Do I need a new alternator?

Question

I just recently replaced the battery because my car wouldn't start, but now I'm having the same problem again..right after i replaced the battery, the car was still not running well. it would shake whenever i came to a stop, but when i accelerated it wasn't as noticeable. It also made a sorta high pitched humming noise..but it would still start. it's been about a week since i replaced it, but it's now sitting in a parking lot. I called my dad, who is currently on vacation, to ask his opinion. i've already replaced the battery, and spark plugs. now he said i should take it to Auto Zone to have them test the alternator. Problem is, I need someone to jump start it and I probably won't be able to do that for another hour or so. When I do get it started, will they even be able to properly test it if I drive the car straight to them or will I need to charge the battery first? I would probably have to keep it running the whole time or else I'd need to jump it again.

I have a small analog volt meter..would I be able to test the alternator with that?

Answer 1

Depending on what vehicle this is (I'm really only familiar with the older GM stuff) you measure the voltage coming off the back of the alternator. Used to be a single wire. It should be above about 12.8V with ALL accessories running, to include the electric fan. (heater, AC, headlights, radio, you name it)



You can then compare the alternator output to the battery voltage. They should be the same, or very nearly the same, if the wiring/cables are good. Voltage should NEVER drop below about 12.8V while the vehicle is running. If the battery voltage is not the same as alternator voltage, the problem is somewhere in the electrical wiring.



Edit: Bearing has NO impact on alternator output. If it was so bad that it did, the alternator would come apart. Don't know about modern stuff, but bearings VERY rarely died in older vehicles...the diode trio would go well before that, and there would be low or no charge problems at that point. So use your voltmeter, and figure out where the voltage problem is. :)

Answer 2

That's a tough one. You can't charge a battery with a dead battery. Alternators need 12-volts to create the electromagnetic field in order to GENERATE power to charge the battery. So, you could slap a voltmeter on the battery terminals after a jump start, see less than 12V and decide it's a bad alternator. Or, you could charge the battery and then do a voltage test and find that it's putting out 13.5 or above. Biggest mistake DIYers make is to think they can charge a dead battery just by driving it or letting it idle.



But since you bought a battery and are having the same problem, you either have a bad alternator OR a computer module is staying awake and draining the battery. Read this post on how to check for parasitic battery drain.



http://www.ricksfreeautorepairadvice.com/diagnostics/checking-out-battery-drain

Answer 3

Stephen, your not going to get anyone like me on here to help you, all your going to get are enthusiasts, and wannabees.

So you want to check the alternator, there are two ways to do this, one is a trade trick, and you start the engine, you disconnect the neg side of the battery, if the car keeps running, the alternator is good, if it dies, it's bad, the other way is connect your volt meter to the battery red to red or + and black to black or -, set the voltage to DC and 20 volts, engine running produces 13.8-15 volts now turn the engine off, check the battery voltage, it should be 11-12.7 engine off.

Next with a volt meter hooked up to the battery, your going to do a battery load test, start the engine and watch the voltage drop when you start the engine, it must be above 10 volts to start the engine, if not the engine just dies or doesn't start.

Answer 4

MENTION YEAR< makee model MILEAGE country of ORIGIN and MILEAGE here! UNFORTUNATELY most CHARGING systems develop ALL THREE issues at the same time! It beigns whent eh BRUSHES inside the alterntor are WORN OUT! It takes DWON the battery adnd makes it OVER WORK itself! and VOLTAGE REGULATOR stops working as well as the BRUSHES wear out! ANY AUTO ZONE can test yoru CHARGING SYSTEM for FREE! I WOULD try to use ONLy the GENUINE FACTORY BATTERIES as most after market onnes are VERY CEHAPLY MADE and fail SOONER! POWER WASH your engine every few months to keeop LEAKING FLUIDS and OIL from getting into the ALTERNATOR and STARTER windings! if your car does NOT start with ONE QUICK Tap of the key SOMETHING IS WRONG!