Your alternator might be causing your brake light to flicker.

Dear Car Talk

Dear Car Talk | Dec 01, 1996

Dear Tom & Ray:
I have a '79 Datsun B210. Two or three months ago I noticed that the
dashboard brake light was flickering, as if it were a turn indicator, while
I was stopped at a red light. I immediately drove to my trustworthy
mechanic. They decided the brake fluid was a little low, filled it, and
that solved the problem -- for a few days.

I noticed that the flickering only occurs at prolonged stops when the idle
speed became very low. I noticed that revving up the motor in idle or
stepping on the brake pedal stops the flickering. They rechecked the brake
fluid level, and there are no leaks.
This week, when I was stopped for a VERY long time at a railroad crossing,
the battery light also began to flicker. Do you have any suggestions? I
love my little car and expect to keep it as long as I continue to drive (I
am 76). I love your column and your radio program. Thanks! -- Sara

RAY: Well, Sara, we don't see many B210s anymore (mercifully), but if I
remember correctly, the brake light and the battery light on this car are
somehow intertwined (ancient Japanese custom, I guess). And what you
probably need -- believe it or not -- is a new alternator.

TOM: When the brake-light circuit is tied into the alternator, a problem
with the alternator can make the brake light come on. And in our
experience, fixing the alternator almost always makes the brake light go

RAY: And output of the alternator would be at its lowest at prolonged
stops, especially at low idle. So the problem could be something as simple
as an engine that's running poorly and on the verge of stalling at idle. If
that's not it, I'd look for a bad diode in the alternator, or a bad ground
(between the block and the chassis, for instance) that's causing poor

TOM: When you find out what's wrong with the alternator and fix it, that
brake light will go off. Then you and this B210 can motor around happily
together for at least another 20 or 30 years. Good luck to you, Sara.

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