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Hi Folks, new guy here with a bit of a bass ackwards TCC in my Sierra.

1999 Sierra
106K miles

I am a long time GM truck owner having had numerous over the years and I recently picked up this Sierra that is doing something very odd with it's TCC. Normal TCC function to me is when at cruise or light load TCC is engaged. THIS truck will engage TCC under acceleration at the normal point but as soon as steady state RPM is achieved the TCC kicks out and it is back in top gear, slight RPM rise. If I have it in cruise control it just stays in Drive gear unless it confronts a hill and the cruise commands more throttle, at which point the TCC engages until steady state is again achieved and it kicks back out. So I have a TCC that basically is operating backwards. When the clutch is engaged it is solid, no indication of slippage so I get the impression it is simply a control function that is out. By the way, it manifests the same thing with trans in Tow/haul mode.

There are no codes in the memory to indicate a situation and the truck is very normal otherwise.
Does anyone have some experience with this problem?
Is there an update?

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