1990 GMC K1500, Designtech ready start 20092

Yes I have tested the AC coming out of the tach wire before I forgot the exact numbers but as you rev the truck the AC voltage goes up just like a tech would. and I hooked a tach up to the wire and that works fine. I can NOT leave the connector plugged into the unit when you crank it starts to smoke the unit and I tried to plug it in wile the truck was running and I programmed the unit to be in tach mode and it stalled the truck and smoked. the unit again. but when I mean smoked the diodes on the circuit board started to smoke but I didn't let them complete fry like the first unit. So I programmed the unit to alternator mode and it's working fine now. but I can't understand why it keeps smoking I know I have the right wire off the coil (white) looked in the service manual and I'm using the Green wire on the unit that is for the rpm signal. I'm really stumped on this one.