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Lately I've been experiencing an issue regarding my charging system. When I measure voltage at the battery terminals while engine is idling, it's a pretty low 12.5v~ or so, when cooling/HVAC fans come on it can go below 12V completely. Revving the engine can top me out at 13V.

I wanted to check my alternator's output directly, so I found the positive cable, uncovered the rubber housing, and put my multimeter lead on the nut, then I grounded it on part of the alternator housing (and again on the engine block) -- the alternator read an output of 14+ volts.

Does this mean the alternator is good but the battery is just not getting enough power? Because as I said the alternator DOES charge the battery slightly, but all of the power doesn't seem to be getting to the battery.

One thing I did notice was the ground wire from the negative battery terminal is exposed, and oxidized (it's green), I have been suspicious of that ground but before I replace it I want to know if that could possibly cause low voltage, or if the solution might be in another area.

So to summarize:

1) Is my method of testing alternator output correct? And does that mean my alternator is outputting good power?

2) Could a bad ground cable be responsible for the described issue?

Vehicle is a 2000 Acura TL.

Thank you

EDIT: Problem was fixed, it was the ground cable. The connection was corroded very badly -- keep your grounds CLEAN. This improved my car 100%, everything in the car is controlled by electronics, now that it has proper supply its great.

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    what happens when you ground the meter on the battery negative? it'd be worth cleaning the battery ground terminal but it probably won't solve your voltage drop issue do some testing on the alternator to b+ cable as well. – Ben Feb 14 '17 at 22:45
  • Hi Ben, do you mean while the car is running go from positive alternator bolt to negative battery? What I just did was test my ground strap, I put the positive lead on the pos terminal, then instead of placing the negative lead on the negative terminal i followed the ground strap to the bolt on the chasis, and the reading was jumping from 0-11.60 (almost battery voltage) and everything inbetween. The bolt was rusted. – Jay Feb 14 '17 at 22:51
  • Correct, you may also want to test the battery positive (b+) to alternator b+ terminal. Clean up that bolt or replace it to get a good reading I bet if you scrapped some of the rust away you'd get a solid reading. You could also do b- to chassis ground testing, disable the fuel injectors and have someone crank the engine, anything over 200mV I'd replace the cable. – Ben Feb 14 '17 at 22:55
  • Sounds like a dead alternator to me. At least a dead voltage regulator in the alternator. The voltage regulator can be easily replaced without removing the alternator most of the time, and usually is cheap so worth a try. Nevertheless, you should definitely clean those connectors and terminals every now and then. – galingong Feb 15 '17 at 9:25
  • Hi galingong, how could the alternator be dead if it's outputting 14+ volts from the positive nut? – Jay Feb 15 '17 at 12:35
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Run the engine at 2k RPM. Turn on all the loads (blower on high, headlights on, rear defrost on, and so forth).

Measure the voltage across the battery and the big bat alternator terminal to alternator case. There are two things that you are looking for. The charging voltage should be between 13.9v to 14.4v. Also the voltage at the battery and the voltage at the alternator should be within 0.5v of each other.

If the voltage at the battery and alternator don't match. For example the battery reads 12v and alternator reads 14v. This means that there is an issue with wiring. While applying the same conditions as above. Put the black multimeter probe on the negative battery terminal (if possible dig it into the battery post, not the terminal). Put the red probe on the alternator case. Your looking for the voltage to be below about 0.5v. If the voltage is higher than that examine all the negative cabling for problems. Preform the same test with the positive side. Place the red lead on the positive battery terminal (if possible dig it into the battery post, not the terminal). Place the black lead on the bad post of the alternator. Same thing as above, if the voltage is above 0.5v then examine the positive cabling for problems.

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