2nd Gen Non-Engine/Transmission Electrical - Trailer Lights

[grimesja]

the current problem; I have no electrical power to my trailer taillights from my truck. The trailer has brake lights, turn signals, and emergency flashers but no taillights. All lights work on the truck.



I had this issue about two years ago the fix at the time was replacing the trailer fuse under the hood. I have checked the fuses and relays; no joy. I do not see any broken wires.



Does anyone have another troubleshooting idea for this problem?

 

[HBarlow]

Do you have a volt meter or simple circuit tester (light bulb on a pointer with a separate ground wire clip)? If so, sit down at the rear of the truck with all lights off and test for one hot wire at the seven pin female receptacle.

When you find it, note or sketch the location. That is the always-hot RV charge line. Then turn on the truck's parking lights and search again with the tester lamp. You should have another hot pin. That's trailer running lights and taillights.

Turn off the parking lights and activate the left then right turn signal lamps, in turn. Using your circuit tester, look for each separate pin to flash hot then off. Then last, have someone activate the manual override on your trailer brake controller or step and look for that pin to become hot.

If you have voltage present at all of those pins, in turn, the problem is in the trailer male plug, wiring harness, or trailer lighting circuitry. Could be a lost ground wire or ground connection in the plug or harness.

It the truck receptacle pins do not show hot as called for, the problem could be in the truck's receptacle or harness. Sometimes old, worn, or corroded pins in the female or male side of the seven pin connector simply don't make good clean contact even though the voltage is present.

 

[SRath]

Like Harvey said: You need to test for tailight juice at the vehicle connector.

YES> You have a broken brown (most likely) taillight wire on the trailer or a bad connection inside the first tailamp the wire goes to if it is wired in series (from one light to the other).

NO> You need to track the vehicle wire to where the juice is and ends.

Just went through this with my wife's S-10 Blazer two days ago, which we almost never use to tow. No Taillights. Everything else was fine.

The diabolical problem was within the molded 4-pin flat connector at her vehicle. It looked just fine. Turned out the wire and connection had corroded through INSIDE the wire insulation right at the molded plug. Could not see it, but the tester don't lie and scraping away the insulation revealed the culprit.

Solution: buy and splice in a new connector using weathersealed heat connectors.

 

[grimesja]

The trailer side is fine, We plugged it into another truck, all lights worked as advertised. It is a truck issue on the running light side.



I had the same issue last year, it was a fuse under the hood.

 

[HBarlow]

Check the fuse panel descriptions in your owner's manual. There are circuit breakers under a black plastic cover under the hood and also a small fuse panel in the outside edge of the dash on the driver's side.

 

[grimesja]

the fuses and relay were the first things which I checked.

 

[SRath]

When you say "checked", do you mean you have juice going both in and out of them?

At some point along the circuit, you will have no juice.

Right now, based on my experience with my own, I would be looking hard at your headlight switch. Mine has melted down twice in the past until I made my own piggyback harness with relays for my lights and trailer connection. The plug on mine (the wiring harness where it plugs into the switch) is still a crispy critter, and is very touchy about making a connection, and I was very pleased to discover the other day that RockAuto offers a replacement.

 

[Road Dog]

If the trailer plug is wired correctly then the running lights are on their own relay and the headlight switch should be fine.

You can't tell if a fuse is good by looking at it. Sometimes they pop and look fine. You need to check the continuity (i. e. resistance).

 

[surfbeetle]

You said that you plugged the trailer into another truck and all was well. I now suggest that you sketch out/notate the receptacle on Both trucks to see if they are the same. If one was miswired and the trailer was as well, it may pop fuses in the truck that doesn't match the trailer. Had this happen with my father's truck, he had wired his trailer and truck with a 6 pin connector that had two wires crossed. His truck and trailer were fine as a matched set. The problem was when I hooked up to it with a 7 to 6 pin adapter.