Troubleshooting Trailer Turn Signals on 1997 Ford F150 With Factory Wiring Harness  

Question:

Helpful Expert Reply:

You can check the mini fuse panel located behind the power distribution box. There should be a 20-amp fuse in location 2 for the trailer tow stop/turn lamps. Also, in the power distribution box, there is a 40-amp fuse in location 16 for trailer tow battery charge and stop/turn lamps.

If this fails to correct the problem, you can troubleshoot. The best way is to disconnect the truck and trailer and test each one individually. If you happen to have access to another truck or trailer that you know functions properly you can test with those to help isolate the problem.

Start with the F-150. You can use a circuit tester such as # PTW2993 to test for correct signals on the vehicle trailer connector. Please refer to the photo provided which shows the various functions for each pin on the 7-way. (I have also linked a helpful article with troubleshooting tips.) Ground the tester and apply the probe to the pin at the 3-o-clock position while the right turn signal is engaged. Repeat for the pin at 9-o-clock while the left turn signal is engaged. If you detect on-and-off (blinking) signals, then the problem is in your trailer.

If you are using a 4-pole-to-7-pole adapter like the part you reviewed (Tow Ready # 30717), you can disconnect this from the vehicle 4-pole harness and test the pins on the 4-pole in the same way. Please refer to the second photo which shows where to apply the circuit tester on a 4-pole. If your 4-pole delivers the correct on-and-off turn signals, then the 7-pole adapter may be corroded or damaged and can be replaced.

It is not at all uncommon for trailer connectors to develop corrosion, especially if they have been in use for a lot of years. Even very slight greenish or whitish discoloration on the contacts can indicate corrosion and often this is enough to interfere with signal transmission. You can try treating the contacts with an electrical contact cleaner.

Next you can inspect the connector on the trailer for signs of corrosion or loose wires. Follow the wiring from the connector along the trailer frame all the way back to the trailer lights. Look for any spots on the wiring that may be rubbing against the trailer frame. Inspect the wiring for cracked or worn insulation and replace or tape securely as necessary.

Pay particular attention to the white ground wire on the trailer. This needs to be securely attached to a clean, rust- and paint-free part of the trailer frame. A weak ground connection may pass enough current to let some lighting function operate but not others, depending on the current drawn from each light on the trailer. If your trailer has a tilting bed, the pivot point may not allow the ground to pass through. The solution to this is to route a ground wire from the connector at the front of the trailer to each lamp assembly at the back of the trailer to bypass the pivot point. You can also apply 12V power directly to the turn light fixtures; this will tell you if the fixture itself or the upstream wiring is at fault.

As you note, it may be that there are fuses and/or relays missing from the power distribution box in the engine compartment. This will depend on the vehicle style. Please refer to the link provided which will take you to our page for trailer wiring for the 1997 F-150. You can use the drop-down menu at the top of the page to select your particular vehicle and see the appropriate wiring kits. Some vehicle versions will require fuses and relays but others will not. As an example, if you have a flareside bed with a factory 4-pole connector you can use Tow-Ready harness # 118242, which includes the fuses and relays required for this particular truck.

expert reply by: Adam R

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