My Cummins friends I'm fighting issues with my speedo. My abs and break light been on since I bought it and have had no issues with speedo The other day my speedo started jumping on my 02 3500 4x4 and been messing with my shift points and shifting into od I been looking at forums left and right. I replaced the vss on the trans right behind the trans pan and still nothing I looked at the rear abs/ speed sensor on the rear diff and it looks to be like it hasn't been plugged inn in years sensor is packed full of dirt but yet my speedo has worked just fine till now probably cause of the abs light. When looking at speed sensors there are three different ones that look the same just different in color my 47re has a white one and I replaced with a new white one even cleaned and plugged in the rear abs sensor just for ***** and still nothing wire seems to be ok also. Just happened out of the blue randomly one day

 

Get your tranny checked.

 

There may be some corrosion built up on the electrical connectors since you say it looked like it hasn't been plugged in in a few years.