Alt Light when I step on the brake

[3Pgarage]

The 90 C2 loves its rebuilt Bosch alternator. It now easily assumes an idle where before the engine would die at low rpm.
So the oddity is- alt light comes on when decelerating and braking. It goes off if I restart the engine.
Alt light does not come on when I use the e-brake
Alt light does not come when I am parked with the engine running and repeatedly step on the brake.
I suspect this oddity was occurring with the previous alternator and I did not fully characterize the condition due engine stalling.
Fresh thinking appreciated.

[juanbenae]

have you checked to see if the brake lights are working since the issue has started?

[3Pgarage]

Good question- Yes, the two lower and one upper brake lights are functional.

[Targalid]

I would put a voltmeter at the battery and check the alternator output. I like the cigarette lighter types so I can monitor alternator behavior while driving:
https://www.amazon.com/Palumma-Charger-Adapter-Display-Battery/dp/B0773BYS6P/ref=sr_1_1_sspa?crid=2UWZJ6F599RKE&keywords=cigarette+lighter+voltmeter&qid=1547747630&sprefix=cigarette+lighter+voltmeter%2Caps%2C217&sr=8-1-spons&psc=1

[offroader1006]

It could be a connection issue, and the load shift under decel causes it to trigger the light.

Where in SC are you?

[stlrj]

You need to let us know what the charging voltage is with the engine running. Don't assume a rebuilt alternator is any good. I personally have installed two that were under charging and finally got one that resulted in the correct output voltage.

Good Luck,

Joe
87 Carrera

[3Pgarage]

Located in Greenville, SC
Load shifting was my thought as well. A 30 min drive of only E-Brake braking does not trigger the alt light and therefore load shifting is not likely the root cause.
______________
New Data- car is stationary
Battery - 12.6 volts, ignition on- 12.5 volts, brake pedal depressed- 12.4 volts, no alt light
__
Battery- 12.6 volts, start, idle- 13.86 volts, headlights on - 13.69 volts. brake pedal depressed, 13.69 volts and alternator light comes on!!!
___
Headlights off-restart, idle, brake pedal depressed (no change in voltage), alternator light comes on.

Contemplating clues

[3Pgarage]

Are there areas on the braking system wiring on this 1990 C2 that you all recommend examining for bad ground, loose connections, lost insulation, corrosion, funky relays, etc? Could it be there is a transient spike when I hit the brake pedal...undetectable by the Voltmeter?

[3Pgarage]

So several months later the problem persists. The charging voltage is 13.7 volts with the alt light on or off. I disconnected the 2 brake light switch leads thinking an electrical short in the rear brake light switch circuit was the problem. The disconnect did nothing to this annoying issue. Pressing the hydraulic brake pedal continues to trigger the alt light.