Flasher Operation

By Norman Nock

Originally published in the Austin-Healey Magazine


The type FL3 flasher commonly found in Healeys is basically a thermal switch that interrupts the current at a pre-determined rate per minute, switching a lamp (a directional signal) on and off. Typical flash rate is between 60 & 120 cycles per second.


This type of flasher consists of a piece of resistance wire anchored at the bottom end to the flasher frame and to a piece of spring steel at the top, so that the wire is always kept under tension. The spring is shaped so that when it is bent, a "buckling" or clicking action will take place in the center. One of a pair of contacts is mounted at this point.


These contacts are normally open. When current passes through the resistance wire, it heats up and expands. The top of the spring initially moves away from the wire, as shown in Figure 1. Because of the click action of the steel spring, however, the center of the spring will move in the opposite direction to the top, causing the contacts to close. Current now flows through the contacts to the bulb, by passing the resistance wire, which cools and contracts, causing the contact to separate, shutting off the lamp. Current then passes back through the resistance wire, the spring moves, the contacts close and the cycle repeats itself.


Flashers also contain a resistance in series with the hot wire as shown in figure 2. This resistance limits the amount of current passing through the flasher bulbs to below that at which they will light, preventing any sudden surge of current from damaging the wire when the indicators are switched on.


Many flashers also make provision for a dashboard warning light, using what is called a Pilot Light Coil. When the main contacts close, current to the indicator light now passes through this coil. When the coil is energized, it moves a small armature that closes a set of contacts in the base of the unit. This completes the lead to the dashboard warning light, causing it to move in sympathy with the indicators. The Pilot Coil is designed so that it will operate the pilot light contacts when 36 to 40 watts of power are consumed. Should the current flow, for example is one of the indicator bulbs fails, the contacts will not close and the pilot light will not flash, warning the driver that one of his directionals is not working.


Three terminals marked “B”, “P” and “L” are brought out to terminals at the base of the flasher. The one marked “B” is connected to the battery via fuse A$. The middle “P” terminal is connected to the dashboard indicator light. The “L” terminal feeds to the lights through the direction indicator relay.


Fitting the flasher


As in the case with most thermal switches, a flasher is susceptible to damage if any dust or moisture gets inside. Additionally, the temperature must be kept uniform to maintain the correct flash rate. For these reasons the unit is hermetically sealed in an aluminum canister. Adjusting the flasher is a tricky proposition that can really only be done at an instrument repair shop. Replacement might be the best solution.


To insure satisfactory performance, ensure that:

  1. The flasher is mounted vertically with the terminals at the bottom. A position free from engine vibration should be chosen.

  2. Use only bulbs with the correct wattage ratings.

  3. No more that 4 amps should pass through the flasher at 12 volts (48 watts). Damage may result if this level is exceeded.

  4. Less than 36 watts will not operate the dashboard-warning lamp.


To test the unit, apply 12 volts to the “B” terminal and connect a pair of 21 watt lamps to the “L” terminal. Connect a 12 volt 2.2 watt ignition-warning lamp to the “P” terminal. You can check the flash rate of 60-120 per minute by counting the flashes over a 10-second interval and multiplying by six. Finally, care must be taken with this lightly built unit both in transit and in storage to prevent damage.


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