I have a 98 2500 cummins diesel. Yesterday I was driving and noticed the speedometer needle was fluctuating up and down. sometimes as much as 30 mph. Then the brake light and the abs light came on. When I first start it up the lights go out as usual and then come back on after a few miles driven.( sometimes 4-5 miles sometimes it will take 100 miles or more. Then the speedo will start working right again after a while. The truck still runs and drives normal and the brakes work ok. If I use the cruise the truck will accel and decel as the needle goes up and down. If I turn off the cruise it drives normal, but speedo still fluctuates.

any suggestions on what this might be.

 

speed sensor on top of the rear axle

 

this is a common problem and yes it could be the ABS rear axle speed sensor, the front abs speed sensors, or it could be the speed sensor that is mounted on the tail shaft of the trans/transfer case. (if 4wd)
or just some plan old broke wires from running something over 5 years ago that is finally affecting it now.
So what you should do is take and get the light to stay on while driving it. and then take it to a auto parts store and have them scan it. that will tell you what one is bad. if they cant then need to find someone that will take it on a road test with the good scanner SNAP ON, OTC/MAC or close to them. Have them run a sensor test while your driving and see what sensor isn't reading correctly. Its easy its the one that will say your doing 0 mph while your doing 50 mph.
to help the auto part store scan it don't turn the truck off until they scan it so the lights will stay on. Don't buy the part what they say is wrong CHECK WIRES FIRST if ok then buy parts.

 

my 98 4x4 doesnt have a sensor on the trans, or t-case, just the one on the diff... but my bet would be the the sensor on the rear diff, it controls the rear ABS and the speedo.

 

My speedo does that when it gets really cold outside (which is now), but never had the brake light problem. I just figured the cold affected the speed sensor (ice buildup or something). I never had the speedo problem above 10 degrees though.

 

Ok, I tried using my scanner. I got no codes, but the lights were out when i scanned it. I will try it again tomorrow without turning the ignition off. Is it ok to do that?

 

should be fine

 

I learned something new today. I didn't know that the older ones don't have them in the case hummm

 

sooooo, I drove 3 hours today and the lights never came on and the speedo never screwed up. Only thing different today is it is warmer and drier than yesterday. When the problem started I was in Michigan on Monday. I am now in Tennessee after driving to Alabama yesterday. I crawled under the truck to look for any frayed or loose wires. Found none. Gonna wait out the storm and go back to Michigan on Friday. Hope I dont have problem again but I am not holding my breath.

 

Is there any connections more forward on the truck along the frame rail? like besides the trans or under the drivers/passengers feet??????maybe the water that is kicked up from the front tires are soaking it with water somewhere else???????