legacycentral bbs

I have 92 legacy nonturbo automatic and when I shift into gear or turn on the lights the rpms drop from 700 to 500 is this normal. I suspect the alternator but I am hesitant to purchase another. I have a Autometer starter, battery and alternator tester, test indicates operational parts but if put the lights on or shift into gear and run the test again the alternator test say low output 20-50 amps output. Again I am asking for confirmation before I spend the money for another alternator. Any thoughts would be great.

Yes, it does sound like the alternator isn't quite as strong as it should be.

I'm curious to know more about how the tester tests it.

Are your cables in good condition?

I actually worked for Auto Meter in Pleasant Grove Utah when I got this tester. I know how they work quite well I have fixed many of these battery volt alternater testers as with everything thing else they sell, as a techy there. To test the alt. connect the unit to the battery terminals then the unit creates a load at the battery terminals and it takes a reading. The reason this can test the alternator with battery connected is the battery has internal resistance and the alternater does not, at least not as much as a battery. So the intial draw of current is all most all from the alternater, depending on the resitance of the battery you have a tiny window to test the alternator before the battery steps in. Basically when the unit creates a load it shorts the postive and negative terminals with a shunt resistor and measures voltage across the shunt and then with ohms law it can calculate current. It works good but it does not know how much the alternater can output, it just knows how much is being output. Well I hope I helped you understand how an alternator can be tested obviously this would be increadably difficult with out the micro processor! With your programing skills and little electronic engineering you could make a test program for a PC! Anyway thanks a lot for your input. I guess I will just keep a close eye on it!
I will also check the cables

Hmm, I don't see what internal resistance has to do with the time it takes for the battery to start supplying current... But I guess as long as Auto Meter found a strong correlation between the current measured that way and alternator performance it's all good...

Oh -- are you sure you're reading the tachometer right? The tickmark that a lot of people think is 500 RPM is actually 750 RPM.

Are your only symptoms that engine speed drops when you shift into drive and that the lights dim as that happens? Because I do think this is normal to some degree... your alternator may be tired but not dying (yet). Maybe someone who knows automatics better than I do can say.