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Answered 2005-08-02 03:59:45

Most alternators need a voltage present to operate. This is because the voltage is used to create the magnetic field used to generate power. If you are directly replacing another alternator, the wiring should already be correct for it. If you think that your alternator is not receiving power (for its field windings) then you should check for the presence of the voltage with a meter when the ignition switch is on. The proper place to check will depend on the vehicle, and some newer alternators will only apply the voltage when a start signal is recognized. This would make troubleshooting much more difficult.


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Voltage flows from the alternator not to it. If it is outputting no voltage it is defective.

To regulate the voltage flow going to the battery.

the regulator takes the power produced from the alternator and converts it to the correct voltage before going into the battery.

Theory of an alternator. To control the voltage the alternator turns of and on very very fast, this is called pulse width modulation. Wiki it for more background. This means that the voltage spikes very very high for an instant and drops to almost zero before it turn on again. To keep working, and it will try, it burns itself out by going to to high a voltage to often.

ceiling voltage is the maximum field voltage that the exciter can withstand before going into unstability

You need to check and make sure the alternator is charging. You could have gotten a defective one or if you got a used one it may be bad. Get a digital multimeter and with the vehicle off check the voltage at the battery. Should be 12+ volts with a charged battery. Start the vehicle and check the voltage again, should be 13-14 volts or better. If no voltage increase check the big output terminal (thick red wire usually going to it) at the alternator and see it the alternator is putting out voltage. If no voltage other than battery voltage then the alternator is not working. This is assuming that the alternator has an internal voltage regulator (most now do). Without year/make/model/engine size it's very hard to give you decent direction other that generic help :-) **I am assuming you have tested the battery**

An alternator will ground out when the normally insulated line that charges the battery( after going through the regulator system) makes contact with the alternator shell. This sends any voltage to the surrounding case and to any surrounding metal thusly grounding the voltage out. This can occur due to vibration or excessive moisture to the casing.

If an alternator light keeps coming off and on, this may indicate that the voltage output of the vehicle is going up and down intermittently. This may mean the alternator may need to be replaced soon.

To get to an internal voltage regulator first disconnect the battery ground ( negative cable ) then remove the alternator. Make sure you mark the cables on the alternator so you can put them back exactly where they are supposed to go. The voltage regulator is inside the alternator and you will have to take it apart. I'm not sure how the regulator is mounted in your alternator, it might have small studs with nuts on them, or small screws or bolts, or it may be soldered in. In the latter case you will have to cut out the voltage regulator and solder in the new one. If you are going to take out the alternator anyway you should probably take it to a parts store where they will test it for free and tell you if you need a whole new alternator or just the voltage regulator.

sounds like you have dirty field points on your alternator or your voltage regulator is going bad. Replace your voltage regulator in your car first, they usually only cost like $20-50. If it doesn't fix the problem take it back. Then have your alternator checked out by a store that only does alternators... This is provided you haven't changed any idle pulley sizes, if you have your under the RPM's required for that alternator.

I'm sorry to say but if you think your voltage regulator is bad you are probably going to have to replace or rebuild your entire alternator. The voltage regulator is usually located inside the alternator and normally cannot be purchased regularly. I strongly recommend just replacing your alternator rather then rebuilding it because rebuilds don't' always have all new parts like new or factory rebuilt alts. from orion_1120@YAHOO.COM

If the only way to start it is to jump start it then the alternator is the most probable cause. While its running check that the battery voltage is 14 volts dc or more. If it is 12 volts or below then the alternator is either bad or the wiring to it is bad. The main wire (the large one that is attached seperately to the body of the alternator) should have battery voltage to it at all times.

sounds like your alternator or voltage regulator is going out most auto zones can test them for free

Yes it is. I am having trouble with alternators on my 95 Aspire. They keep going out and I cannot find the problem.

the voltage regulator in the alternator is going bad it can possibly stick when everything is turn off and drain you battery over night.

