If the alternator is not charging the brake light will remain on.
Note that on any modern vehicle with an alternator, doing a test by disconnecting the battery while the engine is running is very bad. This worked for older vehicles that used generators and did not have onboard electronics, but with an alternator the system depends on the battery to regulate and filter the alternator output. Without the battery in the circuit, the system voltage can rise dangerously high and will have large spikes. The result can be permanent damage to the onboard electronic components i.e. ECU, radio, ABS, airbag controller etc.
The electrical system should be tested only with a voltmeter or, for more accurate diagnosis, a scope.

 

What about the voltage regulator?

 

The regulator is an integral part of the alternator.

 

yes, but doesnt this help filter and regulate the alternators output? And the zener diodes on the recitfying bridge, it takes so much voltage to overcome them i wouldnt think it would allow so much voltage to pass through that would be harmful....you think?

 

If the voltage regulator was bad the system would be charging above 14 volts. If your rectifier bridge was bad you would be getting more of a AC wave, which is bad.

Hey Mulder a scope wouldn't be nessasary because it is a DC wave, and would be the same as a voltmeter. You would need a scope to check the alternator is to see if the rectifier bridge is working correctly.

 

cool, thanks guys

 

I mentioned a scope because it is the best way to diagnose certain problems such as a bad diode, which can cause poor charging and strange electrical symptoms even though a voltmeter check will show a normal reading.
The regulator is designed to work properly with the battery in place, not with it removed.

 

Alot of good info here again thanks, the wrxer is back up and runnnig....

 

I had the same problem a while back. check the wire that connects to the alternator. mine was coroded and was very loose. be careful when tightening the bolt though. its very brittle and might break.