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Old 09-26-2005, 10:26 PM   #1
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Join Date: Sep 2005
Location: Minneapolis/St. Paul, Minnesota, USA
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Alternators - What they are, How to Test them


I replied to a queery on another Forum (yes- GUILTY, I do visit some others) about a guy who thinks he may have a problem with his Alternator. He didn't seem to understand their basics, or how to test them.

Perhaps some of you will find it useful!

To begin, an Alternator is not a very efficient way to produce electricity for a car. First, it produces Alternating Current (AC), which the car cannot use, so Alternators must rely on Diodes to rectify their AC output and convert it to Direct Current (DC) which the Car can use. Second, an Alternator requires two watts of energy to spin it for every one watt that it produces. It's advantages are that it can produce large amounts of current relative to it's size, and it can produce large amounts of current at idle, things a Generator does not do very well.

The Lundell Alternator, technically the proper name for your Alternator, is really three Alternators in one body. Each of the three sections of the Alternator generates Alternating Current (AC). In one revolution of the Alternator it puts out three separate currents. These currents are each out of phase with the other two sections. Since the complete cycle (one revolution) of the Alternator is 360 degrees, each of the three phases are shifted by 120 degrees from the next phase. Each of the three phases has its own windings in the Alternator and each of the windings has its own pair of Diodes. Because the Alternator generates AC current, each of the Diodes in a pair have their own polarity - one positive (+) and one negative (-). The negative bridge of the Diode connects directly to the Alternator housing, which is of course grounded to the Engine Block, while the positive bridge connects to the output stud, which then goes back, via a cable, to the positive (+) terminal of the Battery. This is necessary to smooth out the ripple effect of the current the Alternator produces.

The Diodes' function is to block the power of one polarity while passing through the power of the opposite polarity (this is why you have two of them for each phase - they each function in opposite ways to each other). This is how they convert the Alternating Current (AC) to Direct Current (DC). The Diodes (or Rectifiers, because they Rectify the current), are usually arranged in pairs with all six being located on a single IC board or chip. Because of this, you get some rippling DC in the Alternator's output if everything is operating properly (this was the test the Auto Store conducted). But, the Battery smooths this out to regular or smooth DC.

These Diodes last only so many thermal cycles. Some of the newer Avalanche Diodes are even designed to be self-sacrificial by shunting large output spikes to Ground in order to save the sophisticated on-board electronics found on today's cars. Today's Alternators also produce high output from a very small package, which means that the case is not a large enough to act as a Heat Sink. The high heat which consequently builds up helps shorten the life of the Diode chips significantly.

Now, since each of the three phases has its own windings in the Alternator and each of the windings has its own pair of Diodes. Each of these windings and/or Diodes can fail, one set at a time.

If this happens the Alternator can still charge the Battery sufficiently to run the Car and it's Accessories, but only with a limited current, approximately 2/3 of its original capacity if one system fails. If two systems fail, then it puts out only 1/3 of its rated capacity. What this means is that a failing Alternator can go unnoticed for a long time, because unless you are using the bulk of the Car's Accessories at one time, the limited output of the failing Alternator may still be sufficient to meet the car's needs.

The Alternator will continue to fail until finally, there is insufficient output to charge the Battery which runs the Car and it's Accessories . The first you may notice is the Headlights are dim, or the Dash Indicator lights, or the Stereo starts to whine, or worse, the Car fails to start (as Luck usually has it, typically late at night and in the rain).

Before suspecting a failed Alternator, you need to eliminate other possible causes. Check the Accessory Belt to insure that it is tensioned properly, usually no more than .5" movement when you push it with your finger (car OFF of course). Next, with the car OFF, check the voltage across the battery terminal with a Voltimeter/Multimeter (set to DC scale), it should read at least 12 volts, more is better.

If all these things check out properly, then the Alternator becomes suspect. To begin, start the car, make sure all the accessories are off and increase the RPMs to a fast idle. Set the Voltmeter/Multimeter to the DC scale (not AC or Ohms). Measure the voltage across the battery terminals - red (+) lead of the voltmeter on the positive (+)terminal, black (-) on the negative (-) terminal. The voltage should, read between 13.0 - 14.2 volts. If it reads less than 12 volts you most probably have a failed Alternator.

If the voltage reads OK, (with car still running) turn on the the Stereo, the Headlights, the Rear Window Defroster, the Heater, and anything else that draws power. Increase the RPMs and watch the Voltmeter/Multimeter. It should still be reading around 14 - 14.2 volts. If it reads lower than 13 volts the chances are that your Alternator is not operating properly (or fully, as described above).

Finally, you want to check the Field Voltage. In order for the Alternator to generate electricity (because it lacks the permanent magnets of a Generator) it must first be supplied with a Field Voltage. If you know which wire is the one that supplies the Field (normally labeled 'F') then simply check with a Voltmeter/Multimeter to see if there is at least 12 volts at the Field. Another way to check this is to use a paper clip, small screwdriver, or anything ferrous (iron, steel). Hold it near the side of the Alternator with the ignition switch turned in the ON position, Engine OFF because you don't want to have your hand near the spinning Pulley/Belt. If there is a Field Voltage present then the metal will be attracted magnetically to the side of the Alternator, not very strongly, but you will feel the magnetic attraction.

One last thing to check is the Bearing. The rotor inside the Alternator rotates on Bearings, and these can fail. When they do you will hear a loud grinding noise associated with the Alternator. To isolate the noise take a length of tubing, Heater Hose will do fine, put one end to your ear and move the other around in the vicinity of the Alternator. The noise will be much louder when you point it at the Alternator if that is the culprit. To further confirm this, you may disconnect the Accessory Belt and spin the Alternator by hand. If you hear, or feel a rumble or grinding then the Bearings have failed. Even If you don't hear a noise it may still be the Alternator Bearings since the Bearing might be smooth without the tension of the Accessory Belt. Also, check for side play in the Pulley.

NOTE: NEVER operate the car without having the Battery connected. Without the Battery, or if either the positive or negative cables are not hooked up, voltage can rise to over 40 volts!. This will cause extreme damage to the car's very expensive electronics.

FYI, in an effort to increase their reliability, Alternators are becoming water-cooled. A 5/8" take-off connects to the Car's cooling system. Keeping them cooler will extend the life of the heat-sensitive Diodes. The 4.6L Cadillac NorthStar Engine currently uses a water-cooled Alternator. This is going to become more prevelant as Manufacturer's move to 36 Volt and 42 Volt Electrical Systems, because of the greater efficiency they provide (this is exactly why 12 Volt systems were adopted over 6 Volt systems). This switchover will occur before the 2010 model year. Hope this helps...

Happy Motoring!...Jim'99

Last edited by MNBoxster; 09-27-2005 at 03:30 PM.
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