what should the volts going to tbi drop to when turning over engine 1987 suburban 350

I would check the fuel pressure, and check to see what the spark looks like. Fuel pressure should be 9 to 13 lbs, and the spark should look white, blueish in color. If it is a orange color then there is a weak spark. I think injector voltage should be around 5 to 7 volts.

Another thing to check would be the TBI base gasket, may be bad allowing a vacuum leak.