The Left and Right Turn/Brake Light Circuit
The ground circuit is the first thing to check and repair if you have a wiring problem. The electrical current needs a good return path from all the other circuits in order to allow everything to operate properly. Depending on the trailer hitch to provide this return path DOES NOT work! See the ground page.
Start by making sure that the signal light current is present at the tow vehicle socket. Clip the ground wire on the test light to a good ground on the tow vehicle and probe the terminal at the nine o'clock position (for the left turn) or the 3 o'clock position (for the right turn).(Looking into the plug, toward the front of the vehicle) With the ignition switch on and the turn signal activated, the test light should light up. If not, you have a problem in the truck wiring circuit.
Next remove the tail/signal light lens at the rear of the trailer and check the condition of the bulb. Often corrosion builds up in this area and prevents the current from flowing. Clean or replace the light bulb and clean the light socket interior. (a plastic dish washing scrub pad works well) Don't use steel wool as the strands left behind will short out the circuit.
Next plug the trailer cord into the tow vehicle socket and activate the turn signal. If the new or cleaned bulb does not light, remove it, place the test light clip onto the outside shell and probe the two terminals inside the light socket. Be careful not to allow the test light probe to touch the outer metal shell of the socket as this will create a short circuit and blow the tow vehicle fuse. Remember that the signal light is flashing, so hold the probe on the terminal for several seconds.
If you have no power at either terminal, reconnect the test light clip to a known good ground and probe the terminals again. If it now shows power, then the light socket or the whole light assembly is not grounded and repairs should be made.
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