when I'm cruising with my cummins at a constant speed close to the range where my tranny should be shipping gear,my rpm goes up & down roughly 200 to 300.what controls the shifting is it a vaccum set up or electric?This does it any time I'm cruising in the range it should be shifting.I checked my fuel pressure it's roughly at 12 psi constant no fluctuation.I was thinking maybe a vaccum leak or low voltage.The rpm fluctuates more when shifting to overdrive and keeping a constant speed 45 mph,anything over 55 mph no fluctuation.It you keep a constant speed in the shiftihg range of any gear rpm fluctuates,worst when going to overdrive.If I'm pulling a load there is no slippage.I checked the oil level ( good) and last year changed the oil & filter,the rig as 306,000 KM.

Does the transmission's TC seems to lock, and immediately unlock and repeat? This could be a sign of a ground issue. I know I have read a similar post before. If it's locking and then, immediately unlocking, and repeating this, you may be able to fix it pretty easily.

try an apps resetfixed mine

what do I have to do for an APPS reset?

what do I have to do for an APPS reset?

http://articles.mopar1973man.com/2nd-generation-24v-dodge-cummins/26-engine-systems/258-how-to-reset-calibrate-apps-sensor

I'll try the APPS reset first then if it doesn't solve the problem where do I look for the bad ground?

I'll try the APPS reset first then if it doesn't solve the problem where do I look for the bad ground?

Mikes got a thread on here about isolating the alternator before you go chasing bad grounds. Do that 1st.