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Discussion Starter · #1 ·
Stock '66 Chevelle SS396.

The alternator was making a bit of noise so I decided to replace the front bushing. Everything was working well before I did that. After replacing the bushing (no problems) and starting the car the gauge in the car indicated that the system was discharging. I removed the alternator and took it to my local Autozone and had it checked. It passed the test. I put it back in and still the gauge still indicated that the system was discharging.

Next, I checked the battery no-load voltage: 12.80V. Okay. Then started the car and checked the alternator output (at the alternator): 12.40V @ idle and 12.46V @ ~2000 rpm. No increase in voltage with higher RPM.

Shouldn't the alternator be putting out close to 15V? Does the stock gauge read off of the alternator or off the output of the regulator?

Any trouble shooting suggestions will be welcome.

Thanks,
Greg
 

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Those new brushes sometimes have a tab you need to pull out.

You should be seeing at least 14.2 volts.
 

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I suspect Autozone's 'test' was wrong. When you disassembled and reassembled the alternator, did you install the brushes and retain them with a paper clip until the armature was in place? Did you remove the clip when finished?
 

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Discussion Starter · #4 ·
Yes, I pinned the brushes when I reinstalled then and then pulled the pin after reassembly.

Greg
 

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When you stated it is a stock 66 Chevelle I'm going to assume you still have the external voltage regulator ?.

You also stated, that you checked the battery no-load voltage, so you probably have a volt meter to test other things ?

I would first make sure the two wire plug is seated and connected well on the backside of the alternator as well as the 4 wire connector on the external voltage regulator.

I would then unplug the 4 wire connector on the external voltage regulator and then with the key on, see if you show voltage on the terminal with the brown and then turn the key off to see if that wire goes dead and the terminal with the red wire shows voltage all of the time whether the key is in the run position or off.

You may also want to do a continuity check between the remaining two of the four wires on the voltage regulator plug that go to the two wires on the back of the alternator (the dark blue and white). Maybe a wire is frayed and from unplugging it and plugging it back in the wire or connection has failed.

My experience is, if the battery is sitting there with 12.4 volts, then to charge it while keeping up with the electrical demands, I need to see at least 13.4 volts or a volt over the 12.4 volts and then I'm sure things are good. If the voltage goes up to 15, then this is also fine but if it's over that, something is wrong or not adjusted properly.

Jim
 

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Discussion Starter · #9 ·
I suspect Autozone's 'test' was wrong.
Randy, I'm now tending to agree with you. Jim and Louis, thanks for the feedback.

Since I was only getting ~12.4V at the alternator output, rather than the 15+V I was expecting, I was thinking that the problem is with the alternator and not the regulator. Today I uncovered my '68 El Camino, which essentially has a setup identical to the '66 Chevelle and measured the output voltage of its alternator. It was 15+V at idle -- as expected. I then disconnected the electrical connections to the '66 alternator, ran the car at idle and measured the output voltage of the alternator -- it was ~1.5V. Since there actually was an output voltage, that might have been enough to fool the Autozone test machine.

Next step is to figure out if the '66 alternator has a problem with the rotor, the stator, or the diodes and brush assembly. I've got another working alternator so I'm just going to substitute parts until I narrow it down.

Greg
 

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How did the test of the regulator come out?
Or have you tested the regulator??
 

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The test done with the alternator on the vehicle with the wiring disconnected like you did is not a valid test. Did you read Jim S's post?

Shot gunning parts is not a way to troubleshoot. A lot can be determined just by using a test light to rule things out quickly. There are specific tests you can do...
 

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Discussion Starter · #12 ·
I solved the problem! It was the external regulator.

The reason that I was focusing on the alternator and discounting the regulator was that I didn't understand how the charging circuit works. I assumed that the alternator always put out ~15V with the battery circuit going to the regulator where the voltage would be cut back to the voltage that the battery needed. Therefore, when I wasn't getting ~15V on the output of the alternator, I thought it had to be the alternator's fault.

After I read up on how an alternator works I then understood how the regulator varies the field current of the alternator to adjust the output voltage of the alternator.

The "old" regulator that failed was a solid state unit but it looked quite a bit less heavy duty than the VR-715 that I just replaced it with.

Thanks to all who offered advice.

Greg
 
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