Speedometer drops off while driving

[jcutler]

I have a 2008 granite with a 10 speed split trans. While driving down the road the speedometer will drop to zero and the engine RPM is limited to 1200-14500 RPM. When this occurs I loose the jake brake as well. I have contacted two Mack Dealers and was told to check the ABS sensors and the speed sensor on the back of the trans. I have since replaced the 4 ABS sensors the speed sensor twice. I chased the wiring harness and plugs for continuity and all seems as it should be. I was going to replace the ABS ECM behind the dash. Am I correct in thinking it could be bad? I was advised if the ABS ECM thinks that it has two bad sensors on the same axel it will limit the throttle, I was also advised the speed sensor runs into the ABS ECM which is my thinking that could be bad.

 

Please Help 

[Geoff Weeks]

I had one that drove me mad. It was the tooth gear on the back of the trans (If yours still use that type of speedo). All it was, the yoke nut was a little loose, not enough so you could feel play, just enough that the tooth gear would stop spinning.

Drop the driveshaft and hit the nut with a big gun or torque it with a bar.

 It was weird, you start off the say and the speedo would work fine, then it would slow or cut out. Changed pick=ups, even subbed in another speedo and got the same. Couldn't feel any play in the yoke but just enough not to squeeze the tooth gear between the yoke and bearing.

[Joey Mack]

Let me guess...  an MP-7 with an Eaton tranny?  Have you looked at the wire harness for the speed sensor and tranny for cut or chaffed wires..?  You seem to have put a good bit of parts on it with no repair..   I would say a bad ground or short to ground in the wires is the issue..   Peel back the loom and check..   Just sayin'...