I have a 87 vette that the charging system has been around 14,5 volts. The other day, I started the car and the red batt light was on and volt indicated 11.5 volts. I checked the batt with key off and it was 11.3 vots. I started the car and check the alt output and it was the same as the batt. I went to the parts store and got a new alt. I had the old one tested and it check OK. I installed the new alt and the same thing.....no output.

I did some testing of the three wire alt connection (red, blue, brown). With the car off, the red wire has batt voltage (always hot). When the car is switched on,,,,,the red and blue wire has batt volts. This includes the alt output wire. When the car is running; the connector red and blue wire, along with the alt output registers the batt volts (11.5 volts). The brown wire on the connector has no power either off or on. What is going on here.

Thanks, Allen

 

Thanks for all the help fellow Corvette owners

 

The brown wire lights your charge indicator on the dash to show the alternator has no output by going to ground. That is why you don't measure any voltage on it. The connector red wire goes to a fusible link connected to the battery and the blue wire goes to the coolant fans and the coolant fan fuse which has 12v only with the ignition on.
The alternator output wire is connected to the battery through a fusible link and I would use your ohmeter connected from the battery positive terminal to the alternator output terminal. The ohmeter should measure the same as touching the test probs together (very low resistance).
One thing you likely didn't do was to measure the voltage on the connector red wire when it was plugged into the alternator. If the fusible link on this wire was blown and you measured the wire with the connector unplugged, your voltmeter will likely measure battery voltage because the blown fusible link has high resistance across it and your voltmeter is very high resistance (too low current through the blown fusible link to cause any voltage drop across it). If you take a needle and pierce the connector red wire while it is plugged in (red wire goes to the voltage regulator and the regulator draws several amps,engine running) and the engine is running, your voltmeter on the needle should measure battery voltage (if zero, blown fusible link). Also, charge your battery up, at 11.9 volts it is discharged.
I would also use you ohmeter to prove that the metal case of the alternator electrically goes to ground by measuring the resistance from the battery negative terminal to the alternator case. It should read the same as touching the meter probes together.

 

My Friends Chevy truck stopped charging, it was an Alternator fuse in the fuse box under the dash.

 

87 Corvettes don't have a plug in fuse in the alternator circuit.