Vehicle Specs
2009 Hummer H3, 3.7L Inline 5.
I'm not a mechanic by any means, but I do maintenance and most repairs to my H3.

Good day yall, I purchased my H3 around 2 years ago with around 196000 miles on it. The engine and transmission had both been replaced in the past. Anyway, about a year ago it started having an inconsistent issue when accelerating.
As best as I can explain, when I accelerate from 0 to a max of ~25mph, reach just over 3000rpm, and the vehicle attempts to shift from (I think) 1st to 2nd gear, there is a chance for it to lock in low gear (I think 1st), the speedometer drops to 0 and will not rise above 0. The it returns to normal function after the H3 is restarted, and I have gotten it to work again without turning it off twice, but I can't say what caused it other than maybe shifting it into neutral and stopping. I tried to replicate the issue for a mechanic friend of mine and on my own but never could. No issues come up on the diagnosis pad, nor lights on the dash even when it's malfunctioning.

Best I can guess with my limited knowledge is that it might be sensor in the transmission? Any insight on the issue or someone who has experienced the same issue?

 

Check the wiring and connectors to the Transfer Case Speed Sensor. If the T Case Speed Sensor does not have a proper (uninterrupted) signal it will show erratic speed readings and prevent shifting. Look for any cracked or frayed wires, clean the connection with CRC Electronics Cleaner and snap back together.

If it is not seated to its proper depth, it will also lose its ability to read speed. Like when somebody changes the T Case fluid and does not put it back correctly.

 

You might want to check this thread:

https://www.hummerforums.com/forum/h...-sensor-49079/