Extra Ignition power wire

[fattogatto]

1967 UK version S3. New dash/front wiring harness. A good 95% of the wiring seems to follow the S2/S3 wiring diagrams available. However, it's that last 5% that is worrisome. In particular, I have a (relatively) long white wire with bullet connector exiting the main harness next to the white wire that goes to the tachometer and the coil. I have continuity from tachometer to coil (white wire) and I have a short white wire going to the ignition warning light. And, there is continuity from the ignition switch to the white wire going to the fuse block.

The extra white wire in question is hot through the "run" position (#2) on the ignition switch. Would anyone have any ideas as to the possible usage for this extra white switched wire? The car does not have emergency flashers. Possibly an extra wire not used in my application?

Thanks.

[billwill]

Probably just a spare to use for whatever you want it to do, such as run an electric fuel pump.

Hang on though, you haven't mentioned a white wire which should go from the ignition switch to the tachometer, where it dives inside, goes once or twice around a transformer cor and then comes out to become the white wire that goes from the tacho to the ignition coil.


If that is not connected your car won't start or run.

Diagram corrected, see a message below.

[Andy8421]

While a bit of a picky point, if these things aren't corrected then they perpetuate. Bill's diagram is correct for the wiring that the OP cares about, but the innards of the coil are wrong. Ignition coils are 'auto transformers', the secondary shares the primary, and the secondary isn't earthed. Apart from radio suppression, there is no need to earth the case of a coil.

Good description and correct diagram here.

https://mgaguru.com/mgtech/ignition/ig104.htm

[pharriso]

Hi FattoGatto, take a look at the attached diagram & print it out to refer to :

Now working through your original post:

" In particular, I have a (relatively) long white wire with bullet connector exiting the main harness next to the white wire that goes to the tachometer and the coil."

We'll come back to this...

"I have continuity from tachometer to coil (white wire)"
Sounds like wire #4 in the diagram

"and I have a short white wire going to the ignition warning light. "
That's wire #3

"And, there is continuity from the ignition switch to the white wire going to the fuse block."
That's wire #1

"The extra white wire in question is hot through the "run" position (#2) on the ignition switch. Would anyone have any ideas as to the possible usage for this extra white switched wire?"

OK...so you're missing wire #2 which should go from the ignition switch to Tacho? This is actually the power feed to the ignition circuit. Make sense?

"The car does not have emergency flashers. Possibly an extra wire not used in my application?"

Lotus were pretty parsimonious in their wiring

p.s. Just seen BillWill's posting "Hang on though, you haven't mentioned a white wire which should go from the ignition switch to the tachometer, where it dives inside, goes once or twice around a transformer cor and then comes out to become the white wire that goes from the tacho to the ignition coil." We agree

[fattogatto]

My description was unclear. I do have all 4 white wIres as required. A sudden thought struck this morning. Radio power.

[oldelanman]

My description was unclear. I do have all 4 white wIres as required. A sudden thought struck this morning. Radio power.

Charlie,
You could certainly use your spare white wire to feed the radio but it would obviously only work with the engine running, it's normally fed via a green wire from terminal #4 (aux) of the ignition switch.

Phil,
The ignition switch terminal numbering on your circuit diagram differs from the wiring diagram in the workshop manual and the Lucas notation.

[fattogatto]

Thanks. My ignition switch is like the one shown in the first schematic, not the second.

[pharriso]

Phil,
The ignition switch terminal numbering on your circuit diagram differs from the wiring diagram in the workshop manual and the Lucas notation.

Yup, but not what's actually in our cars. SA157 ignition switches have terminals 1, 2, 3 & 5, there is no "4" terminal.

See https://lotuselan.net/forums/viewtopic.php?f=38&t=24891

Maybe earlier cars had a different ignition switch?

[oldelanman]

Phil,
The ignition switch terminal numbering on your circuit diagram differs from the wiring diagram in the workshop manual and the Lucas notation.

Yup, but not what's actually in our cars. SA157 ignition switches have terminals 1, 2, 3 & 5, there is no "4" terminal.

See https://lotuselan.net/forums/viewtopic.php?f=38&t=24891

Maybe earlier cars had a different ignition switch?

The change may be the introduction of the steering lock.

Earlier cars with the dash mounted switch have the SA47 type - my S4 has it - and the connections and terminal numbering corresponds with the workshop manual. I guess the manual was never updated.

Charlie has a UK spec S3 which probably won't have a steering lock so I suspect he may have the SA47 switch.

