Coil question

[DS74BMW]

I have my tank off and was checking/cleaning connections on my coils, and noticed they are both marked "6 Volt". Given some of the things I've seen done by the PO, I had to check into it : I went to BMW Hucky's site [he has photos of every part he sells], and he does list for '70 - '80 models the 6 volt coil.

Why does BMW use a 6-volt coil on a 12V system?

[20774]

Two 6v coils connected in series equals 12v. The 12v line is connected to the +side of one coil, a jumper is connected between the -side of the same coil to the +side of the second coil, and the -side of the second coil eventually finds its way to ground. Actually, the points are in there as well which is what allows 12v or takes away 12v during the running of the engine. When the 12v is suddenly removed, each coil, as essentially the same time, creates a huge amount of energy as the primary field (coils) (with the 12v running through it) collapses and produces the energy in the secondary field (coils).

[DS74BMW]

Two 6v coils connected in series equals 12v. The 12v line is connected to the +side of one coil, a jumper is connected between the -side of the same coil to the +side of the second coil, and the -side of the second coil eventually finds its way to ground. Actually, the points are in there as well which is what allows 12v or takes away 12v during the running of the engine. When the 12v is suddenly removed, each coil, as essentially the same time, creates a huge amount of energy as the primary field (coils) (with the 12v running through it) collapses and produces the energy in the secondary field (coils).

As usual, 20774, you always come through with an excellent answer! My hat is off to you, sir!:beer

[lkchris]

Coils are usually 6V and if not there's a resistor somewhere to drop them to 6V. Points can't take 12V.

[shire2000]

Kurt, that is a perfect explanaition. I knew the answer, just was not sure how to put it into words. Which is the exact opposite of normal. Give me a few wobbly pops and I can put out all kinds of words about things I have no idea about. :drink:drink:drink

[20774]

Give me a few wobbly pops

Dave -

Had to look that one up...never heard that before... :thumb

[AnnapolisAirhead]

Having just gone through my system to trace a failure, I learn exactly what you so eloquently wrote Kurt. Thanks. :thumb

One thing I didn't understand is exactly how a coil works, the two different windings, iron bar...discovered the difference between primaries (feed) and secondary sides (plug wire), and its the collapsing of the field that sends the spark.

[lkchris]

One thing I didn't understand is exactly how a coil works ...

It's pretty interesting, in fact.

IIRC, the process of firing a fuel/air mixture is one of ionization.

It was fun in the old days to hook up a SUN 'scope to an engine and watch the different (yes different) voltages required to fire each cylinder. The 'scope would show the various peaks required.

Big factor in whether more/less voltage is required is the mixture itself, but also significant is compression, i.e. whether cylinder is mechanically healthy and can compress the mixture as designed.

Advent of electronic ignition paralleled shift to leaner, less polluting mixtures.

So, coil doesn't do the same thing every time. It just does enough to jump the plug gap, and then it's over.

[AnnapolisAirhead]

...So, coil doesn't do the same thing every time. It just does enough to jump the plug gap, and then it's over.

I have that same fear about my own coil as I get older. :whistle

Seriously, I mess with old American and German cars from the 1960's to mid 1990's. There is a remarkable difference in the smoothness of the newer ones. Perhaps much of this could be related to your description of the older days. I recall fine tuning a late 60's tbird by setting a drink on the air cleaner until I got it smoothed out. Todays cars (at least the German ones) are so tight, its hard to find the air cleaner. Most newer cars also use high energy ignition wires...makes me wonder if they just blast a bit more than is really needed for each cylinder--I'd hope its more scientific than that.

[lkchris]

Most newer cars also use high energy ignition wires...makes me wonder if they just blast a bit more than is really needed for each cylinder--I'd hope its more scientific than that.

Yes, all the German cars have decals stating the igntion system can kill you.

In the old days, my Dad had a mechanic that would simply grab spark plugs to find where the miss was. (This on Ford tractors).

It is pretty scientifically challenging to get today's lean mixtures to light.

[seniorasi]

Having just gone through my system to trace a failure, I learn exactly what you so eloquently wrote Kurt. Thanks. :thumb

One thing I didn't understand is exactly how a coil works, the two different windings, iron bar...discovered the difference between primaries (feed) and secondary sides (plug wire), and its the collapsing of the field that sends the spark.

Actually...the collapsing magnetic field induces a voltage into the secondary windings which then feed the plug wire which feeds the spark plug. It is the size of the plug gap and the resistance there in that creates the spark....that may be what you just said...I'm getting old.

[kstoo]

Coils are usually 6V and if not there's a resistor somewhere to drop them to 6V. Points can't take 12V.


There are 12vdc passing through my contact points.
The coils are connected in series; each is essentially a resistor, if you must, and each drops 6vdc.
12vdc from the battery passes through coil 1, coil 2 and goes to ground through the contact points with the condenser squashing the arc.
I am pretty sure that is what 20774 said in the first place ... just don't get mislead by misinformation. Look it up, if you must.