Voltage regulator (external)
 

I have a 1977 911S. The alternator is less that 1 year old and I have a new battery.

The alternator light is staying on after car starts.

Is there a way to "bench test" or otherwise test the voltage regulator (external) to see if it is bad before I start the task of taking out the alternator?

FYI:

Battery with engine off measures 12.4 volts.

When car started voltage drops to 11.

Voltage regulator have 3 terminals:l DF (Black wire) D+ (blue wire) D- (brown wire)

Thanks,

Ron

Certainly seems like a bad alt. Voltages should go up when you start the car. The VR regulates them from going too high, not low. When running you should have between 12.5 & 14.5v.

A new VR is cheap compared to an alternator, get one and swap it out.
I just replaced mine and bought a spare from Pelican....
No real bench test for the new solid state ones unless you have a variable 12-24 volt DC supply.
The old mechanical ones with a tin top can be opened.
When I opened mine the resistors had burnt up. Now running solid state with a spare.

Altanator or Voltage Regulator issue
 

Update,

I had nothing else to do, so I decided to put a slow trickle charge on the battery.

As noted above, the battery with the engine off was reading a little over 12 volts.

After about a 2 hour charge, the battery was reading about 12.7 volts.

I decided to start the car and see what would happen. To my surprise, the car started fine just like before however, this time, the Alternator light went out. I looked at the volt meter, and it was reading almost 14 volts. I turned on the head lights and that dropped the voltage to 13+. I do not know what happened but things looked good. I then went to my external voltage regulator and unplugged it. You could hear a change in the alternator when I unplugged it. I then looked at my meter and it was reading a little under 11.9 volts. I was running off the battery. I then plugged the voltage regulator back in and could hear the alternator kicking in and the voltage was 13.8.

So, I have NO idea what happened. The alternator light is working as it should. When I start it lights and then the light goes out right away.

I should feel good about this but I am concerned if I have a intermittent problem with something.

Lastly, the car had not been started in about a month and is has an alarm that does drain the battery over time.

It's almost like when the voltage of the battery was on the low side that the voltage regulator would not kick in?

Anyone with any feedback would be appreciated.

Thanks again all,

Ron

You can hook up a voltmeter at the cigarette lighter and monitor it as you drive to see what is really going on.

vr
 

i bought an adjustable voltage regulator,,, i had a new alternator and a new battery, and a new stock vr. the alternator would not keep up with the lights etc and the battery would be depleted. i put in an adjustable vr 2 years ago set it for 14 volts and have not had any problems since.
Also I found that if you let the batter discharge, jump start they car and use your alternator to recharge the batter that there is a good chance that you will burn out your alternator,
The alternator guy told me that they are not designed to charge a depleted battery as they are designed to duty cycle maybe 50% of the time, if you have it running 100% to charge you battery you will propably cook your alternator.

Install a battery tender if you aren't using the car for more than a week. The slow drains will kill the battery over time.

I want to thank you all for the ideas.

Got some work to do.

Thanks again,

Ron