Nissan Repair: Automatic transmission, vacuum switch, torque converter

Question
 Terrano 1990 2.7 diesel   Automatic transmission does not hold lockup for more than 30 seconds. Pull the car up and turn it of the turn it back on it goes in for thirty seconds again.
but tacho is not working at all maybe this may have something to do with it, other wise the car runs well all the time.
please any information would be grateful.
Les @ Cherie

Answer
Cherie,

The tach could have something to do with the torque converter unlocking.  Second, you should check the lockup solenoid to see if it is not working properly.  That is, it is a vacuum switch.  The car is at operating temperature and there is no load and then the vacuum switch allows the torque converter to lock.  The tach provides data to the computer to allow for lockup under no load.