Diagnosing and rebuilding alternators.

How it works: The part that rotates is called the field, or rotor. The stationary part is called the armature, or stator. (This is the reverse of the little DC motors we had in electric train sets when we were young, and is the source of endless confusion.)

The field makes magnetism - it is just a big electromagnet. Since it rotates (driven by a pulley), we use brushes and slip rings to get power to the field. This power is supplied by the voltage regulator.

Remember that moving a magnet past a wire induces a current in the wire. (Works the other way, too: You can move the wire past the magnet and get the same result.) The faster you move the magnet, the higher the current in the wire. Also, the stronger the magnet, the more current you get. This is why we need a regulator: At 5,500 RPM the alternator would be putting out 1,000 volts and promptly fry our expensive eurolights and trick stereos. (And climate controls and window motors and trip computers . . . )

(And yes, I am using the words "current" and "voltage" interchangeably here - this is not correct, but will work for the purposes of this discussion.)

The regulator functions by sensing the output voltage of the alternator - now listen carefully - and *reducing* the power to the field (less power equals less magnetism equals less alternator output) as required to keep the alternator output DOWN to the required 13.2 to 14 (approximately) volts.

The armature is the part that makes the electricity. Armatures are nothing more than three coils of wire. They are connected together in two ways: in a "wye" or in a "delta". The wye looks like a letter "Y", and the delta looks like a triangle. This is only important when we start troubleshooting - stay tuned.

The output of the armature is alternating current, but we require direct current to charge the battery and run all the goodies. Alternating current is converted to direct current by a rectifier, which is simply an array of six diodes.

There are no more electrical parts (except sometimes a noise supressor which is nothing more than a capacitor). The only mechanical wear-item parts besides the brushes are a pair of ball bearings. These are usually sealed and require no lubrication. They are cheap, and should be replaced whenever you have the alternator open. (There are some mechanical differences - some alternators have an unsealed needle bearing at the brush end - same difference.) A bad ball bearing in the alternator will whine - or grind, if it is very bad.

Before you take an alternator apart for diagnosis, mark both case halves and stator laminations for alignment. These can be reassembled rotated relative to each other, and while it will go back together, none of the brackets will line up and you won't be able to bolt it back on your car. Also, keep track of all the little bits and pieces of insulating washers and spacers - you NEED to put them back where they came from!

Electrical diagnosis: Ohmmeter or continuity tester required, alternator disassembled.

Field: continuity between both slip rings and no continuity to the shaft.

Armature: continuity between all three "ends" or output wires, no continuity to the frame or laminations. Now here's a gotcha - on the wye wound armature, an open coil will be obvious - one of the tests reads open - infinity - test light doesn't light. On the delta wound armature, an open coil is harder to detect because instead of going through the open coil, it goes around it though the OTHER two coils and will check OK with a test light even though it isn't OK. You need an ohmmeter - if the delta wound armature is OK, you'll get readings of (say) two ohms, two ohms, two ohms between the wire pairs. If one coil IS open, your readings will be two ohms, two ohms, four ohms. Bingo.

Rectifiers: You'll find five connections: three for the armature output, a ground and a hot. There are six diodes in the rectifier array. Each one must measure continuity one way, and when you reverse the wires to your tester, no continuity. Start anywhere you want - I suggest with the ground wire only because you have to start somewhere - connect your meter to the ground wire and any one of the three armature connections on the rectifier array. You will read either continuity or no continuity. Now reverse the wires to your meter - you MUST read the OPPOSITE of what you just read. If you read continuity before, now you must see no continuity. Any one of the six diodes that reads the same both ways is bad. Continuity - continuity means the diode is shorted, no continuity - no continuity means it is open. Replace as needed.

Brushes: Check them for free movement in the carrier and make sure they are long enough (they wear down).

There is a lot to know, I guess about troubleshooting the automobile charging system. The first thing to do is take some basic measurements. Be careful - the battery can deliver a huge current if you short it out, enough to burn you badly or kill you, or weld tools in place until they vaporise! The battery is directly connected to the big alternator stud so go easy down there too.

Before you start, I will assume your battery is a bit low. You must charge it properly before doing these tests, by connecting it to a trickle charger only at least overnight. This is best done out of doors or at least in a well ventilated space.

