Yesterday when coming back home pulling a 29' 8k LB travel trailer and a quad in the bed I had been on the road for about 30 minutes, flat ground in 4th locked just above 1/4 throttle happily cursing 65mph when all the sudden the truck went into what I thought was neutral, I let off the throttle and shifted into N, then noticed the OD/OFF button was lit up. I pushed the button the light went out then I shifted back into drive, truck re-locked 4th perfect and everything was fine for the rest of the 4 hours on the road until Today when I was taking the trailer to the RV dump site I used OD/OFF to lock the converter in 3rd while I pulled a 10% grade, and I swear the converter unlocked for maybe a fraction of a second and re-locked and acted perfect the rest of the day.
Zero other issues with the truck, zero problems when empty.
TPS or ?

 

Sounds like TPS or brake light switch. Either will cause an unlock to happen.

 

I agree, but the OD/OFF being lit up has me puzzled

 

In the 19 years I've owned my auto it's done that same trick about three times, years apart.
I put it off to electrical gremlins and haven't worried about it.

 

yeah its super strange, ill worry less and drive more- but after just one more question:
is it normal for OD to stay locked at half throttle as the truck looses speed on a hill, all the way down to 50mph to where the turbo starts to bark and flutter and the Cummins is vibrating the entire interior?

 

Stock the od unlocks a a much lower speed than it should that's why I had my computer reflashed when it was new for the od to come on at 55. Since then mine disengages at about 52.
That seems about right.
Sort of doubt you could find a dealer mechanic who knows what you're talking about these days.
In the meantime just bump the throttle up a little and it should unlock.

 

No solution but thought I'd chime in. My od light/button kicks the truck out of od about 10% of the time since I've owned old red. I just push the button and runs fine. When I bought the truck it would not go into overdrive at all so that's why I got it so cheap. Typically the dealership told the previous owner it was a blown transmission. With help from big brained guys on here like infidel I replaced the throttle position sensor and old red has run great ever since. That leads me to believe the od button/light coming on would be the brake light switch.