the plug is connected to a solinoid in the trans that locks the engine and trans imput shaft together by locking the torque converter...i discover my daily driver 81 impala coupe has the same trans...it shifts at about 35 into 3rd gear then kick down at 45 (must be the lock kicking in) i found that you can control this type with a brake switch and 2 vacuum switches...heres how the diagram works

12v hot when engine running....goes into a brake switch (normally close..circuit broke when brake is applied)....to a low vac switch(normally open circuit broke) the swtich closes at cruising speed and sends 12v to the high vac switch (normally closed) when u mash on the gas (highvac) is opens and kills the lockup....then to the 12v noid on the tranny

confused yet? no all i need to do is find those 2 vac switches..... anyone have part #?