Driveway test:
Getting 13.86 volts at idle on the 1990 C2.
Headlights on - 13.64
Step on the brake- no change in voltage, alternator light comes on.
Restart the car- no alternator light
Highway test:
stop and go driving using emergency brake only- no alternator light is triggered
brake pedal stop- alternator light comes on.

Any clues guys? I am stumped.

 

Is this a new symptom or has this happened for a long time? If its recent, has there been any recent work done?

Do all of your brake lights work as expected? Could you have a high current draw on the brake light circuit that's triggering the alternator light?

 

Rocket Rob, the three brake lights look normal when illuminated.
The alt light came on intermittently several months ago and I attributed it to a bad alternator/voltage regulator. So I installed a rebuilt Bosch unit. As a result the idle is now smooth and stalling has been eliminated. The alternator light issue is now easier to understand with a stable idle. I am thinking of disconnecting the electrical connection on the master cylinder and see what happens to the odd feedback issue. Any other thoughts welcome.

Moved down from Exeter, NH several years ago. Cars used to spend the winter with Blair Talbot Motors in Dover.
Voltmeter on battery does not detect a drop when brake is applied during idle. 13.69 Volts - steady

 

I agree. I would try isolating the brake light circuit and test again.

Blair Talbot is a great mechanic. I use him when I can't fix something myself.

 

Alternator pulley cross-section. Model 964
After confirming the battery was holding a charge and had plenty of cranking amps I rechecked the alternator output during an alternator light-on event, as triggered by the brake pedal at lower rpm. The output voltage at idle was the 13.85 which was the same idle voltage when the alt. light is off. I began to suspect a momentary slippage situation between the belt and the alternator shaft during braking. Sketching the cross-section of the assembly, I appreciated the torque spec, belt tension and areas of unwanted lubrication. I cleaned and grit blasted the contact surfaces of parts in the assembly followed by reassembly. After several 10 mile outings the alternator light is not triggered by the brake pedal. Is it possible a lurking voltage regulator issue exists? We shall see. Keep you posted. Thanks for the support.

 

Agree, you may have more than one problem lurking. Glad to hear you found and fixed the belt slippage problem. Keep us posted as you drive/test it more.

 

I found this compact set-up very useful during hours removing, installing and trouble shooting the 964 alternator. The tightening of that 24 mm shaft nut is a must as alt. pulley relies on "friction" coupling to the alternator via a stack of bushings under compression.