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Roman, Technician
Category: Audi
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Experience:  over 20 years vw audi and porsche technical repair
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Audi A6: Audi A6 2001 2.4 Quattro, after standing still for

Resolved Question:

Audi A6 2001 2.4 Quattro, after standing still for a while, it seems I have problems charging it. The battery runs low when I run the car, and when the voltage gets lower (like at 11.5V), the battery symbol starts to flash. Do I need a new alternator?


I should add: The car worked perfectly when I put it in the garage in April, no problems. It has not been on the road since. I just started it a couple of times to keep the battery alive, but eventually the battery ran flat anyway (maybe because it was not actually charging when the car was idling).

Submitted: 2 years ago.
Category: Audi
Expert:  Roman replied 2 years ago.
Hello ,
Here to answer automotive questions and make you feel comfortable and become familiar with your product.

1st thing to do is charge the battery, which you did.

Then if you can load test the battery you would need to check.

And finally test the alternator.

It does sound like the alternator is failing, however you will also need to check the wire connections and the cables to and from the alt. to the batt. to ensure the problem does not stem from there.

Finally after all of that you will determine if the alternator is failed.

let me know what you find and if you have any questions?
Expert:  Roman replied 2 years ago.
how is it going? have you made any progress?
Customer: replied 2 years ago.

Hi Roman,


Thanks for your answer.


I have not been home much, so I have not had a chance to perform the tests. I have managed to get a voltmeter borrowed, so I can use that.


If I make it home before dark tomorrow, I will perform some tests.

Expert:  Roman replied 2 years ago.
ok just let me know how you make out
Customer: replied 2 years ago.

Hi Roman,

Finally I had time and opportunity to perform some tests on the car. I will try to describe in as much detail as possible, what I do, since I think that will give you the most information to give me feedback.


This is the multimeter, I borrowed:



I attached the charger about 4 hours ago, and the battery was not flat, as I was able to open unlock the car doors with the remote when I got there. I did not measure the battery at that time.

Four hours later, I went to the car, started it up and measured with the voltmeter. It said 12.3v.

Then I put the red clamps around the thick black cable going to the negative pole (I am unsure if that is the correct term in English - but the minus thingy on the battery) and set the multimeter for amps. I recorded the screen of the multimeter, went into the car and used the throttle for a few seconds. The video can be seen here:

There is no sound, but it clearly increases in amps when I use the throttle and drop back down when I take my foot off.

Am I correct in understanding, that this actually means the alternator generates power and feeds it all the way to the battery? Or am I missing something?

I then proceeded to measure the voltage again and it said 12.1V. I then set the lights to parking, closed the doors and left the car idling.

30 minutes later, I went back to the car and measured the volts again. It was down to 11.0V.

I then kept the voltmeter on, put the camera on it and revved the engine. The volts increased a bit and then fell again when I took my foot off. Video:

Then I set it to amp-mode again, hooked it up to the negative side of the battery and did another video. Clear movement depending on throttle, not sure what to make of the numbers.

I then did a final reading of the battery, it was down to 10.4V. I turned off the car, removed the charger and came in to write this.

So to summarize the volt readings:

18.00: 12.3V
18.05: 12.1V
18.30: 11.0V
18.40: 10.4V

I am pretty confused. This is not my domain at all, but doing software development, I know how to think in logical troubleshooting and here is what it seems to me:

When the car is idling, the alternator produces less power than the car consumes (ie. the voltage drops). If the car is running at higher RPMs, it does seem to charge the battery (the amps change and the voltage increases, at least temporarily).


What do you make of all this?

Expert:  Roman replied 2 years ago.
It sounds as the alternator is failed internally and may only be charging at higher RPM which is not good.

You did very good!! Thanks for the detailed info.
You can double check, Make sure the leads are connected red to red and black to black.

With them connected check all the cables for corrosion and clean if present. Then bend, tug on the cables, if you see the voltage change there is a problem in the wiring if it does not change replace the alternator.
Charge the battery over night on a low rate of charge.

Customer: replied 2 years ago.

I want to do a control-measurement tomorrow of the amps, as the two I did today were so different.


What would be acceptable value for the charging? What should I expect to see?

Expert:  Roman replied 2 years ago.
Hi, alternator output is tested under load with a carbon pile to get a true reading. this usually a tool that the shop provides.

The tool that you have may be able to let you see some change but will not be a true reading for testing purposes.

What you can do is put your amp meter around the POS+ battery cable, start the engine and note the output then turn on the lights, ac, defrost etc. and watch the readout have someone raise the idle and watch read out again.

There should be a sticker or a stamp rating the alternator output on the side. Usually 90amp alternator is used in this car.Out put should be around 85 amps lower than that is not good. Make sure the battery is charged over night.

did your meter come with a card that explained how to do this test? That one looks like it may have that is why I ask.

Lets see what you find??
Customer: replied 2 years ago.

I did another test. First I charged the car for about 3 hours. I then hooked up the multimeter like this:



...around the positive wire.


I then revved the engine and recorded it. It looks like this:


So from 25 to 35 amps and back down.


Then I tried turning everything on that uses power, recorded that:


From 20-25 up to 45 and back down.


There was nothing with the multimeter, I just borrowed it from a friend.


So the readings are nowhere near the 85-90 amps you mention.


To satisfy my curiosity and understanding here... What does this mean?


When the amps go up, that means there is some electricity coming from the alternator to the battery, correct?


If that is the case, would it be correct to say, that the alternator is not dead as such, but simply no longer delivering the power it is supposed to?


So if that is true, it could also be too much resistance in the cable from the alternator? That would give these symptoms, would it not?


I think my next step has to be to call a garage tomorrow and have them come pick it up and they can troubleshoot with proper equipment (and skill better than mine) - and change the alternator or cable, if required.

Expert:  replied 2 years ago.

Very good but let me clear some things up.


You are measuring the amount of current that is being output from the alternator back into the battery not out of the battery.


The max output from the alternator is 90 amps, when you turn on the accessory's the amperage will rise but never to 90 amps. The amperage output on your car is good. It is not putting out enough voltage from the voltage regulator inside the unit. The alternator has an output of between 13.5 and 14.8 volts. If you want to check the cable you will do what is called a voltage drop on that particular wire.


I have a write up on my site about voltage drop testing here. It will answer all of you questions.


I'm sure you will need a new alternator, you can test to satisfy your curiosity but just remember the longer that you run the engine the more you will be draining the battery.

Roman, Technician
Category: Audi
Satisfied Customers: 556
Experience: over 20 years vw audi and porsche technical repair
Roman and 4 other Audi Specialists are ready to help you
Customer: replied 2 years ago.

Ok, thanks. I will contact a local mechanic tomorrow to get it fixed. Thanks for your help. :)

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