Alternator and Charging FAQ

Updated 1/15/2007

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Some of the common charging problems:

  1. Loose belt

  2. Bad battery ground or hot wire or corroded connections

  3. Open charge light in dash (does it light when you turn the key on?) or blue wire to charge lamp broken or unplugged - if the lamp or blue wire is open, the alternator WILL NOT CHARGE.

  4. Worn brushes

  5. Overheated alternator output wire - check the red wire on the alternator for melted or blistering insulation. You will need to solder a new terminal on or in extreme cases run a new wire (#8) from alternator to starter.

  6. Frayed worn alternator ground wire (see picture below).

  7. Bad diodes in alternator - common in alternator rebuilt with cheap parts AMHIK*

  8. Alt light comes on with key off - shorted diode in alternator or see below.

Here's a picture of the upgraded alternator ground wire

*Ask me how I know

Something you might try if you have a sealed or maintainance-free battery is the diode mod (from June 2006 Roundel Marc Calabrese)
Here's the pic, maybe I'll add some text...

Rebuilt Alternator Notes:

After 21 years of service, the original alternator in my E12 based sixer was dieing a slow death. I had the option of installing a new voltage regulator or just going all the way for a remanufactured unit. I decided on the latter and purchased a Bosch piece.

When you get a replacement from Bosch, the model number may be AL41X. The original alt. had a single post which was the terminal for the red output leads as well as the sound supression condenser. The blue lead terminated at a spade fitting and this is the lighting circuit for the telltale dash lamp. The new model is a bit different and a couple of changes need to be made to the harness leads to make sure everything works as it should. The new alternator has two posts and a spade terminal. It all seems pretty straight forwardright, whats to mess up?

Murphy's law is usually in full effect in such situations. It would have been helpful if I had noted the wiring terminations when I removed the old unit. I put a large eye terminal on the red output leads and secured it to the large post. I terminated the condenser to the smaller post and put the blue lead on the spade terminal. When I looked at the dash, the charge light was on; not a good sign. I'll admit right here that I was flustered. I drove the car to an auto electric shop and was told exactly what I had done wrong.

So that you don't make the same mistake here's what to do. Terminate the red output leads to the large post; the red output leads being two red wires on the same terminal. The yellow condenser wire gets the female spade to connect to the one and only terminal of this kind on the back of the alt. The blue wire/s need to have the spade cut off and an eye terminal crimped on. This gets connected to the smaller post. Now when you reconnect the battery ground, the dash charge lamp won't be on. It gets better; the light will come on when the key is turned to start the engine, and go off as soon as the engine catches. If you check the alternator with a meter for output there should be approx. 13.5 to 14.5 volts. If I have saved any of you the five days of down time I went through then my job here is done.

Erich Maas
'81 633CSi (E12 under the skin)

Peter's note: The rebuilt alternator I got a few years ago had spade terminals. I believe Bosch always numbers the terminals so check the numbering of the replacement alternator if it looks different.

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2002, 2007