i have a problem with an 2005 Audi a8, it has an dead battery and when i start the car with a jumper cable it runs but when i take the jumper cable off the car shuts down after couple seconds or minute, thought it was a bad alternator but it wasn't, any of you guys have an idea of what could cause this problem.

 

Something bad in the electrical system. The battery is just there to start the car. The car should run after it is started, even if you remove the battery. If the car dies after it is started, and the alternator is not the problem, I suspect you have some kind if wiring issue.

 

Start with replacing your battery. What you said might be true with the D2 platform, but the D3 tests the battery before applying a charge to it from the alt. If your battery has a shorted or open cell the alt might not charge it at all. Therefore it will not run if the battery is complete crap.

 

i replaced the battery and when i sarted it died in couple secounds also the rpms were like realy low and it keep doing sow until it wormed up a little bit, but still it dosen't recharch the battery, it runs on the battery.

the car had a hol in the front window on pasenger side sow that side was flodded. also hole rear windsheld was broken sow that side was also flooded
please help

 

Torsion'sA8

Not really

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"Start with replacing your battery. What you said might be true with the D2 platform, but the D3 tests the battery before applying a charge to it from the alt. If your battery has a shorted or open cell the alt might not charge it at all. Therefore it will not run if the battery is complete crap. "

can you explain more about it i kinda don't get it

 

Could be the battery and/or alternator. Have the battery charged with a battery charger and load tested. Then have the alternator output tested. Check all your battery cable connections. Lots of battery shops and auto parts stores will do these tests on the car at a low cost or no charge.
Hope this will help you!!

 

Well he already got a new battery. Do you have a voltmeter? With the engine off your battery should have about 12 volts across it, then with engine running it should show about 14 volts at the terminals if the alternator is working right, if not then it has to be the alternator or something in between. How do you know that the problem is not the alternator?

 

had the alternator changed and it still did the same thing

 

Well it sounds like there must be something draining the system somewhere. Does the battery drain while the car is sitting doing nothing? Or just while running? The more you can find out about it the more you can save getting it fixed at a shop, or maybe figure it out yourself. I guess I would start investigating the flooded areas, maybe even pull all the fuses for non-critical systems and see what happens, but I'm not the forum expert, others might have better ideas.

 

on a few other threads about starting, battery & alternator issues, I have read that the long run of the battery cable from the back to the front can short due to liquids..etc...

On the D2, there is a junction box on the driver's side...along the frame area...about the back side of the engine that has caused many of starting issues.

For more info...I would do a forum(s) search...I'm sure you would get a few additional ideas.