground. Check the ground wire at each side near the bumper bracket bolts. This usually only affects the trucks tail lamps, but we have seen several with problems. Also check your fuses under the hood. Just for kicks

The best manual available is the factory GM manual published by Helm. Mine is for sale in the classifieds. You can watch ebay too. They are not cheap, but they are a three volume set covering everything.
Your next best bet would be the big Chiltons manual. It includes wiring schematics, but is not crew cab specific and will not take into account the forward harness we have for the utilities and the truck harness on the back half.

[Grawolf]

Ok, thanks. So it's normal for the turn signals to act the way they do? (Headlights off: unison, Headlights on alternately)

[HenryJ]

Ok, thanks. So it's normal for the turn signals to act the way they do? (Headlights off: unison, Headlights on alternately)

No. That doesn't sound right to me. It sounds more like you have a grounding problem.

[Grawolf]

I'll let you know what I find. Thanks again.

[_STUCKY]

You already found the best repair manual that there is for our trucks. It's called http://www.s-10crewcab.net and it has feedback.

[Grawolf]

Well, sorry this took so long. Life just gets in the way sometimes. Anyway I cleaned all the grounds I could find and replaced the add in harness because it had 2 little black boxes attached that I don't know what their function was. After all that the trailer lights still acted funky. Plugged in a test unit to the truck flat 4 and it worked fine. Tackled all the grounding points on the trailer next, still not working right. For the heck of it I decided to run a wire from the grounding screw at the hitch (on the trailer) to the tail lights. That made all the difference in the world. This is one of those bolt together utility trailers (20+ years old) and I guess the connections between the joints are too corroded to support a decent ground thru the trailer frame. After installing dedicated ground wires to all the lights, everything is working fine. Thanks for all the input. BTW, the front turn signals / corner lights still do their unison/alternating flashing depending on whether the headlights are on or not.

[JaVeRo]

Just to explain the weird stuff... 12v+ going to the left trailer turn signal (brake light filament) can't find ground so it backfeeds thru the tail light filament and goes to the right side tail light filament and still doesn't find ground. Then it backfeeds thru the right side brake/turn signal and goes to the truck brake light to find ground. Then you turn the tail lights on and mess with the backfeeding ground.

It may not happen exactly like that. The point is, as my dad told me 35 years ago, If it's doing something really weird then it's most likely the ground. I had a 74 Malibu that was doing the same alternating thing your trailer was doing.

James

[Grawolf]

Thanks. But in my case the trailer lights blinked both sides together when the headlights were on. The TRUCK front turn signal/corner lights flash in unison when the headlights are off and alternately when the headlights are on. This happens whether the trailer is hooked up or not. Doesn't seem to be a problem. I thought, at first, that might be causing the trailer light problem. Turns out the trailer grounds were bad as both you and HenryJ suggested.