3/4TonYJ, you're right to question my answer above... as I was refrencing more of the "Lock Up" features of a Torque Converter (TC). I did not really want to get into a verbose discource on TCs so I glossed over the mundane aspects of the TC with the phrase "ain't really doing too much." and tried to diagnois the problem from the information given in the post. But since you caught me....
Let's look at the purpose of the TC. It's job is to de-couple and couple the engine to the transmission. I'm not a hydraulic engineer, but I do know a fair piece about electronics and guss what! Electronic and Hydraulic formulas are the same... only the names change! So I have to relate everything I know about TC's to electronics. It makes it a heck of a lot easier to me. The TC kind of reminds me of a slef regulated alternator, it's attempting to keep the balance between the drive load and the engine RPM's and providing enough coupling to keep the engine running. That's a lot to ask of a mechanical system. I find it a wonder!
So, when a vehicle slows to a stop, the car's engine can still run without killing the engine. This is done by the fluids not letting the TC couple all the engine's torque to the transmission. When the vehicle starts from that dead stop the TC increases the coupling to the transmission similiar to you letting up on the clutch. In fact that's what's happening... the hydraulics in the TC and the transmission are engaging and disengaging clutch packs, bands and hydraulic flow all the time to keep the vehicle moving...or conversely... stopping.
Usually a TC when it goes bad will not cause slipping, it shudders. It'll feel like a flywheel in need of resurfacing on a manual transmission. Most often when a TC goes bad the converter clutch will cause a slipping/gripping cylce. It's felt to the dirver like a washboard road when the engine decellerates or accelerates. An internal clutch pack that's worn will cause more even slipping on roll-off. It will get worse with each engage/disengage cycle. This is one reason the AT fluid is coloured red is to help diagnoiss this very problem. As the fluid ages it becomes darker in color. As it darkens, it's ability to work with the TC clutch is compromised. This is one of the reasons you have a cooler, to keep the AT fluid at it's peak longer. If it heats, it dies. Read the end of the post about lugging on a hill, it's death on AT fluid. Simply changing fluids and filtes help. Not because the filter provides more pressure, but because the properties of the new fluid are in effect agiain. That why you're hearing about the newer flush systems that COMPLETELY flush out all the old fluid from an AT. The cost is about $80, and a bargin if it saves your High Buck AOD from destruction.
A torque converer Lock Up is activated in one of two ways:
1. Moving vanes activated by centrifigual force (CF).
The force needed to move the vanes is determined by the engineering of the TC. As the CF increases the vanes move...
Drawback: You have little control, unless you install a shift kit, as to when the transmission's torque convter will engage.
Plus: It's a fairly simple system that is mechanical and it's free of having to know anything about engine load.
2. Electronic Activation.
This is the mode of choice for almost all newer vehicles.
If you've ever been lugging up a hill with a TC equiped transmision, gaining slowly on the hill (Oh ...like say I-84 in Wyoming... long hills that seem to go forever and fairly high elevation changes) just when you finally seem ahead of the game the TC kicks in. The RPM's drop and the vehicle slows down due to the excess load and then you've got to wind up the RPM's again. Then the cycle repeats. You're trapped! That's a lot of wear and tear on the transmission, engine, drive train and you're nerves.
So to avoid all that stuff... the smart engineers now start to figure in load, engine speed, vehicle speed and altitude. As if that ain't enough in some of the vehicles they even give you a little button to disable the TC lock up, if you desire for those long Wyoming hills. That's a nice touch, economy with choice. If the TC is engaged going downhill, it will provide some braking effect similiar to engine braking, less the slippage factor.
So, if I've explained this correctly, you'll have more questions than peace of mind. If not it's a clear as mud.