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Discussion Starter #1
I just had an alternator rebuilt at a professional facility.They stated that it was good and gave me a 1 year warranty on the part. As a precaution I took it to my local parts store to have it tested it on their machine and it passed with flying colors. They tested it three times. I bolt it in my car, and while driving the radio slowly starts to fade away. I stop the car and decided to take a rather unrecommended test. With the car running, I disconnected the power cable from the battery, and the car dies.

Usually when this happens, its a clear sign that the alternator is bad because thats whats supplying power to the car whle its running. The alternator was tested good, but the car still wants to die after being driven for a while.


What do you think could be causing this? :confused:
 

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Are you 100% sure the wiring is correct and the terminals clean ?

I take it this is why you had it rebuilt in the first place, same thing happened before ?
 

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Discussion Starter #3
Are you 100% sure the wiring is correct and the terminals clean ?

I take it this is why you had it rebuilt in the first place, same thing happened before ?
Yes.. I checked and made double sure! Also,the last time I had this problem, It was on an 1985 Plymouth Reilant which had an external voltage regulator. The voltage regulator itself was bad. This alternator I just installed has a built in regulator, and it was proven to be good!
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to do that test you supposed to disconnect negative terminal not positive. but best would be with a volt meter on battery.
I'll go back outside first thing in the morning and try it again with the negative off.

Either way though, the battery is only there to start the car. Once started, the alternator is supposed to keep it running correct?
 

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To re-charge and supply needed electric :yes:

You said the Alt. has internal regulator ? Did you remove the external reg ?
 

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Do not disconnect the battery with the car running to test the alternator! This causes the alternator to over work and may damage it. Find an automotive repair shop that that has a load tester that can check your battery and charging system in the car running.
 

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Well guys, I kept my fingers crossed and took it back to the guys that rebuilt it for me. They tested it and proved that it was still good. I was at a total loss trying to figure out why this car was running off the battery when the alternator was proven to be good. Come to find out that there is a special 80 AMP alternator fuse in the engine compartment that was blown. I replaced it and put everything back together,and now she FINALLY runs fine.

I took a voltage reading with the engine running and cables securely attached to the battery and got 13.5 volts!

Thanks guys :thumbsup:
 

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Do not disconnect the battery with the car running to test the alternator! This causes the alternator to over work and may damage it. Find an automotive repair shop that that has a load tester that can check your battery and charging system in the car running.
Thank you chuck. DO NOT TEST ALT by disconnecting battery while car is running. Not much good can come of it , but many bad things can. As shown in this thread the only thing learned from disconnecting the bat cable while running was either the alt was bad or there was another problem. I think we already knew this. You can cause a spark at the battery resulting in an explosion. I have seen it happen and it isn't funny unless it happens to someone you don't like. It was a good thing there was another problem, because if the alt was charging there would have been a voltage surge in the system. If the alt is working you can damage the alt. You can also damage other electronic components, like computers radios and more. Sort of like lightning hitting your house blowing out the TV radio microwave and phone. The battery acts like a big resistor restricting the flow of voltage from the alt. By removing that resistor you will have a voltage surge. Please test electrical systems with the proper techniques and equipment or have someone else do it. You and your car are worth it.:thumbsup:
 

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Sounds like a bad ground between the alternator shell and the engine to me.
 
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