The hazard flasher is different from the turn signal flasher, although they both operate on the same principle. Each has a heat element and a bimetal strip. Current through the heat element elevates the temperature of the bimetal strip, causing it to bend. On one end of the strip is a set of contacts. When the strip bends, these contacts either open (turn signal flasher) or close (hazard flasher). The current that flows through the heat element also flows through the light bulbs.
Functionally, the differences between the two types is this:
1) A hazard flasher will flash at the same rate regardless of the load, as long as the load doesn't exceed the flasher capacity. One 2 watt bulb will cause the flasher to operate at the same rate as four 21 watt bulbs. The flash rate of a turn signal flasher will vary, depending on the load. The current through one 21 watt bulb is not enough to cause the flasher to work (the lights will stay on), and four 21 watt bulbs will cause the flasher to operate at a high rate (till the flasher burns up).
There is an excellent reason for this difference, and it is not unique to Lucas -- all manufacturers do this. The reason is one of safety. If you turn on your turn signal flashers and one bulb is out, the flasher won't work, giving you notification that something needs to be fixed. OTOH, when you need to use your hazard flasher, you need to use whatever bulbs you have. If one is out, you still want to be able to use the other three. You won't have any indication that a bulb is out, but the next time you use the turn signals, you will.
2) The flash sequence of a hazard flasher starts with an OFF, i.e., OFF--ON--OFF--ON. The flash sequence of a turn signal flasher starts with an ON, i.e., ON--OFF--ON--OFF. This difference in sequence was not a design goal, it just worked out that way.
Electrically, the differences are this:
1) The resistance of the heat element in a hazard flasher is very large compared to the resistance of a light bulb. To the heat element, the light bulb looks like a short to ground. If one bulb looks like a short, placing three more in parallel doesn't really matter: a short is a short! When the heat element raises the temperature of the bimetal strip, the strip bends and the contacts close. The contacts are wired such that they short circuit the heat element when they close. When the heat element is shorted, all current flows through the switch contacts and none through the heat element. As a result, the element cools off and the contacts reopen. Current again flows through the element, and the cycle starts anew. The current that flows through the heat element also flows through the bulbs, but because of the high resistance of the element, the current is much less when the contacts are open than when the contacts are closed -- not enough to light the bulbs.
2) The resistance of the heat element in a turn signal flasher is sized very carefully to the specified bulb wattage for that particular car. If the correct bulbs are used, the current through the element is exactly the right amount to cause the bimetal strip to bend at just the right rate for the flasher. Lower wattage causes the strip to bend too slow, and higher wattage bulbs cause the strip to bend too fast. Just as in the hazard flasher, the current through the heat element is the same current as through the bulbs. The resistance of the element is so low that it offers minimal additional resistance over that provided from the bulbs -- the bulbs light almost as bright as if the element were not there. The strip contacts are wired in series with the bulbs. When the strip bends, the contacts on the strip open, cutting off current flow to the bulbs.
Because the operation of the flashers is dependent on the current flow through them, any change in the voltage applied will also have an effect on the flash rate. An increase in voltage will cause a corresponding increase in the current, which will cause a corresponding increase in the flash rate. A decrease in voltage will have the opposite effect.
Circuit resistance also has an effect on the flasher rate. More resistance reduces the current flow, and less resistance increases it. Barring a short circuit, the only way to reduce resistance in the circuit it to replace the normal bulbs with bulbs of a higher wattage rating. Higher wattage bulbs draw more current than lower wattage bulbs. This is one way of solving a slow turn signal flash rate problem -- replace the 21 watt bulbs installed by the factory in most British cars with 27 watt bulbs used in most American car, bulb # 1157.
Increased resistance is the most common problem, leading to a slow flash rate, or to not flashing at all. Typically, this is caused by bad connections, either in the circuit wiring, internal switch contacts -- particularly the hazard switch, or in the ground connections at the bulbs.
As for testing them, I know of no way other than hooking them up to an appropriate load. For the hazard flasher, any bulb will do, but for the turn signal flasher, the load must consist of the correct number of bulbs of the correct wattage. In my shop, I keep a pair of bulbs handy for this purpose. I have soldered wire leads to them, and wired them in parallel.
Dan Masters danmas@aol.com