99 denali tcc lock up problem

when i am driving and in shifts in to overdrive and just keep the throttle constant it surges back and forth from lock up to non lock up but as soon as i hit the gas more it downshifts out of tcc lock up and is fine but when it locks up agian it will go back and forth from lock up to no lock up it acts like it can't make up it's mind, i have no trouble codes either. any advice woould be much appreciated

It could be the TCC solenoid. The torque converter lock-up is controlled by a solenoid that changes the fluid path to the converter causing it to lock up. If it won't stay engaged it will behave as you've described.

I'm not sure if you'd get a code if it's working intermittently. Actually, I'm not sure if the vehicle is smart enough to know whether the soleniod is working or not. It just knows that it's applied the voltage to the soleniod, not whether or not it's actually engaged.

It's a reasonably easy fix for a DIYer. You need to drop the transmission pan to replace it.

Jim