Now we all know why it is called an IDIOT LIGHT!

[shogun]

The most important:

"

Before we get started diagnosing alternator problems there is one thing I must mention. Alternators use an "exciter" voltage to get the alternator working when you start your car. Now get this! About 90% of the cars made today run that 12 volts through the "battery" or "alternator" bulb (AKA the idiot light). So you need to check to see if this bulb is not burned out. It should light when the key is turned on! If it doesn't then there is a very good chance that the alternator will not put out!! Replace the bulb before beginning the rest of the diagnosis."

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If you have been around cars for a while you might have heard the term generator. Well, those were the old days and the good old generator is history. What a generator did for the old cars, an alternator now does. You see, a car has and electrical system that carries power to such essential things as headlights, the ignition coils, engine cooling fans and other non-essential things as the radio (my son would argue that one), air conditioning fans (my wife would argue that one), and all of those other things upon which we have become accustomed to depend. All of that power has to come from somewhere!! A lot of people might think that power comes from the battery, and that is true to some extent. But the real answer is that the power to run all of those electrical things comes from Saudi Arabia! Huh? Saudi Arabia? Well, maybe Dallas, or Oklahoma. But the point is that the source for all of that energy is the gas tank. Yep. And the link from the gas tank to the battery is that mysterious thing called the alternator. It takes mechanical power from the crankshaft, transmits it via a "fan" belt, (it used to run the cooling fan as well) or serpentine belt as it is called in most of the newer vintage cars, and turns the alternator. So, the main function of the alternator is to convert power from the gasoline engine that drives you along the road, to electrical energy to keep the battery in tip-top condition.

So, what happens when an alternator goes bad? Well, at first, nothing. That is because the battery has some reserve power in it, enough to keep the engine running for quite some time, many many miles in fact. So a bad alternator doesn't necessarily mean a tow truck should be called right away. As long as energy is conserved elsewhere, like turning off the blower motor, the rear window de-fogger, the stereo and the headlights (if possible), you could make it for some distance on just the battery reserve alone.

One major problem which will finally occur as the battery loses its charge is that there will not be sufficient voltage to keep the engine running well. Many years ago I was in California and saw a car coming down the street with its catalytic converter glowing white hot and flames coming from beneath the car. What had happened is the alternator quit, the battery ran down, the engine was not firing on all eight cylinders and the unburned fuel was being burned in the catalytic converter! It had been long overdue for the driver to call a tow truck!

Before we get started diagnosing alternator problems there is one thing I must mention. Alternators use an "exciter" voltage to get the alternator working when you start your car. Now get this! About 90% of the cars made today run that 12 volts through the "battery" or "alternator" bulb (AKA the idiot light). So you need to check to see if this bulb is not burned out. It should light when the key is turned on! If it doesn't then there is a very good chance that the alternator will not put out!! Replace the bulb before beginning the rest of the diagnosis.

So, how do you know when your alternator is going bad? Most of the time the alternator fails in stages. A little techie talk here. The alternator gets its name from the fact that it generates alternating current (AC). The old generators I mentioned before generated direct current (DC). Well the battery can't use alternating current so the alternator output is fed into what are called diodes, which convert the AC into DC. The alternator has a unique feature in that it is able to generate a relatively high voltage while the engine is at idle. The old generators needed to be running at a fast pace before they got up to 13 or 14 volts. The alternator can do this since it is really three alternators in one body. Each of the three sections of the alternator generates its voltage out of phase with the other two sections. Since the complete cycle (one revolution) of the alternator is 360 degrees, each phase is shifted by 120 degrees from the next phase. So in one revolution of the alternator it puts out three separate voltages.

OK, back to the failure mode. 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, 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 that means to you is that you can go a long time on a limping alternator. Chances are if you don't need headlights or air conditioning or other high current using accessories, you would never know that the alternator was in the process of failing! The time you will find out is when it is 10 below zero and you wear down the battery by cranking the starter, then put the fan on high for heat, and then drive in the dark.

So, how can you tell if the alternator is failing without taking it apart and doing some measuring inside the alternator? It's really pretty simple. You will need a simple voltmeter. You can get one at Radio Shack for under ten dollars. Here's what you do - start the car, make sure all the accessories are off and rev up the motor to a fast idle. Set the Voltmeter 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 (ground in most cars). The voltage should, and probably will, read around 14 volts. If it reads less than 12 volts you may indeed have a failed alternator and you can skip the next step. Next, turn on the heater, the rear window de-fogger, the radio, the headlights and anything else that draws power. Now rev up the motor and watch the voltmeter. It should still be reading around 14 volts. If it reads lower than 13 volts the chances are that your alternator is not up to snuff.

