How to diagnose low alternator output

It sounds like your alternator is not putting out full power – or perhaps the alternator output isn't getting to the battery. Often the wiring is set up so that the alternator is connected to the battery and all of the vehicle loads are connected from the battery.

So, if the wire between the alternator and the battery is damaged or the connections are corroded then there is a possibility that the battery will not get fully charged and the vehicle loads won't get adequate power. You can test this by connecting a volt meter between the alternator output terminal and the battery positive terminal while the engine is running – be careful to stay away from belts and fans and anything else that can move. Set the meter on the lowest DC voltage scale and check. The reading sound be 0.00 V – or very close to it – assuming 4 AWG wire and a load of 50 A and perfect connections the drop on a four foot cable would be (based on 0.0004 Ω / foot) about 0.03 V. If you see anything more than 0.05 to 0.1 V I would be very suspicious of the cable and/or its connections.

The same current will also be flowing in the ground path, but this may be much harder to check. Still if nothing else makes sense, you can do worse than to go through the car checking and cleaning the ground connections.

Other possibilities include a failing voltage regulator, or perhaps an alternator pulley that is too small. Check the voltage at the output of the alternator when the engine is idling, it should around 14 V in most cars, maybe a bit lower (13.5 ish) if the battery is fully charged or if the alternator pulley doesn't allow full output at idle. As you increase the engine speed the output should go up a bit and peak around 14.2 V.