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Re: what do I look for?

To:, mgs@Autox.Team.Net
Subject: Re: what do I look for?
Date: Mon, 6 Jul 1998 11:14:00 EDT
In a message dated 98-07-05 22:30:43 EDT, writes:

> when I first got the car the
>  turn signals wouldn't work unless you turned the hazards either off or
>  on, I can't remember which.  Now, the turn signals do not work at all. 
>  When I first hooked them up the left one would flash quickly and the
>  right one wouldn't work at all. 

Becky, In order for the turn signals to work, the hazard switch must be
installed, and in the off position. Power for the turn signals passes through
the hazard switch. When the hazard switch is on, power is cut to the turn
signals to prevent the hazard flasher from back feeding through the turn
signal switch, if it should happen to be left on, and powering every thing
that is powered when the key switch is on. Other wise, your radio, heater fan,
wipers, etc - any thing you left on when you turned the key off - will turn on
and off with the hazard flasher.

Do your lights come on and burn steady, or not come on at all?

If they come on and burn steady, but don't flash, you either have a bad
flasher or some bad connections in the circuit. Most likely, you either have
bad ground connections, or the hazard switch is faulty (the internal
connections are bad). Check and clean all your grounds for the turn signal
bulbs. If that doesn't fix the problem, locate the fuse in the fuse box that
has white wires on one side and green wires on the other. Place a temporary
jumper from the green wire side of this fuse to the green wire on the TS
flasher ( the TS flasher will have a green wire on one terminal and a light
green/brown wire on the other; the hazard flasher will have a brown wire on
one terminal, and a light green/brown wire on the other). If the lights now
flash (with the both the ignition switch and the TS switch on), you have a
problem in the power feed circuit to the flasher. If not, you have a bad
flasher or bad connections in the TS switch and associated wiring.

To see if it is the flasher or the connections, connect another TS bulb to one
of the existing bulbs (one side to the green/white (RH) or green/red (LH) wire
and the other to ground, and try the turn signals again for that side. If they
now flash, your flasher is good, but you have some bad connections. You will
have to trace the wires by hand, checking and cleaning connections as you go,
from the flasher, through the TS switch, to the bulbs, and to ground. If they
don't flash, the flasher is bad (of course, it is possible to have both
problems at the same time - bad flasher AND bad connections).

If it is a problem with the power feed circuit to the flasher, go to the
hazard switch and remove the two green wires. Temporarly connect them
together. If the flashers now work, the hazard switch is bad, and needs to be
taken apart and cleaned. If they don't work, you will have to trace the green
wires from the turn signal flasher back to the fuse box, looking for bad

If the lights don't come on at all, you have some bad or missing connections
in the circuitry, and you will have to trace out the wiring to find and fix
these connections. Follow the same procedure as above to help you find the bad

To help you make sense of all this, I have added a little theory below, a
repost of some comments I made to the Triumph list awhile back.  
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

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.

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
sinal 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 #

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.
I think I've covered everything, and I hope it is of some help.

Dan Masters,
Alcoa, TN

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74

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