Bosch Alternators have 2 posts on them, the big one is for main battery ground, the smaller one is for the exciter or blue wire. When the cars ignition switch is turned on, the Dash light (led and diode) applies 12V+ to the coils to excite them, so the alternator can produce a charge. When the Alternator is producing current and voltage it goes to 13.5-14Volts.
The part # for the Alternator (Automatic trans) is 6H903016L and it's made by Bosh. [Europaparts](https://parts.audiusa.com/p/Audi_2012_A4-Base-Sedan-20L-AT-Quattro/Alternator/48001649/06H903016L.html) sell's it for $499.95 (OEM).
The alternator itself can get bad only if the bearings are shot. If you don't have play on the shaft, and it spins freely then the alternating part of the alternator is fine. What can get pretty bad are the diode rectifier bridge.
If you have the V6 4.3 it's one of the more dependable motors GM ever made. The factory alt is rated at 105 amps. Should be enough for your needs.
I changed out my original Valeo alternator with a Bosch unit at 100k miles. Never had a problem with either one.
I bought a Bosch reman for my X5 about 3 years ago, no issues whatsoever.
I replaced the battery, and upgraded to an AGM battery while I was at it. The problem still wasn’t solved though.
I order a Bosch AL113MX Reman Alternator (yep Bosch reman the Motorola) since that was what was currently in there. The 81 has a big square York compressor mounted right on top of the alternator which means that to get the rear alternator bolt you better have the right extensions and you are going to do it blind. So after at lot of effort I got the old alternator out and the new one in and only then did I notice that the pulley did not line up.
I bought a brand new, OEM Bosch alternator from Rock Auto. A week back I started noticing a bearing squeak sound in the engine on startup... Later got another battery light... Today was driving and lost power steering suddenly and got battery light again. Pulled over as the car was getting hot quickly. Opened the hood and the serpentine belt was shredded. Alternator was completely seized and had burnt belt on the pulley. Got it towed home.
I had a bosch alternator in my Mark 2.. 8valve motor. I noticed the battery was not being charged. so instead of dealing with the voltage regulator or brushes etc, b/c the unit was old I decided to go with a brand new one. making me think i should have ponied up to the Bosch.
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