Technical question on how they work.  I see the electrical connection on the side of the tranny case.  How does that signal get translated into a lockup of the clutch in the converter?

The TC lock up is affected by fluid pressure, diverted into the clutch by a solenoid activated by the switch, which takes it's signal through a couple of methods, rpm, fluid pressure, and speed. I'm no techno writer, and a poor teacher at best. You could look it up in the available on line manuals and get a much better explanation I'm sure.

The Torque Convertor lockup is a mechanical function within the transmission that is generated within the TCU/ECU using speed,rpm and MAP. The TC acts as a automatic torque biased, variable ratio gearbox with ratios of approx 1.8:1 at stall speed and 1:1 at lockup as it converts rotational input power into hydraulic pressure using the ATF. The lockup solenoid blocks the TC bleedoff pressure when a electrical current is applied to the solenoid, thusly keeping the ratio locked at 1:1.

The Torque Convertor lockup is a mechanical function within the transmission that is generated within the TCU/ECU using speed,rpm and MAP. The TC acts as a automatic torque biased, variable ratio gearbox with ratios of approx 1.8:1 at stall speed and 1:1 at lockup as it converts rotational input power into hydraulic pressure using the ATF. The lockup solenoid blocks the TC bleedoff pressure when a electrical current is applied to the solenoid, thusly keeping the ratio locked at 1:1.

I was wondering what it did to lock up the TC.

I think I am reading that there is not an actual clutch in the converter.  Makes sense. I am shopping for a converter and they don't offer a non lockup converter for a tracker, even though the older ones did not have have lock up.

If you really don't want it to be able to lockup just splice into the wire for the solenoid.  Then you can disable lockup, but if you are in low range it should already do that for you.

There are no clutchs within the TORQUE CONVERTOR, the clutch pack is used when the trans shifts between first, second and third gear(and 4th on a 4spd) and is in the body of the trans. Think of the TC as nothing but a hydraulic fluid pump and motor in one unit, another analogy that works is to think of the torque convertor as a fan being rotated by the engine that pushs air against another fan that then rotates and drives the transmission.

Think of the TC as nothing but a hydraulic fluid pump and motor in one unit, another analogy that works is to think of the torque convertor as a fan being rotated by the engine that pushs air against another fan that then rotates and drives the transmission.

I understand the analogies presented here, but not how it locks up.

 The different impellors within the TC act/have a clutch of sorts, hydraulic pressure is applied when the lockup solenoid is activated by the TCU and this pressure forces the impellors to physically lock.

So the solenoid perhaps closes a valve that directs the hydraulic fluid into a different part/set of the impeller/vanes sort of thing  ??

If I watch the tach closely as the auto (AW4) on my Mitsubishi shifts, I can see it go from 1st, to 2nd, to 3rd, to OD, and then the tach - instead of dropping a thousand or so rpms, does a strange "slip" - it drops a few hundred rpm but it's a sort of gradual slip (unlike the gear change which can be both heard & felt) and I've always suspected that was the point at which the TC went in to lockup.

Yep that is exactly what you are seeing. The lockup eliminates the approx 4% loss seen with hydraulic coupling.