Alright I’m at a loss, I’ve searched piles of forums and can’t seem to find my problem. 1990 D250 with a 727, reverse lights are staying on in all gears P, R, N (sometimes D sometimes not) 2 and 1.
ive replaced the NSS, adjusted the shift linkage, chased wires and haven’t found anything that makes me concerned. Removed all the old trailer plug wiring to be sure. Truck is originally from Arizona never seen a winter where I am from.

throw your ideas at me

 

I need to know a little more history on the truck. Is it new to you? Did this problem arise overnight or has it been ongoing, if it's a new issue what were the recent repairs or upgrades? Do the reverse lights come on with the key or just when the markers or brake lights are on?

The reverse circuit is pretty simple and run off a fuse for only that circuit. It should be fuse 13 according to my FSM. Have you pulled that to see if the lights go out?

The reverse lamps share the same ground as the markers and brake lights.

 

I’ve had the truck for 12 years now. This is an ongoing issue I don’t remember exactly when it happened but it was just out of the blue.
the reverse lights come on as soon as the the key is turned and typically only go out when it’s put into drive, but as of last night they don’t always go out when in drive.
pulling the fuse shuts the lights off, as does pulling the nss plug off

 

Does it start when it's not in P or N?

 

No it does not

 

Have you checked the voltage at the plug on the Start/back up switch?

The white wire should be voltage off the key switch.