Bad alternator diodes or what?

[Rigmaster]

Started driving the '87 325is DD/Track car the other day after it had been sitting for several months. Battery was dead, so I charged it up for a while and it started right up and ran fine- but the charge light was on unless I revved the engine up to ~2500 rpm, then the light would *almost* go out.


I checked the alternator output and it was only like 11.5v, so I swapped the alternator for another one from a parts car. The output from the "new" alternator is now ~13.8v, but the dash lights are still on (battery, parking brake and brake light) until I rev the engine.


Anyone had a similar problem? I'm thinking it's a bad diode in the alternator?


Bret

[jlevie]

It is possible that you might have a bad battery that's loading down the alternator. About 13.7-14.5v at 2500rpm would be in the right range for a good alternator. Excessive load could pull the alternator down below about 12.8-13.0v at idle, which would make the lamp illuminate.

[Rigmaster]

Thanks for the reply jlevie.

I think you may have been on to something- I started and drove the car today, and none of the lights stayed on. Must have had a bad alternator AND a dead battery to begin with, now have a decent alternator and a decent battery (charged up) and all seems to be well.

Man, I forgot how much fun this car is to drive!!! I've been driving my M42 318i for a while now, and it's OK, but this 325is is sooooo much more fun.... :)

Bret.

[jlevie]

Yeah, they are just a whole lot of fun...

As an FYI; the life of a battery can be dramatically reduced by sulphation of the plates. That happens each time the specific gravity of the electolyte falls below 1.225 (battery voltage of 12.4v or 75% of charge). If a car won't be driven for a week or more it is best to connect a battery tender to maintain a full charge in the battery. Even when the battery is disconnected you need to do this to avoid self-discharge.