13.6 volts is too low. It's close to the bare minimum of 13.75 volts but it suggests your generator has a bad diode. Have it load-tested on the car for maximum output current and "ripple". Ripple is usually not measured as a voltage. Most commonly it is displayed on a bar graph. One or two bars is okay.
If the generator proves to be defective, replace the battery at the same time to prevent repeat failures unless it is less than about two years old. GMs poor generator design causes them to develop huge voltage spikes that destroy it's diodes and internal voltage regulator and can play havoc with computers and their sensor signals. As the battery ages it loses its ability to dampen and absorb those spikes. The old battery will work fine in an '86 or older model that still uses the older generator design.
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Saturday, October 20th, 2012 AT 7:08 PM