The drawing below shows the electronic circuit inside a typical
3-terminal flasher. Note that there were many variations of this
circuit design over the years.
2 transistors. Q1 and Q2, form a multivibrator which drives the Flash
relay in the collector circuit of Q1. A small voltage is always
present on 49a by way of the 2K resistor (the Load Sense relay has only
a few turns of very heavy wire for its coil). When the Turn
Indicator signal switch is closed, the turn lamps are connected from
Ground to Terminal #49a. Their low resistance drags the voltage
at 49a down to nearly zero which, in turn, pulls the base of Q2
lower. Q2 is a PNP transistor, so more current flows
emitter-to-collector at this time. That current passes into the
base of Q1, turning it fully on and energizing the relay, causing the
turn signals to immediately light.
The 2K resistor is now pulled up to the full 12V level which reduces
the base drive available to Q2. However, the 100 uf timing
capacitor doesn't permit Q2 to shut off immediately. Current
continues to flow until the capacitor has charged via the 800 Ohm
resistor. At that time, Q2 turns off and allows Q1 to drop out
the relay, turning off the lamps. This cycle repeats, with the
capacitor charging and discharging.
The Load Sense relay "measures" the lamp current. If one lamp
burns out, the relay will not close its contacts, leaving the 200 Ohm
resistor in the emitter circuit of Q2. This resistor and the 470
Ohm resistor form a voltage divider supplying the emitter of Q2.
With the relay contacts open, the emitter doesn't pull all the way up
to 12V and the capacitor takes less time to reach the cutoff point for
Q2. This causes a rapid flashing of the signal lamps, alerting
the driver to a lamp malfunction.
The dash/speedo signal lamp for the Indicators is connected from +12V
to Terminal #49a. The resistance of the small dash bulb
also helps to maintain voltage on 49a until the Turn Indicator switch
is closed. As soon as the switch is closed, 49a goes to Ground
level, causing the dash signal to light. A fraction of a second
later, the flasher relay applies +12V to 49a, lighting the Turn
Indicator bulbs. At this point, the dash signal bulb has +12V on both
sides of the bulb; no current can flow so the signal goes out.
This is why the dash signal flashes opposite
to the Turn Indicators.