The alternator output on your Sonoma is controlled by the electronic control module (ECM), which is the computer. 11.7 volts is below that of a charged battery which should be about 12.6 volts. With the engine running the alternator should keep the battery voltage at about 14.2 to 14.6. That said, it's apparent that the alternator is not charging. It's not that unusual to get a rebuilt alternator that doesn't perform well or even work at all. You may want to have the alternator bench test the alternator to see that it's producing enough current. Make sure no fuses or fusible links are blown. If all else is good you probably have either a bad ECM or a bad wire or connection in the alternator circuit to the ECM.
We had the alternator checked before leaving the store. Has to be something in the truck itself. You may be right with the ecm. The poor rendering of a wiring schematic in the book shows a regulator in the ecm for the charging circuit. Does that seem right? I thought they were internally regulated in the alternator.
Somewhere I believe in the first 1990's vehicles GM eliminated the internal regulator in the alternator and went to controlling the output through the ECM. Normally ECM's are pretty bullet proof, that's why I suggested a good look at the wiring and connectors. The ECM controls the amount of current sent to the rotor in the alternator which in turn determines how much current the stator produces.
I see that someone suggested that you remove the negative battery cable with the vehicle running--that's a no no!!! If the alternator is working and the battery is taken out of the circuit the demand for alternator output can cause it to go full tilt which has burnt up many wiring harnesses and ECM's etc. Use a volt meter at the battery.
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Try checking the gauge inside the truck
If it reads wrong - not to worry - just carry on as it is.
Try turning everything on - then CAREFULLY remove the NEGATIVE battery cable - if the truck continues to run WELL - the problem is most likely the gauge.
Check it the local battery store - free.
Gary @ Fix Ya.
We are checking the voltage at the battery and alternator not just the gauge. Thanks for the suggestion.
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