In a message dated 5/8/98 8:04:47 AM Eastern Daylight Time,
RWYGONIK@grove.iup.edu writes:
> My turn signals are molasses slow.
Ray,
The following is a response I posted to the Triumph list to a similar
question. Perhaps it will be of interest to you as well.
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> Can anyone enlighten me as to the function of a flasher module?
Perhaps I can. You are right, 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 minimumal 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. Any added resistance in
the circuit, from bad connections, bad grounds, corrosion, etc, will reduce
the current, slowing the flash rate. When one bulb burns out, the current is
halved, and the flash rate drops to zero.
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.
I hope this wasn't more than you wanted to know!
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Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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