Overcharge, then no charge - voltage regulator?

[blpetry]

My '82 SC last fall a couple times indicated on my aftermarket dash voltage gauge that it was overcharging (16-17v) on short drives before putting it up for the winter.

Then, yesterday I drove it to work and noticed it wasn't charging all the time, voltage would fluctuate between 14.1 and 11.5. Voltage was 12.1 on startup leaving for the day and fell to 9v by the time I got home. Car would not restart after I put it in the garage. I think I'm lucky it didn't quit on me.

Looks to be internal voltage regulator evidenced by the 24mm pulley bolt.

Dash "battery" light comes on when you turn the key on, so its not blown out.

What do y'all think, is there a way to test the alternator independently of the VR?

Also, is the $30 Marchal regulator unit ANY different than the $100 "Porsche" Marchal unit?

Thanks in advance,
Brian

[T77911S]

take it a parts store like advance or Oreilles
they test it for free.

[blpetry]

OK did a little more investigation today.

Sometimes, when you turn on the ignition (but don't start the car) usually when the car has sat for a bit, the battery light will illuminate. When in this state, the alternator charges the battery upon starting (~14.1v).

After driving just a mile or two, the voltage drops down to ~11v and the alternator is no longer charging. If I stop the eninge at this point, the battery light does NOT illuminate with the ignition on. Upon starting it does not charge when in this state.

I've never seen the battery light on while the engine is running.

Seems like a wiring issue somewhere. Bulb socket looks good and spade connectors look good on the warning lamp. Doesn't seem to be a short in the wiring up by the gauge.

Is there a connector in the engine compartment that I should check?

Thanks in advance.
-Brian

[targa80]

The alternator has three exciter diodes along with the power rectifier diodes. The exciter diodes provide the proportionate signal to the voltage regulator which then controls the voltage output of the alternator. In essence it is a feedback loop between the alternator and the voltage regulator. These exciter diodes deteriorate over time from heat and eventually fail. The failure of one of the three exciter diodes will cause the charging circuit to fail. I have replaced the exciter diodes in my 1980 Marchal alternator during a rebuild back in late 1990's. I had to replace them again just about three years ago. After the last repair I added a voltage and amp meter combo gauge to the charging circuit because I had not indication that the charging circuit was failing until the car would not start. Do a search for a post that I made on replacing the exciter diodes.

[blpetry]

Thanks! Checking out your post now.