http://www.autospeed.com/cms/article.html?&A=2672As early-generation electronic transmissions were introduced, it became common to use electronic control for secondary functions - such as kickdown and torque converter lock-up. The torque converter lock-up mechanism (aka torque converter clutch) is used to improve engine efficiency and fuel consumption at cruise. In this situation, the impeller and turbine sides of the torque converter are locked together to eliminate slip.
A page from a locally made vehicle (manufactured 1986- 1988)detailing how to check out the torque converter solenoid: http://forums.justcommodores.com.au/att ... lenoid.jpg This particular vehicle had a lockup button on top of the trans shift to disable the lock/ unlock function to improve driving around town.