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6.5 turbo diesel governor?

The problem I'm having is when I get close to 70mph it will kick down then back up on shift make the engine sound like its screaming for mercy and speedometer will drop down to about 65mph but feel like I'm gaining speed
 
Is this the computer for the truck that everyone is talking about having to retune
 

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It depends. My '94 C2500 HD Cab and Chassis had a speed limiter programmed into the PROM in the ECM from the factory. 82 MPH was it. No, I didn't run out of revs, 3.42 14 Bolt full-floater. Didn't matter if I was in 3rd or OD, converter locked up or not. I could have 5,000 lbs in the back, downhill, with a 40mph tailwind and it would defuel to maintain 82mph.

Which wasn't all that bad. When traveling I-80 all I had to do was mash the pedal to the metal and let it rip at a constant 82, up hill, down hill or on the level. Nice thing was the Speed Limit was 75mph and the State Patrol generally runs a 7mph "cushion". Since it was a Cheyenne base model C&C, no cruise control (or power windows or locks, just a vinyl bench, rubber floor mat, AM/FM) so I could maintain a constant speed just wedging my foot against the hump on the accelerator pedal.
 
The problem I'm having is when I get close to 70mph it will kick down then back up on shift make the engine sound like its screaming for mercy and speedometer will drop down to about 65mph but feel like I'm gaining speed

Errr, it does what again? 🤪

You need to check for codes as ONLY emissions related codes turn on the SES light.

Sounds like it's attempting to shift to 4th or lock up the Torque Converter Clutch, TCC, and something may be slipping. If anything slips in the transmission you loose the lock up clutch as it goes into "limp" mode.

ANY engine problems can also unlock the torque converter. This is by design for out of tune or engines that have problems: can't take the low RPM load for a given throttle = unlock TCC and or downshift so the engine can recover and or continue to run. It also lowers emissions and can save a cat on gas engines as a constant misfire can melt a cat converter down.

As you approach 70 MPH the wind load increases causing weak transmissions to slip, engines to misfire, aka unable to maintain the speed at low engine RPM and causing a TCC unlock and/or downshift. Again Limp mode can also trigger a TCC unlock or downshift.
 
I think I've narrowed the problem down to a bad transmission I've yet to tow anything with it but previous owner was pulling a 1ton forklift. I'm a thinking it burned up the transmission I've fixed the fuel issue it had and changed the pmd relocating it and had the exhaust opened up
 
I was going to say that what you appeared to be trying to describe was the symptoms of your transmission losing OD and/or TCC lockup under load. Certainly not a "governor" issue, more of a running out of RPM's because of gearing vs. speed issue.
 
The 4L80E, while based off of the very stout TH400, is known for having a weak (what is essentially an add-on, not a ground-up, integral) OD section and TCC. The weakest link is the TCC, and when that goes out (indicated by the continual locking/unlocking of the TCC before it fails completely) the first thing it takes with it is the OD section, which will shift in and out, before eventually taking out the entire transmission.
 
I disassembled the 4l80e several times. Every time a direct clutch fails. Direct used for 3.4 and reverse gear.
 
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