91 Chevy s-10 ground short, 12.6 volts across negative battery post to battery

[I might be right]

cable, CABLE NOT CONNECTED!? (91 chevy S-10 4.3 4x4 ) Click on my name and see my previous question for further information, and yes the first thing I checked was all the gounds/ battery cables. The guages on the dash go haywire, coolant temp goes from hot to cold, oil pressure goes through the roof, etc, not good readings as the ground side is getting positive voltage through it and causing inaccuaricies. The main problem is that the alternator charge goes to nothing. I am getting a full 12.6 volts thorough the negative side of the system with one lead on the battery cable and the other on the battery WITH NEGAITVE CABLE DISCONECTED. I can only make short go away by disconnecting the input ground wire from the block to the ecm (tested wire it is good) . I took out ecm case (cpu under the dash), opened ,and looked inside, looks new, and really doubt this is the problem . My question however is is it possible to have the ecm itself tested? ANy suggestions? I have a short tester/ no luck though and full wire diagram