Battery or alternator? Also, brake light switch fault

[LIRS6]

Trying to determine if I have a dying battery, or a failing alternator. Last week I sensed a bit more effort to start the car - caught, but just didn't seem normal. Then a couple of days later I noticed volts at around 13.5, again not the norm. Today, driving home from work, I noticed the volts move down to just under 12 volts. Got home, took a voltmeter to the battery which read 12.31 volts.

Is there a way for me to determine whether it's the battery or the alternator? I don't have a plethora of tools and equipment, and will plead ignorance anyway. I tried to check acid level in the "window" on the battery, but it was tough to get a good view of it due to its position in the trunk, and I didn't want to disconnect the battery to move it.

Any thoughts/advice?

On a separate note (I think), I also experienced a kind of limp-mode acceleration from start as of this past w/end. Checked the rear brake lights, they worked fine. Then I did a scan and got the following fault code:

00526 - Brake Light Switch-F
20-10 - Implausible Signal - Intermittent

So I presume the brake switch down at the brake pedal needs replacement? Anyone have a part nbr?

Thanks for your help

[ttboost]

Bad brake switch will definitely wreak havoc on performance. Cheap part and minutes to fix. Sounds to me like your alternator is dying. Autozone or whoever can test it, if you can get it out.

[kday]

It is a lot easier to remove the battery and have it tested than to do the same for the alternator. So I'd start there, even if the odds favor an alternator problem.

[Jimmy]

Sure sounds like the voltage regulator is bad...re-manned alt from Audi is expensive but the way to go, unless you have someone who can recondition your OE one. For sure test the battery, but also consider a new one while your wallet is open !

[DHall1]

Volt meter on battery with car off.

Volt meter on battery with car running.

Volts should go up with the car running.

[LIRS6]

Thanks Gents

From a search I see that the regulator is Audi part #077903803a - does anyone have a corresponding Bosch part number?

[kevin]

Bosch 0031546506

[LIRS6]

Thank you, Kevin

Jason

[LIRS6]

Well my luck ran out - x-mas tree lights and huge fluctuations on voltmeter spelled the end of my ride home last nite - 10 miles away. Had it towed to my house, will try a DIY. I intend to replace the regulator only and then have the alternator tested while out - hopefully it's just the regulator. I had already ordered a regulator, which arrived today (yes, Bosch).

Will use this thread as a guide: http://www.rs6.com/showthread.php/19720-Alternator-replacement - any other comments/suggestions welcome. Wish me luck ...

[kilian tuning]

...ive had almost same symptoms, in my case it was the battery...

[LIRS6]

Up-date: I spent the w/end on my back, removing the alternator w/o the benefit of a lift (2 floor jacks and a pair of jack stands). The bitch, as others have stated, was prying the unit out from its position. I replaced the regulator only ($36 from RMEuropean.com), and had the unit tested at a shop - it was good to go, so back into the beast it went. Added about 3/4 ltr of oil, re-connected the battery and hoped for the best. All was good - a steady 14 volts over a day of driving to and fro. So as others have stated here, if you have electrical issues it may just be the regulator and not the alternator. All told, I spent about $70 for the repair (excluding the tow)..

I also attempted to replace the brake light switch, but unfortunately my dealer sold me the wrong part. So that fix is for another day.

Thanks to all who contribute to the forum with DIY's, advice, comments from experience, etc. I could never have tackled this job without all of the great input.

Jason

[ttboost]

Congrats on being back on the road. And you didn't spend big money to make it happen...a rarity with these cars!!!