Speedometer problems - Reading 1/2 speed - Out of ideas

Truck is a 98 KW 3406E 18spd. My speedometer is reading half speed sometimes. It is either correct or 1/2 no jumping around. I have replaced the speed sensor in the tail shaft, checked the wires and couldn't find anything broke, or rubbed or pinched all the way to the bigger harness, and nothing pinched on it as far as I could follow it. Also had the speedometer head rebuilt, which it is working much smother but some times I am getting the 1/2 speed reading still. The trip and odometer also count at half. The cruise control works just fine weather reading half or correct speed, and if it changes while I drive I don't need to reset the cruise control, resume works to bring me back to my set speed lets just say 70mph weather the speedo says 35 or 70. I have reached into the tail shaft to try and move my gear but cant with my bare fingers as the gear is pretty slick. It will change from correct to 1/2 speed only every once in a while (maybe 2 times a day) but there is no certain place that it does it. Not like hitting bumps, loading or unloading etc.

Any ideas or other ways of how to check things would be great, cant afford to have a mechanic chasing a sometimes problem.

 

98 W900L. Not being a smart ### at all, how does the break plunger affect the speedometer? First of any suggesting with the break pedal so this may be great.

 

Well he asked what kind of truck and the first thing you said was what kind of truck.

I'm not sure if he read what you wrote.

 

I would venture a guess that the tone ring is slipping on the out put shaft when its reading half speed.

 

How would I go about changing the tone ring, guess I gotta pull the yolk of the back of the transmission?

 

Yes pull the yoke. Sometimes the nut on back of yoke is just a little loose.

 

Thanks for the help. I will get to it this weekend and get a good look at it. Hope that is the problem, getting tired of chasing it.

 

Well got under the truck this morning and used a screw driver, tried to move the tone ring and it was good and tight on the yoke yet, couldn't move it.