One last failure mode is of course noise. The rotor inside the alternator rotates on bearings, normally very high precision needle 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. Other possibilities are the water pump and the power steering pump which are also driven by the engine belt. To further isolate the noise disconnect the drive belt and spin the alternator by hand. If you hear a rumble or grinding noise then the bearings are shot. If you don't hear a noise the problem may still be in the alternator since the bearing might be quiet without the loading of the drive belt tension. Check for side play in the pulley. If you are pretty certain the noise came from the alternator it is a relatively simple task to take it apart and visually inspect the bearings, else swap it in for a rebuilt. Your auto supply store will normally bench test the alternator free of charge and can tell you at that time if the bearings are noisy.

Before you go running down to the parts store for a new alternator make sure to check the connections at the battery terminals and also check to see that the voltage is the same at the alternator terminal (the big fat one with the heavy wire attached) {also, read the article, dead battery}. Check to make sure the belts are tight and not slipping. Replace them if they are cracked or shiny on the side that faces the alternator pulley.

One final thing to check - the field voltage. In order for the alternator to generate electricity it must 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 to see if there is 12 volts at the field. Another check is to use a hacksaw blade or a lightweight screwdriver , anything magnetic, and hold it near the side of the alternator with the ignition switch turned in the on position. 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 it pull the metal to the side of the alternator.

So, what are you going to ask the mechanic when he tells you that you need a new alternator?

1. Did you perform a load test on the alternator? If you did, what were the voltage readings? Were they all below specification?? (mechanics will use a load testing machine instead of turning on all the accessories.)

2. Did you check to see if the belts were old and cracked or possibly slipping?

3. Did you measure the voltage at the alternator connector or at the battery? Were the readings the same at both places or is there a voltage drop somewhere in the system. You can tell him the "Dead Battery" story if you want to.

4. Finally, did you check the price on a rebuilt as well as a new alternator? (rebuilt alternators are just as good as new if they are done correctly and usually cost about 1/3 as much)

Now that you know all about alternators you can feel confident that you can discuss the failure modes with a mechanic and not get shafted. It is also fun to watch the faces of a mechanic when you ask questions like those above. He will soon figure out that you know more about the electrical system of your car than how to turn the lights on!

For yet another description and a different perspective on the charging system go here.

source: http://www.misterfixit.com/alterntr.htm

What does the battery light on the dashboard mean?

Every car today contains a rechargeable 12-volt battery. This battery powers everything electrical: the engine's control computer, the ignition system, the radio, the headlights, etc. So the battery is vital to the car's operation.

Like all batteries, a car's 12-volt battery would eventually go dead if it were not recharged, so your car has a built-in recharging system. Most cars have an alternator, along with a voltage regulator, that keeps the battery charged and also provides electricity to the vehicle when the engine is running. A typical alternator can put out between 500 and 1,000 watts when necessary.

Because your car is so dependent on the battery, all cars have a battery light on the dashboard that is designed to warn you if the recharging system fails. A simple circuit looks at the voltage that the alternator is producing, and turns the battery light on if it is low. The battery light indicates a battery charging problem. If the battery light comes on and stays on while you are driving, the most common cause is a broken alternator belt. Total failure of the alternator is another possibility.

The reason why your car can operate normally even though the battery light is on is because your car can run off the energy stored in the battery. Your car will run fine until the battery goes dead. At that point, nothing in your car will work. When the battery light is on, you can still drive your car to the garage -- you will not damage anything. But you want to get to the garage before the battery fails and your car dies.

These links will help you learn more:

How Batteries Work
How Automobile Ignition Systems Work
How Electric Cars Work
How Hybrid Cars Work
Why do batteries seem to go dead and then come back to life if you let them rest?
What is the difference between a normal lead-acid car battery and a "deep cycle" battery?
If daytime running lights were mandatory in the U.S., and all vehicles had them, how much extra gasoline would that use each year?

