I just replaced my alt because the old one was putting out 13 volts. The new alt is generating 14.2 volts at the post at idle (with a 0.1 volts lost when measured across the battery terminals). However, when I turn on the A/C and lights, volts drop to around 12.6 at the alt post (12.5 at the battery terminals). Is this normal? Will that keep the battery charged? Am I testing this correctly?

 

No, it's not normal. You'll need 13-13.5 volts just to keep the battery charged. You idle speed may be dropping excessively with the A/C and lights on, which can happen if 1) your idle is set too low 2) your BAC is not functional or removed.

 

The idle and BAC were my first thoughts. However, the idle is perfect at 800 rpm and the BAC is holds idle steady. Is there anything else that might cause the volts to drop?

 

Yes, bad connections at any of the above mentioned components (corrosion, loose, bad grounds, damaged wiring even corroded fuses). You are going to want to test voltage drop from the battery toward each component at any junctions, fuses or switches til you find the culprit

 

Electric isn't my area. How do you test for voltage drops?

 

http://www.aa1car.com/library/voltage_drop_testing.htm

 

Do the volts drop with just the lights on? If so how much?

 

If it's also 12.6 V at the alternator then your devices are drawing more current than your alternator can handle and thus causing the voltage to drop. Most likely cause is (another) weak alternator. The alternator will work "fine" as long as you don't run the lights and so on too frequently, but no it isn't supposed to be this lousy. It's common to replace the alternator and get another bad alternator, especially when buying rebuilt alternators on cars this old. You may have to keep trying until you get a decent one, or upgrade to an alternator from a newer vehicle like the FD or the ghetto solution is to carry around a jump box just in case your battery finally does die some day. Hey, it works.

Your replacement alternator should be under warranty so if you can get them to confirm that the current output is too low you can get a replacement for free. Replacing it should get easy. The main issue is driving back and forth to the store 1 or more times.

As others said a bad connection is also possible, though I think in that case the voltage at the alternator would measure normal (~14.5V) with the voltage drop elsewhere. Hope you have a multimeter to check.

 

take it back for warranty, unless by new you mean "used" from a wrecking yard in which case you may have gotten another bad alternator. the stock alternators don't last forever and usually will still work marginally when they get older, putting out as low as 10 amps in some cases but still working.

the lights and A/C could be drawing too much amperage but i really doubt that to be the case, if the alternator was putting out anywhere near the correct amperage then the fuses would be blowing for one or another circuit due to the amount of draws being pulled.

 

The volts drop with just the light to around 13.5, a little more with high-beams.

 

The alt is new (aka rebuilt) from the auto parts store. The alt will probably test in spec which just isn't enough for the car. I'm going to have to upgrade it.

The volt drop is at both the battery and alt post.