I Need Your Help

I have a 12" Infinity 12.1 sub powered by a Wolfenhaug 700 watt amp bridged to 1one channel in a 1994 S-15 Jimmy. I ran 4 ga. power wire & 8 ga. ground wire. When I come to a stop light or stop sign my volt meter drop to about 8 volts, day or night.  Sometimes even a little lower at night with the lights & heater on.  What can I do to keep my voltage from dropping so low?

Does it also do this whenever the vehicle is idling in gear?  What about when you put it in park and rev the engine?  There could be something going on with your charging system, or you might just need to install a capacitor.  But before you get into any of that let's find out a few specifics.

You never actually said that your amp was cutting out, just that your voltage was dropping.  Should we assume that's what you meant?

Finally, does this happen only when you have it turned up really loud?  Or does it happen regardless of the volume?  And do your headlights ever dim?

This happens while I'm in gear with the radio turned up, when I knock it into neutral, my voltage comes back up. The amp never cuts out, and my lights do dim quite a bit. 

Good lord I can't believe I missed that part of your message!  Yes please listen to what they said...fix your ground wire first and then we'll go from there if it's still acting up.