4L80E converter unlocking at highway speeds

[RedRocketZ28]

I have a 2003 1500HD with 56k miles on it.  Recently I have noticed the converter unlocking while cruising down the interstate.  I can be humming along on flat ground and all of the sudden the converter unlocks and won't lock back up for a few miles down the road.  I have tuned the truck via HPTuners and have the converter remaining locked while letting off the gas but of course it still unlocks when hitting the brake as it should.  This is issue should not be related to anything in the tune as I have tuned all of my previous trucks the same way.  Any ideas why it could be doing this?

 

I have searched all over and have come up with a few things.  Brake switch, needing a Case Relearn done, ABS issues, etc.  The brake switch appears to function as it should.  I still need to try a Case Relearn.  I do have a weird ABS issue when coming to a stop though.  When I come up to a stop the ABS will kick on for a brief second before the truck comes completely stopped.  I have found this to be a somewhat common problem but I don't think that it has anything to do with the converter unlocking.  I drove the truck back 10 hours after I bought it only 4k miles ago and the converter never acted like it has lately. 

 

Just looking for a couple of other things to try before I replace anything internally in the transmission.

[Mike GMC]

If the components are similar to the 4l60e, it has TC lockup solenoids.  You may have a bad one.  My was acting up similarly a while back, but it consistantly would unlock at cruise and lock up when I gave it a bit of throttle.  Exactly bacwards.  Turned out to be both solenoids, internal harness and a bad valve body.

[RedRocketZ28]

Thanks, Mike.  I am going to check a few things before I drop the pan and start replacing things.  I am also going to scan it with my tuning software to see what happens with everything when it does unlock.  I can command lockup with it too so we will see if it locks back up when I do so.

[KARNUT]

This was an older truck may not compare. I had a similar problem on a 94 Ram diesel, the converter would keep unlocking, turned out to be the switch on the brake pedal needed adjustment. It was just making slight contact the brake lights were blinking too.


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[redvett]

Check for misfires, if the PCM sees misfires it will disengage TCC.

[RedRocketZ28]

I have the misfire tables maxed out in the tune to check for that very thing.  It didn't change anything.

[sefiroxx]

I have the misfire tables maxed out in the tune to check for that very thing.  It didn't change anything.

Pcm uses tps signal and rpm speed to make shift points (along with current gear and environmental conditions). If you tuned the car, you'll want to check the readout of the sensors as compared to actual to verify everything is at spec for the sensors.

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[carnau]

Does the Cruise control disengage as well? If not it isn't a brake switch.

If the engine see's increased load it will drop out of lockup.

Have you changed anything on the ride of the truck? For instance Tires, suspension components.

The Z ride access will also cause one to drop out of lockup. If the vehicle think's it is on an incline. Most trucks that squat never hit lockup.