Ignition Issues, won't start

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I did a build on my car and I got everything put together today but the car will not start unless I jump the solenoid and the 12 volt source. Also I have an autometer cobalt boost gauge that is tied into my ignition so it doesn't lose power while cranking, and that doesn't even come on. I tested the starter wire while a friend cranked it and it only has 6 volts, car has never had a problem starting ever. Any ideas?

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I think it may be a loose ground some where that's dropping the voltage, but I have no idea where to start. Any Help? Engine Bay ones are good but I think it might be a ground inside.