13.69 volts is just a fuzz below the minimum acceptable voltage. 11 volts is definitely too low. Now the thing to do is to see if the voltage will come back up when you turn off all the loads possible, meaning radio, heater fan, dome lights, etc. If the voltage does go up to 13.75 or higher, suspect a bad diode. It's not very practical to replace them; replace the entire alternator instead.
If it has failed completely and the voltage no longer goes up when the engine is running, suspect worn brushes in the alternator. They can often be replaced without even removing the alternator from the engine. To verify they're worn, use an ohm meter to measure the resistance between the two small terminals in the back. If you get an open circuit reading, irritate the belt a little while measuring. It's common for good brushes to not make real good contact when the rotor isn't spinning, and bumping the pulley a little will create a better contact intermittently so you can take the reading.
Sunday, July 8th, 2012 AT 10:30 PM