Stupid (-) ground question

[N2Clsx]

So after 6 months or so the tranny, transfer can and engine are back in on my sons 72 Blazer. I just wanted to crank the engine over without starting it to run some oil though it and make sure everything is ok.

Well the ground was hooked up to the A/C unit which I cant mount yet until the valve cover is on so I held the ground to the block to crank it over and the ground wire got very very hot and the engine cranked over slowly. Battery read at 12.02 volts. Was this because of a poor ground or poor ground and a low battery? Help.