Dumb question

[bulfin]

Hi,

Here is the dumb question of the day: Why does the wire from the
ignition switch to the coil run through the tach?

Thanks,

Bob
1969 Elan S4

[Elan45]

To make the tach work. That is where the electrical pulses come from.

Roger

[Frank Howard]

Each time the points open, the primary circuit to the coil is interrupted and
every time the circuit is interrupted, the tach is notified via this wire.
The more frequently it is interrupted, the higher the RPMs and the tach
converts the frequency of these interruptions into mechanical movement of the tach
needle.

Frank Howard
'71 S4 SE
Minnesota

[bulfin]

Thanks for the reply Roger and Frank.
I understand the tach needs a signal from the coil,
but why from the ignition switch wire?
Is the wire just serving double duty?

Bob

-

[Frank Howard]

Bob,

Your question is not a dumb one. The coil is fed 12V through the ignition
switch. The power goes to the ignition switch, through the tach, through the
ballast resistor through the coil, through the points, and to ground. If you
disconnect the ignition switch, the coil receives no power and the engine stops.
That's what happens when you turn the key to off.

There's nothing quite as satisfying as giving a lecture to a college
professor!

Frank Howard
'71 S4 SE
Minnesota

[bulfin]

OK, here is why I am asking. I was thinking about doing whatever posible
to decrease the current through the igntion switch (IS). One thought was
to use a relay at the coil, so power comes from the battery cable, but
the IS only supplies enough to trip the relay. Then I thought about the
tach. If I get rid of it (joking) no problem. Otherwise, how do I get a
signal to it. Just running a wire to the tach isn't enough, since those
electrons have to go somewhere and the tach doesn't use them, just
counts them as they are passing by.

As far as lectures to a college professor, it happens to me all the
time, since my wife is also a college professor, and gives me plenty of
lectures.

Thanks for the info.

Bob

[BillGavin]

Bob, I think a lot of modern cars do just that. There should be no
problems with the tach, you'd just be drawing current from the relay
rather than from the switch. A simpler solution might be to use the
relay only for the switched accessories like the heater fan and wipers,
which are the heavy draws. The fuel pump (if electric), tach, and ignition
don't draw huge currents.

I don't think a relay at the coil would be very helpful, the relay draw might
exceed the draw of the coil. If you're running a killer electronic ignition,
they often have a separate supply, and just use the coil lead as a trigger.

A rummage under the bonnet of a modern car in a scrap yard would
probably yield a nice fuse box and relay panel, and a lot of the other bits
needed.

- Bill

[Frank Howard]

Professor,

I suppose you could install a relay that sends power to through the tach, the
ballast resistor, the coil, the points, and to ground, but I don't believe it
would do much besides complicate the ignition system. I think you may be
under the impression that there is a lot of power going through the ignition
switch, but there really is not. The power that goes through the switch powers
the coil. When the coil is turned off, the electrical field in the coil
collapses and that creates a powerful charge that goes through the secondary (fat)
wire coming out of the center of the coil which goes through the distributor
cap, to the rotor, through one of the spark plug leads to the plug where it
creates a spark as it makes a leap from the center electrode on the plug through
the spark plug gap to ground. The powerful part of the ignition system (fat
wire from the coil, distributor cap, rotor, plug wires, plugs) is called the
secondary system. That system is not connected directly to ignition switch.

OK class, now put down your books because there is going to be a pop quiz...

Frank Howard
'71 S4 SE
Minnesota

[Frank Howard]

Bill,

I've tried this. Unfortunately it doesn't work. The relay/fuse boxes in
modern cars are model specific. Certain fuses are connected internally to
certain relays through a series of metal conductor plates that are built into the
boxes. I picked one up at a junk yard and proceeded to figure out which relay
was connected to which fuse using a test light. Some of the relays were
connected to several fuses. Some were connected to one, or two. It was very
confusing. I made a diagram of what was connected to what, then I broke open the
box to find the series of metal conductor plates and every one was as I had
predicted. Kind of a self education. I would up yanking out the relays and
fuses, then throwing away the box.

If you want to build your own fuse box, you must purchase one from a supplier
that handles stand alone boxes like Waytek Wire. On the other hand, you can
use a relay panel from a junk yard car. I found that the Nissen cars have the
most convenient relay boxes. The relay sockets can be moved around. The
boxes come in models that hold 4, 5, 6, 8, or 9 relays.

Frank Howard
'71 S4 SE
Minnesota

[bulfin]

Thanks to all or the responses. I was thinking the wire to the coil had
more current than it does, so I will leave the system as-is. It is great
to have a plcae lke this to float ideas around and get such good
responses.

Bob

[poiuyt]

If you are still using a Lucas geherator, do nothing. The current
through the ignition switch will reduce itself over time!

Steve B

[poiuyt]

Does anyone out there know what the current draw is for the positive
side of the ignition (points closed, of course)?

Steve B

[poiuyt]

I would suggest you buy an after market stand alone fuse panel - I've
seen them up to 20 fuses - and wire everything separately. This is
what I am going to do if the Elan ever needs a body-off restoration
or frame change.

I've had three Fiats and every one had separate fuses for everything,
including a fuse for each headlight filament, etc. If a fuse blows
you know where the trouble is.

I think these British cars will be smoking ruins before one of the
two large fuses blow.

Steve B.

[davidallen]

Hi,

There was a (slightly) later design of Lucas Tach which takes the signal
directly from the coil.

It was fitted to Triumphs including the Dolomite and was said to be more
reliable and work better with electronic ignition.

It looks identical to the Elan Tach

David

_____

[denicholls2]

Steve B wrote:

I think these British cars will be smoking ruins before one of the
two large fuses blow.

-------------------------

Fortunately, Chunky foresaw this risk and used undersized wire in the
majority of the Lotus applications.

Modern automotive technicians now refer to these designs as "fusible
links". Yet another example of the tremendous amount of foresight the
man had.



-- Doug