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Don't understand the why of alternator troubleshooting

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yankee_echo

I'm New Here
On my last flight, I noticed my voltage was showing 12.0V. My first thought was bad alternator so I landed and pulled the cowl. Checked the voltage at the field and B+ and both points indicated 12V which I believe means a bad alternator but I don't understand why.

I'm using a Plane Power AL-EI60/B and its connected just like the Plane Power troubleshooting diagram. If the B+ is connected to the aircraft bus through the output circuit breaker and the battery and alt field switch are on, wouldn't the voltage between B+ and aircraft ground be 12V normally? I'm not really familiar with the internal workings of the alternator other then spinning magnet makes power.
 
Before you decide the alternator is bad check the belt tension. If the belt is slipping you can see a voltage drop while in flight. Tightening a loose belt can be all that is needed to “fix” your alternator problem.
 
With the engine off, the B lead should read battery voltage or -.2 battery voltage. With the engine running, the B lead should read in the neighborhood of 14.2 volts.
 
yankee_echo said:
I'm diagnosing based on the the Plane Power Troubleshooting Guide for internally regulated alternators as well as this post:

https://vansairforce.net/community/showthread.php?t=125262

Unless I'm reading it wrong, both indicate I should not be seeing voltage at the B+ terminal. I just don't understand why a functional alternator would show no voltage.
Click to expand...

Because it is not spinning. The alternator should not show voltage on the B lead if it is not spinning and disconnected from the batt. However, if the B lead is connected directly to the battery it should show batt voltage if the connection is solid.
 
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