Kinda strange don't understand

[alexfisk19]

Ok so I was using my truck in 4 low on the ranch pulling our trees It's an auto Tranny and now it will only go into overdrive with the right turn signal flipped on. Wtf is wrong lol. Supposed to sell this tomorrow.

Just relized it is only doing this I believe with the trailer on...

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[jesilvas]

If it's only with a trailer it'd be the trailer wiring most likely.
All the time would be a wire in the steering column, as that is common.

[madpogue]

Check the center stop light on the cab roof.