SFR Chasing down trailer light problems (on the tow vehicle side)

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Josh

Josh

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My old Ranger has a tow package and flat 4 trailer wiring. It worked at one point, but something has gone sideways and now only one turn signal will work when hooked up to trailer lighting. Brake lights and the other signal don't come on.

Any suggestions on where to start chasing this down? What the most likely culprits might be?

EDIT: Just to clarify, this is NOT an issue with the trailer. I've got a set of magnetic trailer lights to test with and I know they all work (verified on my other vehicles). The issue is somewhere in the Ranger's tow lighting harness/wiring.
 
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I would start with the bulb on the side that isn't working. Next to check would be the fuse.

Tell us the year and model of your Ranger.
 
With car on, and using a test light, check that each lead in the car plug has juice (someone needs to push the brake). Plug trailer in. Remove the light bulbs one by one checking each for power. No juice, follow the wire back to the plug looking for damage. It may be the plug on trailer side. Pick one that doesn't work. Cut small slice to place tester in and see if there is juice just on the back side of the plug.
dielectric grease helps keep my mg wiring working properly.
 
MELinOre said:
I would start with the bulb on the side that isn't working. Next to check would be the fuse.
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To be clear, this isn't a bulb issue. I have a set of magnetic trailer lights and I've tested them on my other vehicles and I know they are working. Truck doesn't have lights out that I'm aware of either.
MELinOre said:
Tell us the year and model of your Ranger.
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1998 XLT
 
Josh said:
To be clear, this isn't a bulb issue. I have a set of magnetic trailer lights and I've tested them on my other vehicles and I know they are working. Truck doesn't have lights out that I'm aware of either.

1998 XLT
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If they work on another setup I'd check the leads on the truck for power and the plug for damage.
 
Josh said:
My old Ranger has a tow package and flat 4 trailer wiring. It worked at one point, but something has gone sideways and now only one turn signal will work when hooked up to trailer lighting. Brake lights and the other signal don't come on.

Any suggestions on where to start chasing this down? What the most likely culprits might be?

EDIT: Just to clarify, this is NOT an issue with the trailer. I've got a set of magnetic trailer lights to test with and I know they all work (verified on my other vehicles). The issue is somewhere in the Ranger's tow lighting harness/wiring.
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Are you indicating that the truck brake and running lights do not work when the trailer is connected, or is it the trailer lights that are not functioning?
 
dep said:
agree with checking the plug connector coming off the truck.
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I'll third or fourth that suggestion. I've always sprayed a little contact cleaner inside the plugs.

Is the the 4 pin plug? They tend to get corroded big time dangling under the truck if not protected

If the wiring harness was installed by a third party, I'd make sure either the connector under the truck somewhere is clean, or if they spliced the wire to the trucks wiring that it is making a good contact.
 
UPDATE: No luck with some contact cleaner and a .22 bore brush. Seems like I'll have to go chasing down wiring connections next time it's dry and I feel like crawling around on the ground.
 
A simple light is a big help in diagnostics. Ground off and see if there is power at the plug, then work back connection by connection to find the hot, then repair or replace. A meter will also work.
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If one turn signal works, I would look more to the power side. If you got hot leads it's the ground side most likely, maybe rust at grounding point.
 
MoreQuestionsThanAnswers said:
Got it. Then it’s the truck side plug or the wiring from the truck wiring to the wire harness. Most likely right at the connection of wire harness to original truck wiring….unless it’s a ground issue, which can do weird things.
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What @MoreQuestionsThanAnswers said. When I checked my harness to truck connection, i found that the idjit who installet it just bared the truck wires with a knife, then simpy twisted the harness wires around to make a connection, and lightly taped them. What a shit job. I'm surprised I wasn't blowing fuses. Maybe the dirt provided some insulation.

You might consider just buying a new harness/plug (they're cheap) and replacing the old one. That may save a lot of troubleshooting time, lets you fix any bad connections, and eliminates a corroded plug as the culprit. Just make sure to do a quality job on the connections. I prefer solder and heat shrink, but waterproof crimp connectors should work also. Just disconnect your battery first.
 
I had a company truck once. The boss taped up the wire splicing to the canopy lights with foil tape. One night coming home from the river got real interesting...
 
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