You're going to need a mechanic to look at this. At "around 11 volts", the head lights should not dim very much. The industry-standard for the starter motor to crank the engine is 9.6 volts. You apparently have more than that. If the voltage is sufficient, slow cranking can only be caused by one open brush in the starter. That kills half the circuit. Current draw will appear to be close to normal, but actually, half of the starter is drawing double the current it should and the other half is drawing nothing. That makes voltages look normal but the starter has only half the strength it should have.
The clues you mentioned don't agree. The "Volts" gauge on the dash shows the battery voltage is much lower than 11 volts. The head lights should be dim during cranking indicating the battery has a shorted cell or a bad connection is restricting current flow. With a shorted cell, the battery will measure close to 11 volts when nothing is turned on. There's no cure for that other than to replace the battery. If the voltage is 12.6 volts but then drops a lot during cranking, that points to a bad connection. Most commonly on GMs with side post batteries, that is right at the battery, and much less likely at the starter.
Friday, October 9th, 2015 AT 6:04 PM