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Transmission issues

Ajbaker1794

New Member
Joined
Jun 25, 2022
Messages
5
My transmission has been slipping for a few months now. I don't drive it a whole lot so I wasn't worried about it. Last week I drove it and it would not shift past 3rd, so I decided it was time to swap the transmission out. I put another transmission and transfer case in the truck now and shifting is smooth and there's no slipping at all. However, it still won't shift past 3rd gear. I've researched this for days now and can't figure out what the issue could be. Any advice would be greatly appreciated.
 
1500 or 2500?
What year?
What engine/tranny setup?
 
I'd check for vacuum lines leaking or not installed:
I don't know if it applies to Avalanches (your Avy) but some automatics had a "kickdown device" to shift it to 3rd when the engine is under heavy load.. My old Chevy Caprice used engine vacuum (lack of vacuum) to kickdown, so a vacuum leak / missing hose could do it.. My older Pontiac Catalina (or maybe it was an old Ford.. I'm getting old and don't recall) had a kickdown lever, when WOT it kicked down the transmission, these are easily bent => mis-adjusted. Good luck, I hope you fix it.
 
Are there any codes stored in the transmission control module? Other error codes related to speed? Engine codes?
It was throwing a code when I first got the new transmission in. It was P0502 if I'm not mistaken. I looked into that and it turned out to be a problem with the vehicle speed sensor on the transfer case, which was also causing the speedometer to not work. I replaced the sensor and it fixed the code and speedometer but didn't fix my main issue. The only code it shows now is for and o2 sensor
 
maybe tranny cable not adjusted correctly?
 
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