Voltage Issues and Perfect Pass

So I will try and explain my voltage issues. I have a 91 Supra Sunsport with a 351 windsor motor. When ever I engage the motor from neutral into drive the voltage drops to zero then slowly works its way up to about 12 volts. The problem is this trips my perfect pass which beeps a couple of times and then resets everything in it. My gut is that the problem isnt the perfect pass but I could totally be wrong. Can anyone help diagnose my issue?? Many Thanks, D