here's exactly what I did:
Car starts, voltage 14.4v
Start driving, voltage immediately drops to 12.6v, 12.5v, 12.4, 3..2..1..etc...
fan/lights on, not much difference
Put on neutral or park, voltage goes back to 14.0v, 14.2, 14.4 etc...
start driving, voltage immediately drops to 12.6v, 12.5v, 12.4, 3..2..1..etc...
Put on neutral or park, voltage goes back to 14.0v, 14.2, 14.4 etc...
repeat again and again, same thing.

I took the 2005 honda civic lx to the dealer. They replaced it with a Honda re-manufactured alternator.

Power on, 14.4v.
Start driving 14.4v.
Driving on highway for few minutes, drops to 12.5v, 12.4, .3.2.1 etc...
Put on neutral, 14.4vdc
Continue driving drops to 12.5v .4.3 .2...
Here's an odd thing: I put the cabin fan on, voltage goes from 12.2v to 13.5v etc...

 

Do you have a problem with the batt. staying charged, can you start the car in the morning?

The ELD, [if you have one] will control the alt. output as needed.

If the batt. light is not on or "flickering" on and you do not have a problem starting the car in the mornings, I don't see a problem, "if it ain't broke don't fix it". 94

 

Sounds like a bad voltage regulator in the alternator. I've had them fail out of the box before. Have you pulled the alt and brought it in to an auto parts store for testing?

 

Well, it happened with both alternators.

Battery does start the car. Does power radio, fan, headlights. The battery is 7 months old. But it did go down to 10 volts when I had it for about a month because of an earlier alternator total failure. At the moment, the battery **seems** to be good. to rule out the battery, what else should I try?

Tell me, should the measured voltage ever be at 12.1 - 12.5 volts while driving?

 

mine suddenly dropped to 12.4 for like 30 seconds and went back to 14.4 after

 

Your alternator is doing what it's designed to do. Run in full output at idle and between 10-50 mph with no load, run in low output.

Completely normal.

To test, drive the car and turn the lights or ac on. Voltage should go back to 14 or so as the alternator goes back to high output because of the load.

You can drive above 55mph and see what happens too.

 

just to be sure...

I just drove locally and the measured voltage is 12.2v. is that still normal?
I never went above 50, drove like grandma.

 

in reality it should be charging at atleast 12.5 but as long as your car starts fine that probably won't be that big of a deal. Again, if you load it up and the voltage goes up it's doing it's job fine.

Here's a complete list of information if you want to read it.


http://opensiuc.lib.siu.edu/cgi/view...text=auto_pres