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Old 03-29-2012, 11:49 AM
jpothof jpothof is online now
Join Date: Mar 2011
Location: Madison, WI
Posts: 29
Default Truck-Trailer Wiring Problem -- Suggestions


I read a lot more than I post here, but have a wiring dillema that I can't get my head around. When using our normal tow vehicle a 2006 5.3L V8 Envoy Denali the 7 to 5 connector hooked up to my 2010 Ranger 621 works just fine. All lights work as they are suppose to.

I wanted the option of using our 2002 Blazer in a pinch so I put on a 4-5 flat connector and hooked up the trailer. The right stop and turn light will not come on.

I left out the 5-wire adapter and the problem persists

I tested the current from the green wire (R stop and turn on the Blazer), and the tester will blink on the vehicle harness and on the trailer harness as far back as I can follow it to the light. I replaced the truck end plug as it seemed corroded a bit, but problem persists

I thought for sure the ground was bad so I replaced the ground completely on the truck right next to the old ground by filing off the undercoat and drilling a new hole and attaching the ground wire with a screw. This didn't help.

Grounds at the trailer tongue and that right light look great with no loose bolts or corrosion.

I'm not sure if its an amperage issue from the Blazer that the Envoy doesn't have or if I'm missing something. Any thoughts from the group?


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Old 03-29-2012, 01:14 PM
B. Fife
Posts: n/a

Jeff. Isn't there some sort of diode protection / voltage polarity protection in the newer vehicles? I think there is and I don't believe you can just hook up a standard flat connector without having that protection incorporated into your connector or wiring.
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Old 03-29-2012, 02:39 PM
Jerryv Jerryv is offline
Join Date: Mar 2010
Location: Minnesota
Posts: 1,154

It seems most likely that it is a poor connection on the green wire on the Blazer. It will allow enough current through to light the tester, but not the trailer light. Are you checking it with the light connected or unplugging the light to use the tester?

Try to get the tester and the light connected at the same time.

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Old 03-29-2012, 05:44 PM
REW REW is online now
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Join Date: Nov 1999
Location: .
Posts: 31,411

1. Inspect your 5 wire connector for signs of corrosion.
Then, just take a new flat 4 connector and plug it into your boat lights.
Connect the white ground wire to the ground on your truck.
Then, take the green wire, and just connect it directly into the appropriate pin of the 5 wire plug. See if all of your lights for the green wire now work.

You can also take the same green wire and plug it into the spot for the yellow wire, and verify that the yellow wire from the truck will light up the green side.

You can also take the same green wire and plug it into the brown or tail light wire for the truck and verify that the brown wire lights up the green side.

If you find that when connecting the green wire from your trailer to the equivalent of the green wire on the 5 wire plug in your truck you find that your lights don't light brightly, but all of the other connections light the lights, you need to follow the wire that goes to the green connection on your truck, back to its origin. It is normally a short run into a tail light harness or similar. If, when you go back to the source for the green wire and connect the green wire to that point by your boat trailer you find that the wire now works, install a new 5 wire plug on your truck. You could easily have a bad or corroded wire internally from your truck harness to the green equivalent wire on your 5 wire plug.

If you happen to have a hand held 12 volt spot light, you can save hooking up your trailer by using the hand held spot light to test your wiring. The spot light will put a good load on your trailer wiring, so that you can easily check for corroded and or bad wiring at the back of your truck.

Most of todays LED testers just check for the presence of voltage and put essentially no load on the wiring when testing. So, if you have a high resistance wire or connection, you will have good test results, but bad lighting results.

Good luck
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Old 03-29-2012, 07:44 PM
oilguy oilguy is offline
Join Date: Aug 2011
Location: Alberta
Posts: 257

Hey Jeff, have you checked the fuse box under the hood of the Blazer? I have a 2002 Silverado that had a bad fuse and I was experiencing the same issues. Once I ruled out the trailer wiring/lights and replaced the plug on the truck I finally realized that there were fuses under the hood I had never checked. It was a relief to have fixed the problem, but I sure wish I would have pulled out the service manual on the truck before going through the hassle I did. I assumed that the signal/brake fuse would be the same fuse for the tail lights of the truck as it was for the trailer plug. Live and learn I guess. It's a good thing I don't make my living as an electrician!
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current , ground , trailer , truck , wiring

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