Hi Rich,

This response describes the Lock-Up function of a 1996 960 AW30-40/43 transmission but yours may function similarly...

The shifting & lock-up functions are controlled electronically by a Transmission Control Module (TCM).

Now, from the Volvo Service Manual,

"The torque converter lock-up, as with the shifting speeds, occurs at different speeds depending on which driving mode and gear has been selected and engaged at any particular time. As well as vehicle speed the transmission and outside air temperature and engine load are taken into consideration."

"The lock-up function is also controlled by the temperature of the oil in the transmission, and, because of the oil temperature sensor location in the transmission a tolerance of +/- 10% must be allowed for when using vehicle speeds contained in the tables. (The oil temperature sensor is not located in the hottest part of the transmission)."

"Lock-up engagement normally occurs only in 4th gear in the D position and in 3rd gear in the 3 position, but lock-up can also be engaged in 3rd and 2nd gear at high transmission temperatures. Lock-up is always disengaged if the throttle opening is below 3% regardless of the vehicle speed."

"Lock-up engagement and disengagement tables for the three driving modes together with the engine variants can be found in the Pocket Data Booklet 850, 900 1996-."

Hope this helps.