Hold on there. Your engine WILL increase RpM if you are coasting or going down a grade if you downshift, with a corresponding deceleration of your truck.
I would imagine some transmissions won't actually downshift to a certain gears if your speed is too high, so that you don't over rev and blow up your engine.
This is a 6L80 transmission, right?
Throttle Position Sensor? Do the DBW throttle body even have that? I thought those were on the older DBC?
If you are not getting a code for throttle body or accelerator pedal, I would think you are good here.
Why do you suspect your throttle body is related to the downshift issue? Because of the RpM?
You will need to hook up a scanner with live data to see what the throttle body percentage is, and compare to RpM and gear commanded, then go from there.
Scan for any transmission codes too.
Electrical signal throttle bodies ARE the throttle position sensor, one and the same.
On DBW throttles, your foot makes a request, and the pcm looks at a bunch of criteria before deciding how much / how fast to open / close the throttle.
If your foot demands 00.00%, obviously, the throttle will close as much as possible, but it would never quite fully close it.
(On physical mechanical throttles, an Idle Air Control Motor would open a 'nostril' as the main throttle approached about 5% throttle or less,
so that when you closed the throttle, the engine'd inhale through the IAC alone.)
Idling is correct but when going downhill the rpms keep going high even if I shift down to 3 or 2 or 1. the ability of the engine to slow the car disappeared.
Sounds like you've a 4L60E. If you manually downshift, the RpMs will eventually go up for that reason.
If the RpMs do not begin coming down after the downshift, your overrun clutches may be wearing out.
The 4L60E has a preprogrammed top speed limit for 1st gear. Faster than that, it will NOT downshift into 1st.
All of the above aside, there's also a chance that your TCC is not unlocking properly.
When was the last time you changed your ATF?
