Turn Signals on 2009 Nissan Frontier Do Not Work When Trailer is Connected


I have a 2009 Nissan Frontier, and when I connect my trailer using a 4 prong plug a few things happen that I need to troubleshoot... note that I bought my truck used... and it had a electronic break system installed that I dont need ,nor know how to use... Im wondering if its causing problems? Anyhow, when I hook up my trailer wiring already installed the running lights on the trailer work, as do the break lights on the trailer. No issue here. The problem I have is two fold. So although the running lights and break lights are all working fine, I have two issues 1 my TRUCK turning lights dont seem to work - they just dont turn on. I have to turn on the left turn signal about 6 times before it starts blinking on my truck...so eventually it will start blinking if I keep playing with it 2 After I can finally get the left turn signal working on the truck, the RIGHT turn signal on the trailer starts working... and vice versa when I get the right turn signal on the tailer left starts blinking Im no auto expert but if someone can point me in the right direction, that would be really appreciated

Expert Reply:

Typically a 4-pole flat trailer connector on a vehicle carries only lighting functions (stop light, running light and turn signals). Electric trailer brake signals from an in-cab brake controller require an additional circuit (and an additional wire) to send those signals to the trailer brakes. Usually a 5-, 6- or 7-way connector is employed to include this function. Please refer to the linked article on trailer wiring; this shows the various vehicle-side and trailer-side connectors and how the functions are typically assigned by wire color and connector pin. A 4-pole flat vehicle connector does not normally support the braking function on a trailer.

Most likely the brake controller in your Frontier is an aftermarket unit. These can be wired into a vehicle either with a vehicle-specific plug-and-play harness that simply plugs in to both the brake controller and a connection port under the dash board, such as with part # 3050-P, or by a hard-wire method. If the vehicle was not built with tow package wiring in place to allow use of such a harness then the hard-wire installation is required. Since you do not need the controller you might be well-served by either unplugging the harness (if so equipped) or by cutting the wiring and removing it if a hard-wire install.

The simplest troubleshooting step is to substitute either a different tow vehicle or a different trailer to see if the same problem occurs. If you can, try your Frontier with another trailer known to function correctly. If you have the same issue then you can be fairly certain the problem resides in your trailer wiring. If you can try a different tow vehicle with your trailer you can also isolate the problem in this manner; if the truck lights behave normally then you know the issue is in your vehicle wiring. If you cannot substitute then you can use a circuit tester on your truck 4-pole connector to see if its various functions are operating correctly. You want to do this with the trailer disconnected. You will need a circuit tester such as # 3808 or # 40376.

Apply the tester ground to the exposed ground pin on the truck 4-pole. With a helper in the truck to activate light functions one at a time apply the tester probe to each wire while the truck functions are engaged. With the left turn signal on the truck activated, test the yellow wire: you should detect a blinking (on and off) signal on this wire. With the right turn signal engaged test the green wire: you should again detect an on-and-off blinking signal on the green wire. Next turn on the vehicle lights and test the brown wire: you should detect a steady signal until the lights are turned off. If the 4-pole behaves as described then you can focus on the trailer as the potential cause for the issues. I have included a photo showing where to apply the tester. If your Frontier uses an aftermarket trailer harness that connects to your tail light wiring, rather than an OEM connector, then this could be damaged.

The most likely causes for the vehicle lighting behavior you are seeing are 1) a bad ground connection on the trailer and 2) an overload of the wiring from the trailer lights. Inspect the trailer wiring from the connector all the way back to the lights. Make sure the white ground wire is firmly attached to a clean bare-metal surface on the trailer. Any rust or paint at the grounding point can be sanded off to provide a good bare-metal contact for the ground wire. Check all wires for any signs of cracks or wear in the insulation and for any points that may be rubbing on the trailer frame. Routine vibration can cause wear in the insulation that can lead to a short, or to a loose ground wire. Also look for any signs of corrosion on the connector pins; even a very slight greenish or whitish film can indicate corrosion that could interfere with proper operation.

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expert reply by:
Adam R

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