If your grounds are all good, then it should keep running with the negative cable removed. That's how I've tested alternators in the past, successfully - though I haven't tried it on the Solstice.
I'm going to ramble out some wild-ass guesses:
If it was only putting out 12V when running, then either something is pulling the voltage down (most likely by drawing a ton of current) or the alternator isn't doing its job properly. That could also trigger the battery light (which is really, I think, a charging system light).
said, a loose alternator belt could cause low alternator output. But, given how simple the tensioners are on this thing, it's pretty difficult for it to have a belt that's *not* tensioned correctly. You said that you used the accessories from your old engine. Did you also use the old tensioner? Is it properly tightened to the block and keeping the belt tight? If the tensioner is bad, you should be able to see some play in it. The belts should be normal "belty tightness." If you reconnect the battery and start the car, can you see significant bounce in the belt or tensioner?
Did you recharge the AC yet? Make sure the AC is off when you're running this. If it was improperly charged and the clutch is engaged, it *may* put too much load on the system. (Again, I'm guessing here.)
If you have a cheap scanner, like this:
And a smartphone app like DashCommand (or Torque for Android people), you can see what the ECM thinks the voltage is, among other things. But, I'd expect that to be the same as what your multimeter is telling you.
Blue-ish 2006 2.4, Werks stage 1 turbo, Borla cat-back, DDM braces, Spec aluminum flywheel, Spec stage 2 clutch, Werks aluminum radiator, some gauges, RKSport hood, Morimoto FX-Rs, GReddy Profec, Norm's fenders
'07 GXP, RPM Stage 2, Solocatless downpipe, TCE Wilwood 6 piston front brakes, 4-piston rears , Stainless brake lines, slotted/drilled rotors, BC Racing BR coilovers, Performance Autowerks intercooler, DDMWorks CAI, charge pipes and braces
Last edited by raygun; 03-25-2019 at 09:03 AM.