Recently I have been having alot of problems with my charging system. After replacing the battery and having the alternator rebuilt, it seems I am still having problems. Here is the problem. I checked voltage of battery with car off, read 12.6 volts, with car on only read 12.4 volts. It seemed as though the alternator was not supplying enought current to the battery. But when I disconnect the negative battery terminal, voltmeter reads 14.4 volts at idle. Went to mechanic and explained the problem. He told me that voltage regulator does not supply between 13.6 to 14.4 volts to the battery constantly because that would overcharge the battery. The battery hopefully should have been fully charged, because the entire weekend I drove only during the day, with no accessories on whatsoever. He informed me that it is completly normal for the voltmeter to show low 12s as a voltage reading, if the battery is fully charged, to avoid overcharging the battery. But the problem is that the more load I put on the electrical system, compressor at max speed, headlights, radio, with 2 500watt amplifiers, and rear defroster, the closer that 12.x gets to 12.0. With all these systems on, the alternator read 12.1 volts. This seems like the opposite way the alternator should be functioning, correct?? The higher the load, the more voltage needed to maintain battery supply. Alternator shop said that something is wrong with the battery and that I should have it swapped out, but the battery is only a week old?? What is going on here??

What RPM are these reading?

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Readings were taken at idle, about 600rpm.

What a load of BS, depending on rpm, load and charge in battery you should have a reading on your volt gauge of at least 13 v, anything less is not acceptable. If you have cleaned and checked all your terminals and grounds, next step would be to check voltage output at the alternator (the big fat terminal) anything less than 13 v would suggest alternator has failed.

First thing is NEVER unhook a battery while the engine is running. 2 things could happen and both are something no one would want. First the battery could make an arc and ignite the gases around the battery and cause it to blow up. Second, if there is a problem with the regulator and it is OVER charging you could actually TOTAL the car by distroying all the electronics. SO NEVER unhook the battery while the engine is running.

Now on to your problem.
The voltage at the battery with the engine running and everything else off and fully charged battery should be 13.0 to 13.8 Volts. Domestic cars will be 14 to 14.8 volts.
With the engine at idle, check for AC voltage. It should be less than 1V. If it is more, than the rectifiers are bad in the alternator. Now back to DC volts at the battery, turn on the lights and the AC fan to high. The voltage will drop but it should stay above 12.5V. Rev the engine to 2000 RPM and the voltage should go back up to 13.0 to 13.8 V. If not, check the alt belt first. It should be tight enought that you can't turn it by hand.(Be careful, it will be hot). If the belt will slip when turning it by hand, it is toooo loose. Tighten it and recheck.
Let us know what you find.

Paul's Auto Electric

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84 500 SEL (307,xxx miles)

With the engine at idle the AC voltage read .1 volts. After switching the headlights on and with the fan on defrost, the DC voltage on the battery only read 11.8 volts. At 2000rpm, voltmeter only read 12.1 volts. I was not able to test/tighten the alternator belt tonight, but will have that done by tommorrow. The thing is that I bought the battery only 10 days ago, had the alternator rebuilt 3 days ago, and checked the wiring/made my own wiring harness but got the same results. Alternator shop suggested that something was wrong with the battery, but now I just don't know what is going on.

I agree with G-Benz 100%. If any car I drive isn't charging at 14.5V I start to wonder what's wrong. Keep in mind that even some all oem cars cannot always keep up at idle, with all the accessories and lights on. It sounds to me like you're more knowledgeable and mechanically inclined than you mechanic, I bet you solve it on your own. It sounds to me that your alt is just simply too weak for your car. Maybe a different more powerful alt, or having the rebuild shop boost yours?
I like doing things different and on the cheap, I’d put a high amp unit from a junkyard chevy in there. The boneyard by me charges $18 for an alt.

I think 14.2V or even 13.9 should be enough.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

True, but I like to be in tip top shape.
Last week my RV was putting out 19 Volts, that is until I realized my multitester had gone crazy. A check with a new tester and I was right at that sweet 14.5

When I had the alternator rebuilt some of the internal parts were upgraded, not sure of all the names but, shop informed me they could boost output to about 90 amps. How would I go about verifying this. And I think that my vacuum lock pump is draining my battery and causing all my problems. Would it be a proble to disconnect fuse #13 and test the battery voltage, or is their a fuse for the lock pump seperately?? And about my audio system I have 2 Alpine MRD-M500 amplifiers, which produce 500 watts continuous power each and 900 watts max. The amplifiers are turned down though, because my subs can only handle 300 watts continuous. I have a Monster 1 farad cap that supplies the voltage to the amplifiers after it receives it from the battery. About the wire, Moster 4 gauge wire an 80 amp fuse. I would really like to make this alternator work, seing as how I just spent $150 to have it rebuilt last friday. I did look into a custom alternator but after all the custom fabrication necessary to mount it, it seemed a bit too expensive for my tastes, probably close to $500. I have unplugged the amplifiers for now, until I get this whole electrical problem straight. I am kind of surprised that you all are oppossed to the idea of amplifiers in the car. Subwoofers are the best. It is like torture listening to music through those puny 3.5 speakers!!

You could take it to a parts store chain like Kragens or PepBoys, they usually have testers. That or you could probably rig some guages and do it yourself.
90 amps is pretty good if it can really do it, but I doub't it, plus you do have a lot of stuff going on. I can't say about that pump since I don't really know what you're talking about, but I'd disconnect it.
I've put non oem alternators in cars before the ones I did weren't too difficult. Some just needed some washers to shim 'em up right, others need a custom nount. I made a custom job just a few months ago, it came out awsome. I also made an AC compressor mount for my truck; Toyota Tercel AC on a Chevy V8.

I love amps myself, butt news flash: I've never seen an amp draw what its rated output is. My last car/amp setup was 700 watts which would require maybe 60 amps or so of draw. The fuse was 30A and never blew.
I'll never forget my very first amp and its lesson in wattage; it was rated at 300W, but came with a 5A fuse!

I agree! Sounds like you did your homework as far as your system is concerned.

I had to quit stuffing subs in my cars when I got married and started a family. I don't particularly care for the pathetic OEM systems offered by MB (or anyone else), but I get to have my butt kicked every weekend by my arsenal of bass cabinets when I'm playing with the band! So, it's a tradeoff.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car