Cummins ISC Throttle behaves like an on/off switch

2009 Kenworth T370
Paccar px8 / Cummins ISC with weight reduction kit
1.1 million miles / 29919 engine hours (rebuilt in 2021 at 780k miles)

Been dealing with this issue for a while. Talked to both Kenworth and Cummins and no one seems to have any idea what's up

Throttle pedal acts as an on off switch when trying to accelerat slowly or for slow driving. When stopped, the pedal has a small dead zone at the top where it does nothing, but after that, if you try to accelerate and hold the engine rpm at something like 800 or 1000 rpm, it will take off untill about 1500 rpm. I can hold any speed lower than that unless the engine is stone cold and fluids are thick. I can hold about 1200 then but nothing less. While driving with a load on the truck but at slow speed, it will accelerate and almost hold at around 1300 rpm, but then the turbo usually kicks in building boost and thus accelerating the truck more, all while barely touching the throttle.

I can set the idle with cruise controll at what ever speed I want with no issues. And driveing with cruise control, the truck doesn't seem to lurch forward after coasting and coming back into power. It can do that slowly. If I coast down a hill and try to gradually come back into power, there will be a sharp hit in the drivelive as if you stabbed the throttle hard. And yes drive line u joints and carrier bearings are tight. The acceleration is just so violent you hear and feel it everywhere.

So as you can tell if I'm trying to just slowly drive through a parking lot or stop and go traffic, the truck will just put h forward and accelerate. I usual just have to put it in the tallest gear possible and coast with the clutch out. This is wrecking havoc on my drive line with the shock when it accelerates

The pedal is a floor hinge accelerator with a 5 wire Throttle Position Sensor. I've replaced this thing several times and that does not help. There's supposedly a calibration process where if the key is on, you press the pedal down slowly and then release slowly 3 times in 30 seconds or something like that but that does not fix the problem. Looking at insite, there's no way to calibrate the throttle there either

Anyone have any ideas? Truck is old and has a lot of miles so it could be anything. But I'll take any advice as this is insanely annoying.

Thanks!

 

Dang no one has any ideas? Lol seems to be the running theme no matter where I go. Ah well. One of these days I'll figure it out

 

Pedal is harness is exactly the same as the floor hinged versions in the older t800s and what not according the KW if that means anything. I have looked at the harness there and it seems to be in great shape all the way till it disappears behind the foot wall going behind the dash. That part I can't inspect in any way. Does anyone have possibly voltage numbers? If there's any form of resistance, I can check that with a multimeter but I would need to know what's the nominal range for those

 

I did actually just get insite so I'll have to do that. I'm assuming a sign of wiring issue will be a voltage spikes or dips when slowly pressing or depressing the throttle?