Clutch or Trans causing my speedometer fluctuate under full load?

You may have an issue with the engine wiring harness. Had a truck that the speedo bounced when the truck hit a pothole or ripples in the road then it would start to shake like it was a drive line issue. The dealer had it for a month trying to figure out what the problem is. While I was at the Cummings dealer, I ran into an old fart who had his motor home there for maintaince and I mentioned what was going on, he said he has a truck that did the same thing and it was the engine harness. So I called the dealer and told the mechanic just replace the harness .... it fixed the problem.

 

Interesting...thinking something is loose or out of balance. Somethi g screwing with the speed sensor so,it cant calculate a smooth reading. Cracked transmission mount,bunch of mud caked on drive shaft,throwing it out of balance,ear shaved off something. Id probably start with "looking at rear transmission mount,then driveline.Even trann mount loose or engine mount loose.

 

Well not sure but comparing to "cars" loose or starter "chipped off" flywheel teeth could affect balance.JMO

 

Harnesses can be defective. It may be the accelerator pedal to the ECM or a bus problem with the instrument cluster or something stupid like that. The sensor on the trans goes to the ECM and then the ECM reports that to the speedometer if I am not mistaken.

ALSO ... the best thing to really be done is a dealer hook up the computer and go for a ride with you. See what is going on to rule out electrical issues.

 

Have you tried a different ECM or factory tune?

 

Any luck with your issue? I'm having very similar problems