Dead battery - Is the alternator bad?

[dghii]

Dude,
Quit checking molecules and stuff. You have a bad voltage regulator. Disconnect your battery. Remove your alternator and replace the regulator.

You will not be able to detect a bad regulator with a DVM. It will not spot a bad diode within the regulator. Your problem is that the voltage coming out of your alternator (the output is the regulator) is all over the place and makes the electronics in your car do really weird stuff.

Your car did not develop fantom current draws overnight. Don't overthink this. This maybe a Porsche, but its really just a car.

[Ian c]

Originally Posted by dghii

Dude,
Quit checking molecules and stuff. You have a bad voltage regulator. Disconnect your battery. Remove your alternator and replace the regulator.

You will not be able to detect a bad regulator with a DVM. It will not spot a bad diode within the regulator. Your problem is that the voltage coming out of your alternator (the output is the regulator) is all over the place and makes the electronics in your car do really weird stuff.

Your car did not develop fantom current draws overnight. Don't overthink this. This maybe a Porsche, but its really just a car.

Ahhhh ....
The good old "diagnose with your wallet" theory

[cas951]

3point2,
Your chrging seems fine to me. As long as the Alt output is 14+ volts with no load you should be fine. If you want to test further let the car idle, turn on your lights, AC, and other accesories to see what voltage you have at the battery.

You should have 13+ volds.

What you have is a drain when the car sits for awhile which drains your battery. This is why Ian and I suggest you use the voltmeter and pull your fuse out 1 x 1.

If you don't find the culprit from the fuse the likely cause is either the starter or Alternator or bad Ing switch.

[Ian c]

Indeed .
I suspect he's decided to give up and go get drunk

Hopefully he'll be back tommorrow and we can carry on with the online diagnosis

An alternator and voltage regulator can be working fine yet not put out enough volts to charge a battery .

The fact his is putting out enough on its lowest reading (2nd test = dropping) , and his battery DCv reading went up indicating a fuller charge , leads me to look at other things as I mentioned in my "best guess" post .

I had a read back earlier, and depending on the mA drain test results , the ignition switch is edging its way towards the number one on my list ....


(Radio fail)

[dghii]

Originally Posted by Ian c

Ahhhh ....
The good old "diagnose with your wallet" theory

No. A 3 time Boxster owner who had the EXACT same problem 3 months ago and who fixed it after diagnosing the problem.

You want to verify you have a bad regulator? Get an oscilloscope and look at the output of your alternator. You'll see that you're missing (at least) one of your diodes.

[Ian c]

Ive had a bad diode draining the battery , also a bad connection on the alternator , a bad battery connection , bad radio , bad ground , bad ignition switch , bad alarm , bad immobiliser , bad door switch etc etc etc .

You could well be correct , you could well be wrong .
You could scope it to test , or you could meter it .

I too had the EXACT thing happening within the last 3 months on my boxster ....
It was the ignition switch ...

I'm used to diagnosing before ordering parts and scheduling labour ....
It's what I do for a living , and have a huge amount of test gear at my disposal ( each Instrument tech working for me carries about 20Ks worth in their toolkit)
If I did it by guesswork and throwing money at it , no matter what past experience tells me , I'd be claiming unemployment

This is me diagnosing using experience .
Just a pity the guy had allready been to the main dealer , then 2 specialist that diagnosed with his wallet first ....
http://www.volkszone.com/VZi/showthread.php?t=850130