Troubleshooting Trailer Lighting with 4-Pole Connector and 2008 Jeep Grand Cherokee  

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Expert Reply:

The most common causes for trailer lighting issues like yours are corrosion on one or both of the connectors or a bad ground connection at the car or trailer. The linked article may be helpful to you as you troubleshoot. When troubleshooting trailer lighting it is usually easiest to disconnect the trailer from the vehicle and test each individually.

Inspect both car and trailer side connectors for even slight greenish or whitish discoloration on the contacts. Clean as required; you can try a spray-on contact cleaner like Caig De-Oxit or fine grit sandpaper to remove any corrosion. Also make sure the ground wires on the car and on the trailer are both secured to clean bare-metal surfaces that are free of paint, primer, grease or dirt. If your contacts are clean you can keep them that way longer by applying dielectric grease # LT37534. If these checks do not correct the problem then you can move on to testing.

I suggest you first test the trailer. You can use the car's battery (or a spare battery if you have one in the garage) to apply 12V power directly to the contacts on the trailer's plug. This testing method bypasses the car's wiring harness. You will connect the battery negative terminal to the ground pin (which has a white wire). Then one by one you'll connect the positive battery terminal to each of the other pins to see if each function operates correctly. On typical 4-pole trailer wiring the yellow wire will activate the left turn/brake light; the green wire will activate the right turn/brake and the brown wire will activate both running lights. If the trailer lights work correctly then you can move on to the vehicle's harness.

To test your Grand Cherokee's trailer socket, you'll need a circuit tester such as # PTW2993. You'll need a helper to activate the car's lighting functions while you apply the tester to the 4-pole. Please refer to the linked video that shows how to use a tester like these. You'll ground the tester to the exposed ground pin on the car's 4-pole. Then have your helper activate the left turn light and then the brake light while you test the pin for the yellow wire. Both the turn and brake should activate the tester. Repeat for the green wire's pin while the right turn and then the brake are applied. Both should activate the tester. Finally, apply the tester to the pin for the brown running light wire; both running lights should activate when the car's headlights are turned on.

If your trailer harness has 5 wires it is likely that it has two brown wires both connected to the same pin on the 4-pole plug. This is called a wishbone harness because it simply has dual running light wires to make it easier to make the connections at both sides of the trailer. This avoids the need to run a jumper wire between them. An example is part # A20WB.

If you find that you need to replace the wiring harness on the car and/or trailer you can use the linked page to choose a harness. If your car has a factory tow package use harness # 118381 as a direct plug-in replacement 4-pole. If it does not, then instead use Tow Ready part # 118425. Both are no-splice installations that simply plug-in to existing connectors on the car. For a replacement trailer-side 4-pole you can use the wishbone harness noted above. If your trailer has a 5-pole, then instead you can use # A25W5B. Do note that a 4-pole and a 5-pole can join together and provide the 4-pole's functions (just not the additional circuit function that the 5-pole has).

expert reply by: Adam R

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