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Old 01-24-2021, 12:43 PM   #1
1 Rivet Member
2018 Interstate Grand Tour Ext
Huntsville , Alabama
Join Date: Nov 2020
Posts: 8
2018 Interstate: All trailer lights work except brake lights.

I've searched this forum and asked Dr. Google but haven't found someone else with exactly the same issue I have with our recently-bought 2018 Airstream Interstate EXT GT. I need to tow a trailer that uses a flat-4 wiring harness (just tail/running lights, blinkers/flashers, and brake lights). The flashers, both blinkers, and the running lights work... but the brake lights don't. (With this trailer's setup, a single bulb/filament is used for blinkers/flashers and brakes.) I tried this trailer with our Honda Pilot (which has a wired-in flat-4 harness/plug), and it works fine. I tried a different similarly-wired trailer, and it worked fine with the Pilot but not with the Interstate. I've tried two different 7-to-4 adapters on the Interstate, one of which claims to have a lamp-out bypass (as I gambled earlier on that as the problem/solution).

I've worked on wiring/lighting issues on a few trailers and vehicles in the past, but this situation has me stumped. It's as if the harness/plug isn't doing its job of sending 12V to the two turn/stop bulbs/filaments when the brake is pressed. I started testing the 7-pin receptacle with a voltmeter, but apparently there's a "smart" system onboard to detect the presence of a trailer - a system that appears to send little or no voltage to the pins when no trailer is attached. This is my first experience with such a "smart" system. Any ideas?


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Old 01-24-2021, 02:09 PM   #2
1 Rivet Member
2018 Interstate Grand Tour Ext
Huntsville , Alabama
Join Date: Nov 2020
Posts: 8
One more thing: no trailer lights are LED; all are incandescent.
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Old 01-26-2021, 02:02 PM   #3
1 Rivet Member
2018 Interstate Grand Tour Ext
Huntsville , Alabama
Join Date: Nov 2020
Posts: 8
I just figured it out. The van only sends brake light signals/power to the trailer plug/connector when the key is in the ON position (not just ACC). In the ACC position, it will still send running light, blinker, and flasher signals/power to the trailer - but not brake lights. In the ACC position with the brake pedal pressed, the van's brake lights will be on, but the trailer's brake lights WON'T.

Both 7-to-4 adapters I bought work, by the way. So now I have TWO working adapters. :-)
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Old 01-26-2021, 02:49 PM   #4
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2015 20' Flying Cloud
Kingsport , Tennessee
Join Date: Aug 2015
Posts: 230
Although I don't have an Interstate, I have to say that I appreciate that you posted your discovery instead of leaving it hanging... that way in the future, others can find and see the complete story.
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