measuring alternator current output

I posted this over on engineering/tech.....

need help with theory. 84 cavalier has 78 amp alternator. Voltage light keeps coming on so I'm trying to test with a multimeter. Volts check okay.

But my multimeter has DC amp setting of like 50M (micro amps) 500M 50u etc and a single 10A (I think the max. )

If I connect between alternator grnd and alternator battery out terminal, how many amps should come through? I assume at this point its DC right? THis multimeter won't measure 78 amps.

It really just depends on how many loads are being put one the alternator. If you already have the volts theres a formula to convert to amps. Ill try to find it for you. If you just want to know if the alternator is still good start the car and check the battery. It should be around 13 - 15 volts

'K, if yer gonna use an ammeter:
A.) Ya can get one in an auto parts store or out of an older vehicle that used one. Shunt and all.
B.) The ammeter needs to be wired in SERIES in the hot lead from the alternator-no ground about it.
If you connect your Multimeter like you suggest, the only thing you'll see is smoke as the internal shunt in your meter burns up.
Couple formulas:
Power(Watts(equipment rating))=Volts x Amps
Amps= Watts divided by Volts
Volts= Watts divided by Amps

Best way to check your alt ouput is to do it with a voltmeter.
Set it on the scale that is just a bit higher than the voltage you're measuring.
Take a voltage reading with the engine off/key off.
Should be 11.8-12.5 volts thereabouts.
Now start the engine, voltage should increase-depending on ambient temperature- to about 13.8-15v. You usually only see higher (15v) readings if it's really cold out or the vattery has been severely discharged.
With the engine running-turn on lights, heater or A/C, etc. Stuff that draws heavy current.
If the voltage reading fluctuates lower briefly and then recovers to 13+ volts-everything can safely be assumed to be working OK.
If the voltage reading drops and stays dropped-either you got bad/dirty connections or you need a new voltage regulator.
If you see low voltage at the battery with the engine off and then higher volts with it running-chances are your battery is shot.
Running with a bad battery puts a helluva load on the alternator constantly and that doesn't do it any good.

If you really wanna check your alternator's current output-many auto electrical places and some like Autozone will load-test your battery and alternator for free with a heavy tester.

thanks three,

did not see your post til' after the fact but you spelled it out exactly as I ended up doing... Also, I found an article at about.com that kinda gave me the theory (copied below for other folks)

I checked voltage with engine running and sure enough it dropped - very slowly - first 13.5 finally got down to 12.5 over about 15 mins. but that confirmed that the alternator was not keeping up.

So, I went ahead and put in new alternator. Voltage light no longer stayed on (dim) and voltage at battery now sits at a steady 14.5 volts even with all going full on (lights on high beam, radio, A/C).

Conclusion is a weak alternator or regulator.

Not bad though - a 20 years old Cavalier with only 40K original miles (little old lady's car)



some extra info for the record:


found this on ABOUT.COM author Vincent Ciulla

"Part 2: The Alternator

So far, all we have talked about is voltage. Now we'll discuss the amperage output of the alternator. The state of charge of the battery controls amperage output. When the battery has a full charge, the electro-motive force of the voltage lowers the amperage to almost zero.
As the battery charge wears down, the electro-motive force is not enough to stop the amperage, so it flows into the battery and charges it again.

As you use more electrical accessories, such as lights, heater etc., the electro-motive force decreases and more amperage flows into the battery to compensate for the added load. It is extremely important that when alternator efficiency is checked, both voltage and amperage outputs are checked. Each alternator has a rated amperage output depending on the electrical requirements of the vehicle.

In modern alternators, the voltage regulator and diode pack are inside the alternator and are not serviceable by most DIY's and mechanics. If any one of these parts goes bad, the whole alternator assembly needs to be replaced.

Testing the alternator is simple and basic. You need to check the voltage and amperage output. The voltage is easy enough to check with a voltmeter. If the voltage is between 13.5 and 14.5, it's a good bet the alternator is good. To check the amperage output, you'll need some special equipment. You'll need an ammeter and a load tester to check the amperage.

When the alternator is load tested, a simulated load, usually ½ the Cold Cranking Amperage rating of the battery is applied to the charging system. Then the engine is run at 2200 to 2500 rpm and the amperage is read on the ammeter. The reading should be close to or at the rated amperage of the alternator. If it is, then it's good. If it's substantially lower, then it is weak and should be replaced.

There are some signs you can look for that will indicate a problem with the charging system. If the headlights brighten or the blower motor picks up speed when you rev the engine, you may have a weak battery. This indicates the battery may not be holding enough current to run the car and is working off the alternator. If the charge light is on dimly at idle, this could indicate a weak alternator. If all the warning lights come on while driving, this could indicate a bad diode pack in the alternator. It may or may not still be charging, but you don't know unless you have it checked. At any rate, it will need to be replaced.

If you notice a strong sulfur smell, it could indicate a bad voltage regulator. The battery acid boiling inside the battery is causing the smell. This could lead to a battery explosion so be very careful when checking it.

Today's charging systems have come a long way and are very reliable. A new alternator can last seven to ten years and a rebuilt alternator can last five or more years. It's a good idea to have the system checked when you have the car serviced at every 30,000 miles to make sure it is in good shape and it will greatly reduce your chances of getting stuck with a dead battery."
http://autorepair.about.com/cs/elec...aa122700a_2.htm