measuring alternator current

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.

There is no way to check that many amps with your multimeter. Take it to any parts store chain and they can test it in the car. If you have it out they can test load and voltage, but the best way is for them to use their equipment to test the actual amperage flowing through the wire. They have an inductance clamp that measures amps by the magnetic field generated by the current in the wire.

You are right, its all DC. Its hard to answer how much amperage you should have since its partly determined by the load you're putting on it, but the parts store folks can test everything else with that same machine and find out what's going on by the process of elimination.

They have an inductance clamp that measures amps by the magnetic field generated by the current in the wire.



hey curtis73 Thanks for the quick response. Heres my long reply....


This question came up because the Chilton manual troubleshooting flowchart for the 84 cavalier says to place an ammeter in series between the alternator and battery. I had it hooked up with my multimeter, ready to test. Right before I started the engine I started looking for the right setting on the meter. I realized nowhere does Chilton say what amps to look for!

As the multimeter reads DC micro and milliamps (m and u), with one setting for 10 amps, I started to wonder what amperage actually come through under DC.

The alternator is stamped with 78 amps but I assume this is what it generates AC before rectifying it (through the alternator's diodes) into DC voltage.

To add to my confusion, I'm looking at ammeters on Sears.com - about $100 for a "pro" ammeter that has the clamp style and measures up to 600 amps - but it says its a/c amps thats why I'm trying to understand.

So then I found an automotive style ammeter on northerntool.com that rates up to 1200 amps DC and sells for 180. I'm always working on somebody's car so I'm game for a new tool but not if I can work around it. Some threads here say check the voltage at the battery under load etc to see if it above 13.5. I will try that later today. This supposedly proves if the alternator is charging enough or not. So why does Chilton say to check the inline current with an ammeter?

here's more I just found this on ABOUT.COM author Vincent Ciulla

This mostly answers my theory questions so hopefully others can understand this better. There were some threads that said you don't need to check current but Ciulla says you may need to...


"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/electrical/a/aa122700a_2.htm

Final follow-up. Will copy this over to Cavalier forum.

Did not get into checking current flow and it would be nice to have that capability. I'll keep looking for a less expensive ammeter.

Meanwhile, I checked the voltage at the battery with all the accessories on and lights going full up. The voltage was about 13 volts but after about 15 minutes it dropped by 1/2 volt. I went ahead and put in a new alternator and checked the voltage. THis time it was up at 14.5 volts and held steady. Plus, the dim voltage light went out. Problem solved.

It appears that the alternator was not exactly dead but rather becoming weak. So the car was running fine but had less voltage and so the voltage light was on but dim. Now the car seems to have more pep.