Electrical - alternator - voltage regulator

[donnieknoxville]

1987 911 Carrera Convertible

For the past several weeks, I have been having this problem where the headlights (and all interior lights) suddenly get brighter and dimmer. It is totally random, despite my best efforts to determine a cause and effect relationship. What's particularly odd is that the seatbelt warning lamp on the dash (above the radio) illuminates almost every time the lights go from dim to bright - then ater a few seconds, the warning light goes out. A few seconds later the process repeats. The battery is good, but I'm wondering if the voltage regulator or alternator mught be going bad. One night a few weeks ago, I switched to high beams and when the power surged, I blew out both headlights.

At real low RPMs, I have been able to make the battery warning light illuminate, but this has only happened a half dozen times in the pat 4 or so weeks.

The one thing that does seem to make the power surge (i.e. seat belt warning light goes on and headlights and interior lights get brighter) is when I'm at a constant speed for a while and I lift of the gass just slightly. Doesn't happen every time but enough to notive a correlation.

Any thoughts would be appreciated.

Don

[RobertsStole2nd]

Don,
I have an '86 Coupe that had the exact issue you are having, including the seatbelt warning lamp illuminating and the alt light coming on at low revs. While driving, my instrument lights and headlights would randomly dim for a short period then brighten. I installed a new voltage regulator, problem solved. I would test the battery and the alternator to check their condition, but this is what worked for me, fyi. Also, the vr install was relatively straightforward and you can get it right here at PP.

Todd

[donnieknoxville]

Todd,

I was really suspecting that but couldn't figure out what was triggering the seat belt warning light. At this point I willing to learn that later. Thanks a ton.

Don

[Wil Ferch]

Classic symptoms of a bad voltage regulator.

Check voltage when car is "off" ( fully charged battery is 12.6V...half charge is 12.2 volts). When car is at idle up to 2000-3000 steady RPM....voltage should go to 13.8-14.2 or so.

If it spikes at any time to 15-18 volts .. it's a bad VR.

Wil

[fuelie600]

I had the same, noticed the lights first, then tested with a multimeter. Voltage would ocassionally jump to 18+ volts!

I ordered a new VR from our host. Problem solved.