Troubleshooting Trailer Taillights Where Both Turn Signals Light Up at Once  

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Expert Reply:

If you are using a Blazer trailer light kit C6424 and both turn signals flash at the same time when you activate either one, then this usually indicates a ground problem on the vehicle's wiring harness. A weak ground can cause all sorts of odd behavior from trailer lights. An example of a wiring harness for your Explorer is part # 118345

Check that your Explorer wiring harness ground wire ring terminal is firmly attached to a clean bare metal surface free of paint, primer, grease and dirt. You can sand that spot if needed to get a clean bare metal surface. Or you can relocate the ground to a better spot but be careful when you drill to avoid any delicate components. You can also run a ground jumper wire right up to the battery's negative post. Re-attach the ring terminal and test your lights again.

Another possibility is that there is dirt, water or corrosion in one or both of your trailer connectors. You can use a spray-on electrical contact cleaner and a small brush to remove any discoloration (sometimes greenish, sometimes whitish in color) and use dielectric grease like # 11755 to protect those connections.

If neither of those steps fixes the problem, you'll want to use a tester like # PTW2993 to test the pins on the Explorer wiring harness to see if it is damaged. Test the wires going into the box for the correct signals while you activate each light function on the Explorer. If the signals are going into the harness as they should then test the output wires on the right side. If these signals are going in but not coming out, then the harness is bad and needs to be replaced.

expert reply by: Adam R

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