grabo

1998 Audi A4 B5 2.8L, 30K (NOT a typo) miles. One-year-old battery not getting charged. When the car was running, voltage at the battery wouldn't go over about 12.8V when charging, and fell to about 12.2V when car was off.

Charged the battery with my other car. It still settled around 12.3V, so I replaced the battery (under warranty, no questions asked)

Battery voltage when charging still ugly, so I figure it's time for a new alternator. Might just be the regulator ($50), but have to remove the alternator ($200) to replace the regulator, so may as well just replace the whole thing since I gotta do the "bumper/lock-carrier" dance. May as well also do the timing belt, water pump, thermostat, serpintine belt, and flush the coolant. (Also replace the outside temp sensor, since it randomly says it's negative in the summer)

Ordered the parts. Spent a few afternoons doing all the work. Put the thing back together. Car started.

But... with the car running, battery voltage looked no better. First drive around the block was fine. Second drive a few hours later greeted me with the charging system idiot light, so I went straight home.

B+ on alternator follows battery voltage exactly... so wiring is OK.

When car is "on" but not running, D+ is ~0.6V, and the idiot light is lit (again, wiring OK). Start the car, idiot light stays on, and D+ on the alternator hovers around 5.5VDC. If I set my meter to AC, it sees about 5VAC as well. (A few millivolts of AC is seen at the battery)

I'm thinking that one or more of the isolation diodes is shot, which would explain the low D+ voltage when the car isn't running (I think it should be higher, to account for two diodes in series), and also explain the low DC and high AC voltage when the car is running.

I'm just looking for confirmation that I need to go through the bumper/lock-carrier dance again... to warranty replace my NEW (well, rebuilt) Bosch alternator (came in a Bosch box, not some no-name rebuild)

4Driver4

Autozone alternator?

grabo

Ha! I knew I missed some detail in my original post.

No, not AutoZone. It's a Bosch AL0721X (remanufactured by Bosch, in a shiny new Bosch box). Bought it from FCP.

grabo

Annd... yanking it out the second time was a lot easier. Put the front end into the service position, but just stretched the radiator hoses a bit and held it in place with wood blocks. Was able to weasel it out of the bottom of the engine, instead of the top (which I did the last time, since the radiator hoses were also out of the way)

Quick tests on the new alternator - the stator windings are shorted to ground. Either that's due to a bad diode or some other short, but either way, it's on its way back to FCP, and a replacement is already enroute to me.

Mental note - test the heck out of alternator (diodes, etc) before installing it in car. I also rigged up a homebrew alternator test rig, with a 12V power supply, 12V lamp, drill (to spin the alternator), and a couple voltmeters.

grabo

Just following up for the sake of completeness - new alternator tested out fine, and after installation and start-up, puts out 13.8V, and the car is super happy now.