What's the ECM Voltage suppose to read?

[jorgensensc]

Ok guys, dumb question of the day. What is the ECM voltage suppose to read on the scantool? I'm trying to narrow down why my l92/6l80e went into limp mode last week and won't get out of 3rd gear. I've got no codes in the scantool, I've checked all fuses and checked voltage to the tranny with both key on and key off. All appears fine. After research I see that people are saying bad alternator or battery can cause this same problem. The battery seems to be staying charged at about 12.5 volts with the truck off. With the truck running at idle the alternator shows 13.5 volts at the junction block and the battery. However, the ecu varies drastically from 12.02, down to 11.5, all the way up to 13.5. It will literally bounce around while my scantool is hooked up and driving. Tends to hover around 12.02-11.7 while driving. As the rpms go up, or when first started in the morning tends to be around 13 or so.

When this happened I was literally driving just fine, stopped and got gas, then when I left the gas station it was in limp mode.

So where should the voltage be at? I'm thinking bad voltage regulator? Ideas, opinions, or suggestions are always welcomed!
Thanks all!
Shawn