GMC Repair: GMC electrical, wiring schematics, battery terminal

Question
I have a 1990 GMC 3/4 ton pickup with a 5.7 litre engine originally equipped with cruise and air. The voltage at the alternator reads between 4 and 8 volts. I have installed several alternators (new and rebuilt) - all give the same result. If the voltage regulator plug, which runs to the instrument cluster is unplugged the readings return to normal battery voltage @12 - 13 volts. I have cut the wire and left the plug installed - result; battery voltage. I have traced the wire and it appears intact. With the wire cut I have run the wire from the plug to the positive battery terminal. Once contact is made voltage at the alternator drops to @ 4 volts. I have replaced the instrument cluster with a used one and the results were unchanged. I have checked with numerous mechanics including GM authorized and none have ever come accross the problem. Every time the vehicle is started there is a drop in votage reading in the cluster. Once the reading nears 9 volts at the dash voltage gauge it quickly drops out of sight. Once the vehicle is shut off it will require a boost to start. Even with a new battery the readings are the same. Any assistance you could give would be appreciated. Thank You  

Answer
I don't have access to 1990 wiring schematics right now, but I will try to get a look at some soon. The only thing I can think of off the top is that you have a ground issue. Somewhere there seems to be a loose or missing ground that is trying to back feed into the alternator. Also check the alternator bracket since the alternetor is self grounding through thr bracket.