MisterFixIt.com: Alternators - what are they, how do they work and what breaks?
MisterFixIt.com: Now we all know why it is called an IDIOT LIGHT!
FamilyCar.com: Understanding your dashboard gauges

http://auto.howstuffworks.com/question324.htm

Now we all know why it is called an IDIOT LIGHT!
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I came across something that all GM owners should know about. One of my neighbors came over to seek my advice on a baffling problem he was having with his GM (I think it was a Chev) and its charging system. He was having dead battery problems (sort of like Don was in another story) and had taken his alternator off to replace it. Fortunately he had taken it to "Wheels" the former National Auto store, and they had tested it on an alternator testing machine. There was nothing wrong with it. This neighbor, I'll call him Artie, had the foresight to do some diagnostics ahead of time and determined that there was no output (14.5 volts) at battery or at the alternator stud, so a logical conclusion was that the problem was in the alternator - it has a built in voltage regulator so logic prevailed. Replace the alternator and solve the dead battery problem, right? Nope.

I walked over to see what I could see. We had the service manual open to the circuit describing the charging system. I traced the wires and determined which one was the field and which was the bat and which one was ground. Then I noticed something strange. I thought it was a typo - the wire from the 12 volt source to the field winding went through the idiot light filament, that's right, the bulb is in series with the field supply. Alternators don't have a true field supply like the old generators had with external voltage regulators which regulated the output by switching the field winding on and off. But they do need something to start the system working. Once the system is putting out its current the system becomes sort of self sustaining.

I took a short length of wire and connected it to the field terminal while the engine was running. Suddenly, the voltmeter across the battery went to 14.5 volts! I took the wire off the terminal and sure enough the system continued to charge at 14.5 volts. Fixed, right?? Nope.

Artie shut the engine down and restarted it again - nothing - jump the field, 14.5 volts.

I asked Artie if the idiot light was on while it was running and not putting anything out. Nope. He shut it down again and we tried it all over again, only this time I sat in the driver's seat and Artie did the jumper thing. Same results, only I did notice something strange. "Just where is the "alt" idiot light on the dash panel Artie? I didn't see one - just an engine light and an oil pressure light."

There was no idiot light for the charging system! I took a flashlight and held it at an angle to the dash panel. There, not visible under normal lighting conditions, was a place for the "bat" light. Yet it did NOT light when the switch was turned to the "ON" position. Then it hit me like a ton of poop! Remember I thought that the circuit diagram was in error - that the idiot light was shown to be in series with the field terminal on the alternator?? Well, that was not a typo! The bulb in the dash panel is part of the charging circuit. It is REQUIRED to be functional to supply the initial field voltage to get the alternator started doing its thing. IF THE BULB IS BURNED OUT THE SYSTEM WILL NOT CHARGE!!

It took us another half an hour to get to where the bulb should be but it became obvious that it was going to take a LOT more than a half hour to replace the bulb. It looked like part of the instrument cluster was going to come out before we got to the bulb. Plan "B" went into effect. I found the wire under the hood that supplies the idiot light and wired in a small socket and a bulb under the hood. I connected the other end of the wire to the field terminal and started it up. Voila! Charged just like new. I taped the bulb and socket to a handy vacuum hose and closed the hood. To this day that GM product is still riding around with a bulb taped to the vacuum line under the hood. I would love to see the face on the mechanic who discovers that bulb and wonders what the heck it is doing there!!

http://auto.howstuffworks.com/framed...m/idotlite.htm

[genphreak]

Great story and explanaton. Best I've read- thanks Shogun.

I was once baffled by an alternator in an e30 325i. It was not charging (making good voltage) at idle. The battery was not healthy and the alt light was coming on dimly and going out at higher revs. Loading the cars circuits made the light come on stronger. It turned out to be a diode pack, one or two of the three. Once new diodes were soldered in, it was cleaned, re-greased and reinstalled, she was back on the road again.

[tnsguy]

Great write-up Shogun. Thanks.

My daughter's '93 318i was once consistently draining the battery, but voltages all measured ok. Turned out that the ground strap from the alternator to the chassis was so corroded that it wouldn't allow enough charging current to flow. That one took a while to figure out!

[shogun]

as today I just repaired an E34 with the same problem, up with this thread that people know how important the idiot light is.

[whiskychaser]

Great article though I am not sure I like my youth being referred to as 'the old days' :-)

I read recently that 90% of new alternators fail because they have been hooked up to a flat battery. Best trickle charge it first.

[Katmandu]

A co-worker's son drove his 99 328i with a BIG RED light ON on his dash for over 3hrs. Dumbazz eventually overheated the engine and blew out the head gasket. His parents were co-dependent enough to go out and spend $5000 on repairing it for him!

If it was my kid, he'd still be walking ! LOL!