Was reading another thread and some details about a high HP conversion on an X300 XJR, and one of the issues also faced was to reprogram the GM 4L80E transmission controller (TCU) to accommodate. Along with changing the shift points, etc, the torque converter lock up was modified along with the capability to lock it manually. This improved snappy acceleration "feel" (like a manual transmission at speed) and in theory could improve fuel economy.

This sounds very interesting and I wonder if the 4L80E torque converter lockup function is just a signal (high/low) on a single wire/pin in the harness? If so, it would be easy to allow the TCU to normally control lockup, but also allow manual lockup if desired.

So a few questions:

1. Do we know what the lockup RPM is for the XJR 4L80E transmission?
2. Do we know if that stock lockup RPM is really ideal, and if not, if a lower lockup RPM might be more desirable from a performance and fuel economy (versus comfort) perspective? Tastes and fuel efficiency goals have changed in 20 years after all
3. Do we know how the TCU triggers lockup, or if the TCU is even involved versus a mechanical lockup in the torque converter?

I did some (very, like 5 min Google) quick research on the 4L80E and at least in big truck applications there seem to be quite a few owners that have modified when to lock up the torque converter by a switch on the appropriate pin. So that would make it seem that torque converter lockup is electronically controlled on the 4L80E if one wanted to and it made sense to fiddle with it. I'm not sure the latter is actually the case, and is desirable, hence the start of this discussion.

But the technical part of my brain now is curious....

So, anyone thought about modifying the lockup RPM engagement or making lockup manual, and why?


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Did you ever find out? I'm wanting to put a 383 in an x300 xjr.