TC lockup problems

[99Firebird3800]

Okay, so I've got a '99 Firebird, 4L60E. After about 30 minutes of driving and acting normally, it will simply drop out of TC lockup. Trans temps are right about 195-200, which is about where the engine runs, so I don't think it's getting too hot. Trans doesn't act like it's slipping at all, no harsh shifts, and hasn't thrown a P1870, trans component slipping. It has plenty of fluid in it after a recent top-off (was about a quart low, no leaks that I've seen). It WILL go back into TC lockup when climbing a slight incline such as an overpass, but as soon as it gets back to level ground, it again falls out of TC lockup.

Any ideas? I am guessing that it's either a problem with the TC itself or the TCC lockup solenoid. If there's something else I should be looking at, please let me know.

I also have a '99 GMC Jimmy with the same trans, but when it falls out of TC lockup I generally start to get harsh shifts and a P1870. This only happens on hot and humid (80+ degrees, 40%+ humidity) days. Spring, fall, and winter we can drive for HOURS in stop and go traffic and have no issues. Summer is the problem. I believe it's heat related, and have a trans cooler that I intend to install, but if there's something more I should be looking at I'd love to know about it.

Thanks for your help guys/gals!

[msfyter]

Do you have a custom tune or modified anything(tps)?

[Joe 1320]

The TCC is part of the open/closed loop operation. Do you suppose it's switching between the two modes in the ECM?

[99Firebird3800]

No. Not that would affect the computer, anyway. I simply made it so the fans would run for a little while after the key was off, and lowered their turn-on/off temps on the firebird. I did nothing else. I can't tune the Jimmy.

I'm pretty sure it's the TC lockup solenoid on the Firebird. I do not have the service history on the car, but before I bought it the lady said a mechanic looked it over and said it was fine (I'm starting to doubt it, either that or her son abused it REALLY bad in the 3 weeks he drove it). I do not know when the last time a filter was put in the trans, so I plan on doing that soon. When I do that, I'm going to spend the $30 and put a new TC lockup solenoid in it.

As for the Jimmy, I left something out. It only comes out of TC lockup and gives the P1870 on hot humid days WHEN THE A/C HAS BEEN ON. If the A/C is left off, it will drive all day and give no problems, even in stop/go traffic. I'm convinced this one is a heat issue. I may need to do a coolant flush on it after I put the trans cooler in.

[Christopherrr]

Drop the pan, change the filter and do a complete evac and refill. I've read that 4l60e are known to have tc lock up issues when they get hot. I know mine did.

The only difference is that mine had a seriously wicked 1-2 shift. So quite possibly you have a different issue...but I'd start with a filter & fluid change.

[Kevin - Blown 95 TA]

Also if the fluid isnt at fault, it may be a lockup solenoid getting hot and dropping out, and check the brake switch too just for fun in case it is marginally adjusted.