Nissan Repair: td27 d21 nissan, lockup torque converter, pathfinder 4x4

Question
Hey i have a D21 TD27 turbo Nissan terrano\pathfinder 4x4 has a 4 stage auto box i have a problem with the torque conveter lock up   it changes gears fine  selects overdrive fine etc but in 4th it locks up too early and wont unlock under acceleration,  if i take my foot of the gas and put it back on it unlocks for appox 3-5 seconds and then locks again, I am a mechanic but have no experiance with automatics only a rough idea of how they work. Would this have anything to do with the TPS? and what controls the lock up is it internal fluid pressure or some sort of sensor etc. Any help would be gratly appreciated  

Answer
Dave,

The lockup torque converter works with a couple of parameters being met.  First, the engine is at operating temperature.  Second, there is no load, that is from the vacuum sensor.  Third, there is enough speed that the vehicle is in overdrive and then the transmission computer will allow lockup of the torque converter.  I would almost bet the vacuum sensor is defective since this is what I find on most Nissan vehicles.  Check it out and email me back.