Troubleshooting Light Bar That Only Functions In The Accessory Ignition Position

Updated 04/17/2025 | Published 12/13/2021

Question:

I installed a LED truck tailgate light bar on my RV trailer with all LED lights to supplement rear visibility. Everything works as it should with the truck off and ignition in accessory power mode. However, as soon as I start the engine, the tailgate light bar stops working, however the truck and RV all still work fine. Why would starting the truck make any difference to the light bar but nothing else?

asked by: Michael G

Expert Reply:

It sounds to me like the light bar was wired to the running lights which is why it works with your truck's ignition in the accessory position but cuts off when you start the engine. You can test this by checking the wire functions with a circuit tester like # PTW2992. Or simply turn your headlights on in your 2018 Ford F-350 Super Duty and check if the light bar is working. If this is not the case let me know and we can keep troubleshooting further.

expert reply by: Kevin C

RV With Light Bar
(click to enlarge)

Jim T.

6/17/2024

Good day. I have the identical problem. TV is a 2019 Ford F250. Trailer lights are LED and work fine. Light bar works fine when truck key is in Accessory position. Light bar turn signals/break lights do not work when ignition switch is on (engine running). Running lights on bar continue to work in either Accessory or On key positions. Have tested voltage at connector without the light bar connected and all is good. Once light bar is connected, the voltage reading across either of the turn signal connectors good from 0 VDC (or close to it) to Over limit on my Fluke digital multimeter (also tried to see pulse voltage using meter's Min/Max recorder and still get Over limit results). Is there a fix for this?

Kevin C.

6/17/2024

@JimT When the truck's ignition is just in accessory mode it sends back 12v power and isn't searching for the trailer. When the ignition is on the truck is looking for the trailer and not getting enough of a load from the light to properly operate. To test this you can poke into the wiring for the light with a circuit tester that has an incandescent bulb # PTW2993 and see if the light functions operate correctly (i.e. if the incandescent bulb flashes when the turn signal is on). If it works with the incandescent bulb then you just need to splice in a resistor before the light for the truck to sense a large enough load to operate the light properly.

Michael

4/11/2025

@KevinC Good afternoon I am having the same problem I have a Ford F-150 2015 and I install a tailgate light bar everything work perfect when the truck is not started but once I started the truck up the blinkers left and right started hyper flashing and then stop working i hardwired everything and I know I connected all the wires to its proper connection but once I disconnected the left and right turning signal the reverse and parking light work fine it's just the turning signal for the left and right side that is having an issue once I turn the truck on can you help me out or let me know what I can do I done all the tests but I'm still getting the hyper flash in the left and right turn the signal is still not working properly wants the vehicle is started

Michael

4/11/2025

@KevinC. my 2015 Ford F-150 has the blindspot system which has no light bulbs in the tail light could this be a reason why it's not working properly I done a circuit test on both left and right blinkers and it worked fine The tester indicated that the blinker was active on both sides but once the truck is turned on both blinkers stopped working but inside the truck is hyper flashing just wanted to add that part