Here's my situation: When driving in mud, I have a hard time distinguishing wheelspin from normal torque converter slippage(I'm still stopped, but engine rpm's are rising). Normally, the torque converter locks up around 40mph, but I'd really like to have the ability to lock it manually at much lower speeds. Has anyone ever done this before? Or should I just start looking for a 4WD with a 5-speed?

This was discussed at some length and tried by someone in Australia quite a while back.. for the purpose of getting back the loss in engine braking that is normally there due to the torque converter freewheeling.

To make a long story short, sounds like a good idea, but too many down sides.

The major down side is that the torque converter lock up clutch has a VERY thin lining and limited torque carrying capability.. As you know if you put very much torque loading in overdrive, it unlocks to let the torque converter do its torque multiply thing and in the process protects the lock up clutch from overload. Using it for other than what it was designed for would probably lead to failure.

The second major downside is if your transmission were to inadvertantly shift gears with the torque convertor locked it's a major torque shock to the transmission. I know about this personally as I have a Ford farm tractor with a 10 speed forward/2 reverse (automatic style, planetary transmission, but with no torque converter and of course manual control shifting). Its major weakness and failure mode is shearing off shaft spines from the shock loading that occurs from making gear shifts that you can make (per the manufacturer) without using the hydraulic manual clutch.

When your vehicle comes to a stop, you of course would also have a way to disengage the lockup just like using a manual clutch. I believe the guy in Australia used a switch that kept it locked only when the switch was held down.

Lastly, your rpm climbing while in mud (after reaching TC stall speed) should not be due to the torque converter.. Once the engine speed reaches the stall speed of the converter, any increase in speed after that should be due to your driveline and wheels turning faster in the mud.

For more on TC stall speeds go here.

Modified lock up TCs for engine braking applications are built but as shown, looks like there's a bit of engineering work to be done to make them work and be reliable.

btw there is a very interesting chronology of the Aisin Warner A340 series transmission that Toyota has used for years in this article. Scroll down to the article "The Evolution of the A340". I inadvertantly found this while google searching to see if I could find any info about the lock up mod that the guy in Australia tried.