Trailer Lights - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Mike Quillici · 09-03-2012, 11:42 PM

I have a boat trailer that I know is wired correctly, because I have towed it with several other vehicles, and have never had a problem. When I hook it up to my 72 C-10, however, something isn't right. Turn signals appear to work fine, tail lights appear to work, brake lights appear to work, but when the tail lights are on and I press on the brake lights, the trailer goes dark. When I let off the brakes, the tail lights come back on. When the tail lights are off, the brake lights work fine. The only other thing that I have noticed is that the brake lights and turn signals are dim. . .Ground is solid.

I'm not sure what's going on, but clearly something is crossed on the truck side. Any ideas?

**mr48chev** · 09-04-2012, 12:11 AM

Do the other rigs you tow it with have turn light bulbs that are separate from the stop light bulbs? If the trailer is set up for those it "might" cause an issue.

2. How big of a basket of snakes do you have under the back of the truck for trailer wire hookup. It seems that on almost every old truck I buy I end up cutting all of the previous owner's trailer hookup out and rewiring it to make it work.

Mike Quillici · 09-04-2012, 08:16 AM

1. No, I just have my boat trailer.

2. And I just cut out about 10 feet of useless wiring and wired up my own wiring harness. I did a little research, and apparently this is a very common problem associated with a bad ground, so my statement in my first post may be incorrect. I am going to re-ground both the truck and trailer tonight, so we'll see what happens.

PanelDeland · 09-04-2012, 08:52 AM

I'll put in that the symptoms are those of a greound problem.It sounds like the lights are grounding thru one of the other circuits when power is applied.If both circuits are powered,there's no place to ground.If you have a "flat 4" plug,they make a tester for it that only costs a few dollars.The 4 wires should be
Left turn
right turn
tail lights
ground
The brake lights work thru the turn signals.They function as brake lights until the flasher (for turn signal) interupts the circuit causing them to flash.This happens one at a time with turns and all at once with emergency flashers.

Hope this helps.

Mike Quillici · 09-05-2012, 12:30 AM

It was a bad ground. All fixed and reattached. No more problems. Thanks everyone.

09-03-2012, 11:42 PM	#1
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Trailer Lights I have a boat trailer that I know is wired correctly, because I have towed it with several other vehicles, and have never had a problem. When I hook it up to my 72 C-10, however, something isn't right. Turn signals appear to work fine, tail lights appear to work, brake lights appear to work, but when the tail lights are on and I press on the brake lights, the trailer goes dark. When I let off the brakes, the tail lights come back on. When the tail lights are off, the brake lights work fine. The only other thing that I have noticed is that the brake lights and turn signals are dim. . .Ground is solid. I'm not sure what's going on, but clearly something is crossed on the truck side. Any ideas? __________________ 1972 Chevy C-10 ~ 188,000 miles give or take

09-04-2012, 12:11 AM	#2
mr48chev Registered User Join Date: Jul 2007 Location: Toppenish, WA Posts: 15,595	Re: Trailer Lights Do the other rigs you tow it with have turn light bulbs that are separate from the stop light bulbs? If the trailer is set up for those it "might" cause an issue. 2. How big of a basket of snakes do you have under the back of the truck for trailer wire hookup. It seems that on almost every old truck I buy I end up cutting all of the previous owner's trailer hookup out and rewiring it to make it work.

09-04-2012, 08:16 AM	#3
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Re: Trailer Lights 1. No, I just have my boat trailer. 2. And I just cut out about 10 feet of useless wiring and wired up my own wiring harness. I did a little research, and apparently this is a very common problem associated with a bad ground, so my statement in my first post may be incorrect. I am going to re-ground both the truck and trailer tonight, so we'll see what happens. __________________ 1972 Chevy C-10 ~ 188,000 miles give or take

09-04-2012, 08:52 AM	#4
PanelDeland I am a Referee of life. Join Date: Jun 2000 Location: Greensboro N.C. Posts: 13,993	Re: Trailer Lights I'll put in that the symptoms are those of a greound problem.It sounds like the lights are grounding thru one of the other circuits when power is applied.If both circuits are powered,there's no place to ground.If you have a "flat 4" plug,they make a tester for it that only costs a few dollars.The 4 wires should be Left turn right turn tail lights ground The brake lights work thru the turn signals.They function as brake lights until the flasher (for turn signal) interupts the circuit causing them to flash.This happens one at a time with turns and all at once with emergency flashers. Hope this helps. __________________ The 47-present Chevrolet and GMC Truck Message Board Network,it's owners,moderators,members,and associates of any type should not be held responsible for my opinion. You can't fix stupid,not even with duct tape. "My appearance is due to the fact that "GOD" does punish you for having too much fun!" Barrett-Jackson has perfected alchemy,they make rust into gold! "You can lead a horse to water but you can't saddle a duck" "Cleverly disguised as a 'Responsible Adult' "Sometimes your Knight in shining armor is just a retard in tinfoil"

09-05-2012, 12:30 AM	#5
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Re: Trailer Lights It was a bad ground. All fixed and reattached. No more problems. Thanks everyone. __________________ 1972 Chevy C-10 ~ 188,000 miles give or take



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