Electrical party

["Paul Albertson" <albertson@domain.elided>]

Regarding:
From: Luca <nodoubt@domain.elided>
Subject: Electrical party

"... When I switch the turn signal, the battery warning light comes in
flashing for one second or so, then just disappear as it should be.
What's the point from which I should start my troubleshooting
procedure?"
***************************

Hello Luca,

I do not own a 75, so my advice is more general for this problem.

Short answer: 
  Try replacing the turn signal flasher. It is small, about the size of
a stack of large coins. It makes a quiet "click" sound when the turn
lights go on/off. Also check the electrical connections to the flasher
and alternator. Make sure the alternator belt and ground are good.

Long answer (for those with time to kill)
   The battery light going on means the charging system voltage has
dropped below the battery voltage. Something is drawing more current
than the alternator can supply. Because you said it happens when the
turn signal starts, and it only lasts a moment, I suspect your old
flasher is slow to start flashing, drawing too much current at first,
dropping system voltage, and making the battery indicator turn on. Once
it is flashing, the flasher is heated up and drawing less (normal
amount) of current. 

   This could be normal if this only happens when the engine speed is
near idle or when several other accessories are on. You didn't mention
this, so I suspect there is also another problem contributing to this,
as the charging system should be able to supply even this extra current.
You may have a weak diode in the alternator, reducing it's amperage
capacity or a weak electrical connection at the alternator, or a loose
belt. If this weak charging system theory is true, the problem will get
worse if other items are also on, such as headlights, fan etc.
 
   The old mechanical turn signal flasher is a thermal switch. It has a
bi-metallic contact arm (strip of copper bonded to strip of steel). When
it heats up, the difference (copper vs. steel) in thermal coefficient of
expansion makes the copper side expand (get longer slightly) than the
steel side. Because these are bonded together at each end, the
combination results in the arm curving as it gets hot from passing
current through the arm to the contact at the end of the arm (and then
to turn signal lights).

  Once it curves, the contacts separate, the current stops, and the arm
cools. As it cools, it straightens and closes, the current flows to the
turn lights, and then arm begins to reheat, gets hot as it passes
current through the contacts. This on/off cycle repeats. After many
years, the metal in the bi-metallic arm fatigues, and may be slow to
start flashing. This will draw more current at first, which could drop
system voltage enough to turn on the battery indicator, for a moment.

Paul A.
Oregon USA
1979 Sprint Veloce (new track car, new car in general)
1958 Chevy Apache truck (for sale)
1950 Pontiac Silver Streak
1948 Chevy Sport Sedan (old PIR track car & driver)

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