RPM
 

Have some questions when going down the road about 60 mph, ounce in a while my tach starts going up and down and it seems it confusing the tranny. Any idea. I cleaned the cables, changed the batteries not sure what's next. Need advise. It also does at any speed.

Converter locking and unlocking? I had a 3500 Dodge work truck with a gas motor and auto. It would do that until it got hot. The tach would rize and fall 100 to 200 rmp and speed would stay the same with the same throttle.

When my tach sensor was ripped off it did a similar thing and at times my tach would lay flat at 0 or the other end. Maybe clean up the sensor a little bit or swap it with a buddy for a couple day's to see if that fixes it. It is located right above the main pully, has two nuts that hold the wings on. The plug for it is up near the top front of the motor.

Is it just your tach going up and down? Or is the actual engine speed fluctuating?

What trans do you have and does it only do it in od. If so the governor pressure regulator could be the problem

I just got done messing with this problem myself. After doing a lot of research, this works. Your TPS is not sitting in the right position. This site tells you how to fix it.
Dodge diesel maintenance tips

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Read about the TPS obviously. Throttle Position Sensor