Here's a strange one. Start up in the morning and the torque converter locks up in in all the right places. After the car warms up and I take an extended highway trip the converter will not lock up at cruise speeds (any speed where the converter SHOULD lock). If I accelerate lightly, say 55 to 65mph the converter will lock...then unlock when I maintain speed again. And then (there's always a "and then") after cruising for about 45min to about an hour the converter will fail to lock even under light to moderate acceleration as it did before. Any ideas? A neighbor who used to be a tranny mechanic suggested a seal on the input shaft. I'd like a few more answers/suggestions/myths before I go tearing into this thing. I have a TH700R4 lying around as well. Are any parts swappable?

The car: 93 Formula. Bone stock with roughly 136,000mi. I bought the car roughly 2000mi ago and the previous owner kept every service record. As far as I know no service has been done to the transmission except fluid/filter changes.

 

It sounds like a TCC lock up solinoid (Torque Converter Clutch) and I do believe that the famous TH700R4 is NOT one of the trannys that the TCC solinoid can be changed just by dropping the pan. Also it very well could be a torque converter it self but i wouldnt think so. A way to find out if its the TCC solinoid is to find the vehicle speed sensor and unplug it and see if it stops, you will get a check engine light when you do this, but if it stops doin it dont keep it unplugged for a long time ie (a couple of months) because it can cause excessive heat within the trans, but a day or two shouldnt hurt anything to see what it does.

 

The fact that it starts to fail as the tranny heats would lead me to lean more towards a seal. More heat...fluid begins to thin out a bit. I dunno. Keep the suggestions coming. I plan on pulling the unit out and replacing the clutch packs anyway. Maybe even throw in a shift kit while I'm at it. I don't like to do the same crap more than once so just throw your suggestions out here. If someone thinks it's a solenoid I'll replace it. If someone else thinks it's a converter I'll swap it out.

 

I had the exact same problem with my 4L60E and it was computer related, so in your case won't apply being you have a 700R4. I though I would still share my problem for any others out there that are having a similar problem.

The torque converter would lock up and unlock randomly as you described at many different speeds. I checked the mechanical parts as mentioned in previous posts like the TCC solenoid and a sensor, don't remember which one. My problem turned out to be the computer was picking up a miss or random cylinder misfire, which I didn't have because the car ran very well and a scanner showed no miss. I talked with Bryan at PCMforless and said it was a common problem on LS1 cars with larger cams but very rare on cammed LT1s. He desensitized a parameter in a certain table, I don't know which one, and it fixed the problem. The torque converter locked up flawlessly the remainder of the trannys days.

This took me about a years worth of headaches to finally find. I was ready to take the tranny out and send the tq converter back to be checked.