[erps]

Bad grounds cause some very interesting problems. I remember working on a big rig truck working with a test light. I had power going through the firewall from the engine compartment but my tester showed all the wires were dead on the other side of the firewall until I ran a jumper wire to the cab to make sure my test light was grounded. I'm a big fan of jumper wires.

 

[erps]

You'll find the problem. Usually when all you get is click click when you turn the key it's the solenoid, but the strange voltage readings make it a little more complicate. The engine voltage should drop while the motor is cranking. That it remains 12.5 Volts tells me the high current can't make the loop through the starting circuit. Bad solenoid contacts would do the same thing though. A pair of jumper cables from the battery to the engine block and the big positive terminal on the starter would point back at the solenoid if the problem persists while the jumpers are hooked up. If it starts well, then disconnect one half of the jumper cables at a time and crank again to narrow it down to the positive side or the negative side. Good luck.