Just purchased a used 2003 silverado HD 6.0 with a 4l80e. Truck takes off and throws hard shifts into 2,3,4 but once 4th kicks it’ll kick back down to 3rd unless iM really easy on the throttle then it’ll shift back to 4th but no overdrive what so over and the overdrive button doesn’t work on the shifter and no light on dash.
basically i can feather truck in 4th but anything more than 1/8 throttle itll kick back into 3rd
it isnt slipping, no check engine, speed gauge is jumping around after 40 mph until 75 mph

noticed it has a new throttle body put on and wire up with a new pigtail,

 

A. this is not a ford...the button on the shifter is not an overdrive button. It's a tow/haul button. (unless you have an option package in a truck that I've never seen before...in which case, COOL! post a picture) FYI the wire for this button very commonly breaks in the steering column. It's not a hard fix to repair the wires, just kind of time consuming/cumbersome
B. Read the codes. No light on dash doesn't mean there aren't any codes...it means there is no light on the dash.
C. speedometer jumping all over is a clear issue. Don't keep driving it without finding out why the gauge is bouncing. Is it the dash? Is it the speed sensor? is it the connection to the pcm from the trans?
D. Check live data and see if 4th is being commanded when it falls back to 3rd. If the PCM is commanding 4th but the trans is in 3rd anyway...that's a clear indication of an issue.

Do a little more checking and let us know what you find.

 

My obd scanner doesnt have live data. Im dropping truck off right now at my tranny builder, and ive scanned codes and nothing comes up, ive purposely turned on the CEL by unplugging maf and it through a code for that, ive checked connection and even applied wire dryer to remove any moisture if any, ive heard About the shifter problem, need to look into it. Ive ordered a replacement instrument cluster

ill post and update on what my tranny guy finds

 

So he found on my transfer case the bushing came out and is dangling around my driveshaft and its been run like that for awhile by what it looks like, transfer case housing is eaten up beyond to where i can install a new bushing or even seal, speed sensor is ate up, so i can be going 70 and the speed sensor will be throwing random MPH variables out like 30 then jump to 85 then back down and so forth. Would that trick computer into downshifting and acting how it is

 

absolutely it would