[Prof. T. Soggmeyer]

Can someone explain what occurs in the torque converter when the lock-up is engaged on a 700r-4? This inquiring mind wants to know.
Brad...

[cobalt327]

Short answer is a hydraulically operated clutch engages to lock the trans input to the flywheel, w/o any slippage coming from the torque converter. On my TH350C, this amounts to about 200 or so rpm drop at cruise speed. There's a solenoid that when engaged, lets the TC lock up clutch see line pressure, when de energized (when the vehicle slows below a preset speed or the brakes are used), the pressure is released and the TC goes back to normal.

I've heard there are two types of lock up used on the TH700R4, the later one I think is pulse width modulated and that's all I know about the 700 TCC. This is a short Crankshaft Coalition wiki article on the wiring/control part of it.

[Prof. T. Soggmeyer]

Thank you Cobalt. I just installed my lock-up kit and while laying under my truck I was thinking about the mechanics of the lock-up and realized I did not know what occurs after the solenoid activates.Now maybe, just maybe I will appear smarter to my boys.

[blwn31]

If I remember correctly, once 12v is applied to the lockup solenoid, it closes off the the converter oil return passage. This causes the pressure to rise on the clutch side of the converter. When this happens the converter internally will lock together. So the converter is then locked to the input shaft, which is really called the turbine shaft. This is why when towing with an OD Trans, it is recommended to tow in D. This is to a avoid overheating the Trans as there isn't any fluid flow though the converter.

Keith