I saw a post with similar issue as mine, only my truck is older...a 1992 dodge D150 (in fabulous condition) with 318ci and 4spd auto w o/d. It does shift into O/D while driving, but I can't turn it off with the button, and it does NOT light up when pressed. I'm about to put a camping trailer behind this great older truck and need to sometimes lock out the O/D while pulling. Any easy ways to test?

 

You want to be in the first gen Ram section.

Welcome to DF!

On the 2nd gen trucks, pushing the button just sends a momentary signal to the PCM to disable O/D. So, check and see if you are getting reference voltage TO the switch, and if you aren't, find out why. If you are, see if you are getting signal voltage back to the PCM. Same deal applies here. Gonna need wiring diagrams for this exercise though.

Moving this to first gen Ram section for ya as well.

 

Didn't see I was in 2nd gen area until after the post... thanx for moving me to 1st gen.. will try those things this weekend.
Anyone know of a good source for wiring diags for this specific year? I forgot to mention that I DO have a check engine light that came on a while back, but there's no obvious issue and the truck runs great. On these older trucks, how do you check codes for dummy lights?
Thanx in advance.

 

Should be OBDI, so, Stick a paper clip in the "A" and "B" ports on the obd connector, and the cel will flash codes. Write 'em down, there might be a few.

 

http://www.allpar.com/fix/80s-codes.html

No paper clips.