I was receiving a cam shaft position sensor code. Got that replaced. Then truck started throwing a crank shaft position sensor code immediately after. Then replaced that... Our trucks RPMs are all over the place. I cant tell if the transmission is actually shifting or if the engine is just reving. It does it the most around 40-45 mph. If I drive it hard and put my foot into it, I can keep it from doing it. Does anyone have any ideas on what could be going on? Does it sound like maybe a transmission issue or could it be related to the failed sensors... I will do my best to answer any questions. I'm posting on behalf of my husband but I drive the truck more than he does. I came here as a last resort to see if anyone has any input. Thanks in advance. My truck is a '99 Cummings 3500 dually. This is the second time the crank shaft position sensor has been replaced.

 

TKARUSH,
You haven't given us much to work with but let me see what I can do. Are you saying that if you are driving gently in third and fourth gear, the engine RPM seems to jump up and down for no reason and if you put your foot into it, it smooths out?
If so, that is exactly how my truck behaved when I was having torque converted lockup issues. If you search on this thread there are only about 400 posts about the problem. The problem comes from electrical noise between the PCM and the transmission.
The first thing to do is to clean all of your grounds - I think there are 6 of them. Check where each ground coming off each battery goes, remove the ground and polish up the metal nicely and apply a little bit of dielectric grease.
If that doesn't work, go to Geno's Garage (a sponsor of this site) and by a transmission noise isolator. They are inexpensive and for most of us who haven't solved the problem by cleaning the grounds, then the noise isolator fixes it.
If neither of those fix the problem, let us know and we will get you the diagram of how do a bit of rewiring around the PCM that will fix it once and for all.

 

after replacing the sensor did a new code show up? did you clear the old code prior. are there any after market tuners , chips etc on the truck. by chance is your volt reading moving when this occurs.