Hi Tony,
Yes, lock-up on torque converters is transmission-temperature-controlled on
1990-on MY cars (whether ZF or GM trannies) - ie, lock-up doesn’t work until
the tranny temp reaches a pre-determined level (in order to avoid snatching
and engine labouring, which are both detrimental to engine and transmission
life when the powertrain is below optimal - ie approx. 160-200C -
temperature).
Cheers,
Graeme Hoy
1993 XJ12