[billwill]

While a bit of a picky point, if these things aren't corrected then they perpetuate. Bill's diagram is correct for the wiring that the OP cares about, but the innards of the coil are wrong. Ignition coils are 'auto transformers', the secondary shares the primary, and the secondary isn't earthed. Apart from radio suppression, there is no need to earth the case of a coil.

Good description and correct diagram here.

https://mgaguru.com/mgtech/ignition/ig104.htm

Yes, it's an old diagram, which I have corrected in it's latest version on another topic.
viewtopic.php?f=38&t=48627

With only 3 terminals to an ignition coil, it MUST HAVE the condensor/Capacitor to make anything significant happen.



The coil inductance plus the capacitor form a series resonant circuit to get a rapid succession of sparks. Contrary to most common expectations, there isn't just one single long spark, there are a series of very rapid short-duration sparks. I published some pages from an electronics magazine somewhere on this forum some time ago, which shows the oscilloscope trace of the high-voltage spark line.


Here's a corrected diagram sketch.

Classic car ignition circuit, with no ballast resistor.

[Craven]

While a bit of a picky point, if these things aren't corrected then they perpetuate. Bill's diagram is correct for the wiring that the OP cares about, but the innards of the coil are wrong. Ignition coils are 'auto transformers', the secondary shares the primary, and the secondary isn't earthed. Apart from radio suppression, there is no need to earth the case of a coil.

Good description and correct diagram here.

https://mgaguru.com/mgtech/ignition/ig104.htm

Andy8421
I have read an ignition coil described as an autotransformer before but I think this is misleading although technically a case can be made.
A quote,
• An ignition coil has two separate windings, a primary The primary winding is made of thick copper wire with approx. 200 windings (diameter approx. 0.75 mm2)
• The secondary winding is made of thin copper wire with approx. 20,000 windings (diameter approx. 0.063 mm2)
The fact that the windings have one end connected together does not follow the operation of a transformer with a single winding tapped at various points, an autotransformer, a part of the load current is obtained directly from the supply and the remaining part is obtained by transformer action.

[fattogatto]

I see my confusion.

My tach only has two white wires coming out the back. One is piggy backed to the ignition light and it goes to the ignition switch (position 2)

I only have one other white wire which disappears into the potted input on the tach which also comes from the ignition switch (position 2). According to the descriptions I have there should be another white wire exiting the tach and going to the coil. The extra white wire I have coming from the harness has continuity to the coil.

Can I simply splice that wire into the wire coming from the ignition switch? Or, should there be another white wire exiting the tach?

The tach configuration is as I received it from Nisonger.

Please forgive my confusion.

[Andy8421]

While a bit of a picky point, if these things aren't corrected then they perpetuate. Bill's diagram is correct for the wiring that the OP cares about, but the innards of the coil are wrong. Ignition coils are 'auto transformers', the secondary shares the primary, and the secondary isn't earthed. Apart from radio suppression, there is no need to earth the case of a coil.

Good description and correct diagram here.

https://mgaguru.com/mgtech/ignition/ig104.htm

Andy8421
I have read an ignition coil described as an autotransformer before but I think this is misleading although technically a case can be made.
A quote,
• An ignition coil has two separate windings, a primary The primary winding is made of thick copper wire with approx. 200 windings (diameter approx. 0.75 mm2)
• The secondary winding is made of thin copper wire with approx. 20,000 windings (diameter approx. 0.063 mm2)
The fact that the windings have one end connected together does not follow the operation of a transformer with a single winding tapped at various points, an autotransformer, a part of the load current is obtained directly from the supply and the remaining part is obtained by transformer action.

I would agree that an ignition coil is a special case of an autotransformer, but I think the description is valid - the primary windings are shared by the primary circuit and the secondary circuit. Lucas also thought it was an auto transformer. Bottom of page 10:

http://www.da7c.co.uk/History%20Section/LUCAS%20ACHIVE/lucas%20coil%20course%203.pdf

[pharriso]

I see my confusion.

My tach only has two white wires coming out the back. One is piggy backed to the ignition light and it goes to the ignition switch (position 2)

I only have one other white wire which disappears into the potted input on the tach which also comes from the ignition switch (position 2). According to the descriptions I have there should be another white wire exiting the tach and going to the coil. The extra white wire I have coming from the harness has continuity to the coil.

Can I simply splice that wire into the wire coming from the ignition switch? Or, should there be another white wire exiting the tach?

The tach configuration is as I received it from Nisonger.

Please forgive my confusion.

Did Nisonger convert your Tacho from RVI to RVC?

[fattogatto]

I could answer if I knew what RCI and RVC mean. The car had been converted to negative ground when I got it. I did not request anything special from them.