Lockup is electronicly controlled on the TBI models.I have slightly altered the lockup points by adjusting the TPS on these trucks,as well as TV cable.You might have to live with it or change the computer chip.Lockup is affected by TPS,coolant temp,road speed(VSS)and MAP sensor readings.A couple of springs in the valve body to. If your computer wont allow lockup until 53 mph,adjusting the TV cable or TPS is useless.Most people would be happy to have TCC lockup at 55mph,most chug to much at 40mph in lockup in OD.under light throttle my 89 GMC 2500 will lockup at 31mph in 3rd,shift to OD at 35 mph and stay locked up chugging along at idle speed.<p>----------<br>John D<br>