Just bought a 99 Silverado and my first highway trip with it I noticed that the converter wasn't staying locked all the time. Oddly enough it seems like it wants to lock in and stay locked when pulling hills and towards the top of the hill, going down the back side, and on flat ground its constantly wanting to unlock and relock. There was no check engine light and the fluid looks and smells fine. Its got Mobil 1 synthetic transmission fluid in it that was put in recently before I bought it. There truck has never been abused, all highway miles driven by an older fellow. The previous owner put 3k miles on it in the last 3 years so its done a lot of sitting. It does not act like a valve body issue to me. As fast as its switching from locked to unlocked and back I think the command signal from the PCM is being lost or interrupted for some reason. I know a certain coolant and trans fluid temp has to be reached before lockup can occur. I'm curious if that could be it. Like I said its not turning on the check engine light. Anyone ever heard of this happening before? Its a 99 silverado NBS 5.3 and 4L60E and it is a 4wd if that makes any difference. No bulletins about this specific problem either.

 

A miss from a bad plug or plug wire?

 

Converter locking and unlocking is what ate my transmission pump.

 

Its got those nice bosch platinum 4 prong pieces of crap in it that are going to find a new home in the trash but no its not a miss. Its got a flowmaster on it so with the AC off and blower off going down the road I can hear it pretty easily when it does lock and unlock. I can see it on the tach too. I can hold the throttle steady and press the brake pedal when its unlocked and the rpms don't change so yes it is unlocked.

 

Possible TCC Solenoid in the ol' valve body?

 

when my TCC solenoid took a poop, it would lock and then unlock, but would never re lock....

You say it only unlocks going down hill? are you sure it isn't DFCO you are feeling? it feels like the converter when it is operating... What is your Throttle position when it acts up?

 

A solenoid will either work or it won't. A near dead solenoid isn't going to like being cycled on and off like that and I highly doubt its the solenoid or a mechanical issue in the trans at all. The control circuit is obviously working and there is no check engine light set so I'm thinking for some reason the pcm is commanding the converter to unlock for whatever reason due to a bad input. Trans temp, engine coolant temp and I just found out after talking to Justin, misfire data can all cause it to unlock. Its got those stupid bosch plugs in it so I may just go ahead and throw them away and get some good plugs in it.

Its not DFCO. It does it most when the load is reduced like on level ground or going down the back side of a hill. Throttle position doesn't seem to have any bearing on its locking and unlocking.

 

is it all stock? my stall would do that until i modified the tcc pwm min and max tables. only during light load cruise.

 

100% stock

 

Most places I have read the tranny guys say the 4l60e doesn't like synthetic fluid. I doubt that is entirely your problem, but if a trans flush was recently done and it is the first and only one the truck has had then that may have toasted the tranny. They seem to be picky about that sort of thing if the fluid isn't changed regularly and then it gets changed after a bunch of miles it tends to flush out all the crap that was holding it together.