alternator questions...

ive read so many different alternator troubleshooting guides that im about to puke. i dont think many take into account new cars that dont necessarily follow the old ways. my question is this, could a car
stop running in the middle of operation and it not be tied to a bad alternator? of course please assume that everything else (engine wise) is sound...spark, compression, ect... im mainly concerned with the options being either a bad battery or bad alternator. Arent these new cars designed to sense a bad battery and for the alternator to quit charging the battery? so it doesnt damage the alternator? and also, does the cars computer need some type of constant voltage/ amperage from the battery, and once this falls below the required limit, the car can die while running down the road? Its a 2005 Chevy Trailblazer. its a friends, and im trying to convince him that these symptoms dont always mean a bad alternator. but i dont know enough details. the belt prolly needs changing too, i always thought if you can turn the alternator pully by hand, then the belt is too loose, and the alternator might not be putting out the correct output. can anyone explode this and elaborate? thanks.
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Dogbyte wrote:

There are a LOT more items than a bad alternator that can cause a vehicle to shut down.
I don't know of any new vehicles that will stop charging into a bad battery. Could this cause an alternator to fail? Yes. However you will usually see an indication on the gauges or an idiot light if the alternator is failing. Yes, the computer needs a constant voltage to operate.
If he thinks it's the alternator, pull it off and take it and the battery in to an AutoZone or Advance type store and have it tested.
A 2005 TB has an automatic tensioner on it. Being 2 years old I doubt the belt is bad but it may be.
--
Steve W.
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----http://www.pronews.comoffers corporate packages that have access to 100,000+ newsgroups oops sorry its a 2004. bout 90k miles. but if the alternator couldnt keep the battery charged (due to a failing battery), and the required voltage/amperage from the battery fell below the minimum the computer needed, would that make the car stop while driving?
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If the available voltage from the battery drops below a minimum level, then the PCM or ECM will stop functioning. You may go through a period of rough running, but it will eventually stop cold.
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----http://www.pronews.comoffers corporate packages that have access to 100,000+ newsgroups

Maybe, But not real likely unless the bad battery destroyed the alternator. The battery starts out at about 13 volts. You start the engine and the action of the starter motor and the electronics in the vehicle will drop the voltage some. The regulator sees the voltage drop and raises the control voltage to the field in the alternator to bring the voltage in the system back up. This also charges the battery back up. Now if you have a bad battery the alternator will still try to charge it until it is either charged, blown apart or the overload burns out the alternator or wiring. While the engine is running the alternator will keep putting out voltage and the engine will keep running. That alternator can put out well over 100 amps, that is FAR more than the computer requires, Even a dead battery won't stop it, unless the alternator fries.
How has he been starting the vehicle? Jumping the dead battery?
Or does it start up and run OK until he starts down the road and dies? The bad battery/alternator happens once. Then you replace the battery/alternator and your ready to go.
When it dies, how does it shut off, like you turned off the switch? or does it start running real crappy, idiot lights come on and shuts down?
The other items that can cause this problem are a plugged fuel filter, excessive water in the fuel tank, faulty fuel pump, intermittent fuel pump connection, for a start.
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Steve W.
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I agree, lots of things can cause the engine to die. A low voltage to the computer can and will do it, but there are many more possibilities.
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Yeah - but you're stacking up your if's here. Just stick a volt meter across the battery and see if it is putting out the required voltage. From what you originally posted, and what you're posting here, it seems you are spending a lot of time hypothesizing this and no time troubleshooting it.
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-Mike-
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