Attach the "-" wire of your voltmeter securely to a ground other than the battery terminal. Prepare to take notes. Put the meter on a scale that will read up to 20 volts or more, and measure the voltage at the positive battery terminal and the big alternator output bolt (it has the fat red wires going to it). Turn the key "on" but leave the engine off and measure these again, and also the voltage at the small alternator connection (this wire is usually blue on Audis). Do the same with a bunch of electrical draws turned on - high beams and high fan should do it. Turn them off and note the voltages again. Now start the engine, and make these three measurements with it running - being careful not to get your clothing or test wires or finger caught in any spinning parts! You can even measure the three voltages with the engine running and electrical loads on the system, but chances are wihtout revving up the engine a bit the voltage will drop because at idle the alternator doesn't have enough output to keep up anyway.

Now with your meter set to resistance, on as low a scale as possible, very carefully measure the resistance between these various ground points (make sure to get a good clean reading): battery negative terminal, alternator housing, engine block, and the car body. You can expect readings in the range of 0.1 or so ohms, or less if you are lucky, any that are much higher than that indicate poor connections which must be disassembled, cleaned, and protected from corrosion and dirt.

These numbers contain most of the information needed to troubleshoot your charging system. If you are going to ask someone to help diagnose your troubles, at least have this data ready for them.

With the key off the battery and alternator should both read the same and this reading should be over 12 volts - the battery is a 12.6 volt device. Anything lower indicates a poor charge. Unequal readings indicate poor ground connections, which you will have identified with your resistance measurements.

With the key on, engine off, the voltage may be a tiny bit lower due to vehicle circuits being on. The small alternator wire should read this same voltage, although it will almost certainly be a bit lower due to the resistance of the wire and the load it is feeding (the alternator rotor). If the small wire is not reading a voltage close to the other two (say within .2-.3 volts) there is a problem with the circuit supplying this wire - it basically runs off the "X" or Load Reduction Relay in your car. When the electrical accessories are turned on, the voltages will certainly drop, but it should stay above 12 or at least in the high 11's. If it doesn't, your battery may be in need of replacement. The best way to test it is with a device used by most auto repair shops, which puts a very high fixed load on the battery while monitoring its output voltage. They will gladly sell you a new battery.

With the engine running, the three voltages should all be higher than 12.6 volts. This indicates a charging condition. If the voltage does not rise, the alternator is not charging for some reason. Clean any poor grounds previously found and start again. PS the volt meters that many cars are equipped with in the passenger compartment are nice to have, but not adequate for these measurements, due to their lack of accuracy and the amount of wiring between them and components we are testing.

If the trouble appears to be your alternator, yank the regulator with the alternator in place (it's on the back, held in by two bolts) and check the brushes. If they're worn down to less than 5mm or so, replace either them or the whole unit, making sure to clean up the contacts nicely. The ground is at one of the bolts, and the positive is a tab on the regulator which presses against a spring contact in the alternator.

If you would like to test the functioning of the voltage regulator, you will need a variable power supply that can vary from 0 up to about 15 volts at about 3-5 amps, and a voltmeter. Attach "-" to the connection under one mounting bolt hole and "+" to the tab on the other side of the brushes. Set the voltage at zero and turn on the power supply. Hold a parking light bulb so it is being energised by the two brushes. Start to raise the voltage - the light will come on faintly and slowly get brighter. As it reaches around 14 volts, the light should suddenly go out. Lowering it back to around this voltage should cause the light to come back on again. Now drop the light bulb, as it has become quite hot! Please don't hurt yourself or your tools - you should only be doing this if you have experience working safely with electricity.

You should also check and clean all the connections again -

battery ground strap (at block and frame as well)
starter stud (hot)
alternator output stud (hot)
alternator grounding - usually this is the mounting bracket and hardware
alternator signal wire stud
alternator signal wire at fuse box (if you can find it)

These should be clean, free of corrosion, dirt and fraying, and tightly attached. The first four can be checked with an ohm meter for quality without disconnecting anything. The fifth and sixth should have battery voltage present when the ignition is on and nothing when it is off. This wire carries about 3-4 amps when running, to make sure it is capable of it disconnect it from the alternator, turn the key on, and run a parking light bulb with it - the bulb should be bright!

Cleaning connections starts with the obvious - disconnect them, remove grease and dirt, and carefully file, sand, or emery paper them til they are shiny. Then, you should smear a bit of silicone dielectric gel (a tube is about $10) on both surfaces before reassembling them. I like to shoot the exposed studs and such with a quick dose of battery corrosion preventing spray paint after this.

Amateur rebuild Voltage regulators Alternator primer Charging system