The Audi 100 won't charge the battery. Looking around online, it seems like a problem with the wiring from the alternator to the battery.

Instead of fixing the old wire, I ran a new wire, rated for 105 amps, from the alternator directly to the battery. Current alternator is 90 amps.

I was told the battery and alternator were replaced a month or so before I bought the car. The alternator looks like a referb unit.

When it is hooked up, the engine drops 50-100 RPM's or so, so the alternator seems to be working, but I only get 12.5v on the battery, which is the same as when it is off.

I did a few tests to find out what the problem might be.

Voltage of the battery with the car off - 12.7v
Voltage of the battery with the car running, but alternator not hooked up - 12.4 - 12.5
Voltage of the battery with the car running, alternator hooked up, engine @ 2500RPM - 12.5
(The voltage actually goes down the higher I rev it, with the alternator hooked up, by .05v from 2000 to 4000 RPM)

Voltage at the alternator, not hooked up to the battery, while the car is running @ idle - 20.9v
Voltage at the alternator, not hooked up to the battery, while the car is running @ 2500RPM - 15.5v
Voltage at the wire I ran next to the battery, from the alternator, but not hooked up to the battery @ idle - 20.8v (only a .1v drop over the wire, fine with me)
Voltage at the wire I ran next to the battery, from the alternator, but not hooked up to the battery @ 2500RPM - 15.5v

I don't understand, a direct connection should bring the battery to around 14.5 volts @ 2500RPM or less. Is this a bad alternator? Bad belt? My wiring solution crap?

 

Yep this is confusing.

The lack of change in the battery voltage suggests that the connection is not being madeeven with your handy dandy wire, but I am wondering if the diode is bad. I suspect that when it sees any voltage from the the battery it assumes it dosen't need any current so it idles the alternator. The 20 volts at idle vs 15.5 at 2500 RPM could be some sort of funcky feed back to the diode, causing it to slow down the output. Remember - in fluid flow terms voltage is pressure and amperage is flow. Amperage is what you really need to measure!

I know their assistance is sketchy but maybe a trip to Autozone could help.

 

Wish it was as easy at going to autozone here. They can't help. My friend is having a similar problem with his '89 200 qt. the alternator is brand new as it the battery, and the damn thing still drains the battery. it's like the alternator isn't even charging the battery. We even ran new alternator lines from the alt. to the battery as well as new wire from the alt to the start, and new ground lines. and it still doesn't charge the battery. anyone else have similar problems?