It means that the voltage is going up as the alternator spins faster. 4 posibilities; 1 Your battery is dying and the alternator is working harder to charge it when you accelerate. The lights are comming up to full brightness. 2 You have a bad connection at the battery terminals or frame ground and the battery can't hold the voltage down. The lights are receiving too much voltage and are more likely to fail. 3 The voltage regulator on the alternator is failing and overcharging the battery. The lights are receiving too much voltage and are more likely to fail. 4 The battery needs water. The lights are receiving too much voltage and are more likely to fail.

Ah, you have made one of the most common and easiest mistakes to make in automotive maintenance. Because the 'battery voltage' idiot light came on, you assumed that the battery was bad. Did you have an automotive electrical specialist test the electrical system on your car? Did you have the battery tested at a battery shop? Did you know that there are 3 primary parts to an automotive electrical system, and anyone of them can be responsible for the system operating below required voltage levels? Most modern automobiles generate electricity with a device called an alternator, which is driven by a belt from the crankshaft. The alternator generates Direct Current, at voltages up to 20 volts, in some cases. To keep the alternator from frying the battery, a Voltage Regulator is used to control the output of the alternator. This insures that the battery is only charged with a current of 18 volts, or less. Most modern alterators have the voltage regulator inside, but on older cars, the voltage regulator is mounted on the fender or the firewall, somewhere in the engine compartment. So, the alternator generates the electricity used to charge the battery, but the voltage regulator controls the output of the alternator. If the alternator is going bad, it may not be generating enough voltage to charge the battery. The battery is usually a 12 volt battery, and must see more than 12 volts to charge. Or, the alternator output might be reduced to an unusable level by the voltage regulator. Before you replace the entire electrical system, take the car to an automotive electrical specialist for testing. They can determine exactly which component is not working, and repair it. (Note: If the voltage regulator is bad, this DOES NOT mean that the entire alternator must be replaced! In almost all cases, the voltage regulator can be replaced by disassembling the alternator. If the specialist insists that the alternator must be replaced, I would recommend getting a second opinion, even though testing is often not free. Voltage regulators are fairly cheap, and can be replaced by a competent technician without too much difficulty. Alternators are expensive.)

* Voltage Regulator * Defective Battery * Dirty Battery Post Connections * Alternator/Generator problems

Battery fails to charge, alt light comes on, sometimes only at low revs. Check with meter shows low or no charging voltage (close to or less than battery voltage) voltage does not increase to charging volts with engine revs increase

i have a 1993 Toyota Camry and I'm having a problem with the alternator whenever i start the engine the voltage is at 13.5 volts a few minutes later voltage drops between 11.5 to 12.7 volts,i had the same problem 6 months ago with the same car and when i was going to replace the alternator the belt turned out to be have cracks on it,so instead of replacing the alternator i replaced the belt and that fixed the problem,but i don't think is the belt this time any good answer that could help?

Check pigtail connection at alternator (possible loose), or voltage regulator going out in alternator, take to autozone or other and have tested, replace.

If your battery is always going dead a day or two after a boost, then it most likely the alternator. Two surefire ways to see if the alternator is bad is to take the positive (red) battery cable off after starting the car. If the car still runs, alternator is good. If the car stops running, then it is likely the alternator. The battery is just to turn the car on and then the alternator regulates the initial charge coming from the battery and that powers the car. Another thing you can do is to run a voltage check at either the battery or on the alternator with a Volt Ohm Meter (VOM). If the voltage comes back as 13.5V or better, than the alternator is good. Less than 13.5V, and it is likely the alternator. Be advised, though that what could look like an alternator problem could also be your ignition switch or starter.

Sounds like the Diode or the Voltage Regulator is going out on the alternator. I would recomend replacing soon. When one of those 2 componets go out, it really over heats the alternator. This causes the bearings on the back side of the alternator to fail, and sieze. This has cause some fires in GM cars and trucks.

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