General question about distributor points...

[Arthurdaily]

Okay, so I know the basics of the electrical system on the engine now - but have a quick question about distributor points. Do the points OPEN at the point where the rotor arm passes the spark plug connection 'nipple' on the distributor cap? If so why? Or are they SHUT at that point? In other words what is the relationship between the points opening / closing and the rotor arm 'distribution' of power?

[bmcecosse]

The spark is created when the points open - releasing the energy stored in the coil primary into the secondary and causing the spark. However the condenser also stores energy - it's not just there to quench points burn - without a condenser the system doesn't work...

[Arthurdaily]

Right, so there is no spark between the points? The points opening thus disrupts the earth signal from the engine, causing the coil to 'fire', is that right?

[mogbob]

Yes the low tension side of the distributor electric circuit is broken by the points opening, allowing the high tension side to fire.

The coil discharges it's stored energy, down the HT king lead down through the distributor cap, via the carbon brush/ pip to the rotor arm and is directed to the relevant spark plug, via the plug HT lead, it is pointing to.

Rotor arm rotation continues..points close, low tension now a complete circuit, activates the coil again,next cam on the shaft breaks the circuit ( points open ) and the next spark plug fires, etc, etc
Bob

[Arthurdaily]

Ah right, so - with the points CLOSED the low tension wire from the distributor is EARTH, and it meets the positive from the other side of the coil and basically energises the coil? THEN the cam opens the points breaking the circuit - the wire from the coil to the distributor is then LIVE (hence you can get a bulb to light up attaching it to an earth)... then the coil fires (I don't understand how that happens when the coil suddenly doesn't have an earth?? But probably don't need to).

Brilliant, I'll be an auto electrician before you know it!!!

[bmcecosse]

Skips over the role of the condenser - but otherwise quite good..... http://auto.howstuffworks.com/ignition-system.htm

[Arthurdaily]

Really good link, thanks! One little thing, having read some sections of the manual. It says to test the low tension circuit attach a test bulb between the low tension wire on the distributor and the CB side of the coil... now surely by adding the wire / bulb you will be REFORMING a negative connection to the coil and thus the bulb wouldn't glow anyway? I'm getting a headache - I think I'll draw a diagram for myself...

[bmcecosse]

The bulb is very high resistance.

[Arthurdaily]

Right, so - if I now finally understand (ish) is this right:

If you attach a bulb between (and instead of) the CB side of the coil and the distributor low tension it will LIGHT UP when the points are CLOSED, but NOT when open as it takes the energy from + & - before the coil does so to speak.

However, if you attach a Bulb between either the Battery and CB sides of the coil, OR the distributor low tension and general engine earth then it will be opposite - .i.e. the bulb will light ONLY when the points are open - as in these two instances the coil is allowing the positive current (my car is positive earth) to pass as it's not energising.

A simple no or yes would be great!
Thank you

I'm going to ask an electronics teacher about this...

[bmcecosse]

Yes - but my head is spinning now...you are over thinking it... And what's the point of the bulb anyway??? Just get it running - then set the dizzy by road test.

[Arthurdaily]

Haha BMC - the point? Actually this whole question for me now is NOTHING to do with getting the Morris running well or anything - I'm just extremely interested in the ignition system and why wires are one moment positive, then negative etc...

Call it academic interest.

[bmcecosse]

Wires are never positive and then negative - except in an AC system of course - ie at home !! Not in a car - not yet anyway - although I guess AC may come some day soon.

[MarkyB]

Lots of zeros and ones in the wires these days!

[Arthurdaily]

ha ha... these electrical thingies are difficult to understand - I have found an original Lucas training manual on-line... I'm going to study study study...

[katy]

Wires are never positive and then negative

There is some exceptions to that rule.
On a negatively earthed vehicle, with the key on, the LT wire between the coil and the distributor will be positive when the points are open and negative when the points are closed.
Also, the wires to the lights will be negative when the switch is off and positive when the switch is on.
I hope this makes sense.

[rayofleamington]

Not in a car - not yet anyway - although I guess AC may come some day soon.

It seems you're already many years out of date!
Hybrids and electric vehicles are not exactly 'unavailable'...
The days of the DC-motor milk floats are long gone (unless you're one of the US DIY fanatics making home-brew EV's)
AC - especially 3 phase - is here to stay.

[bmcecosse]

Yes but that's for power. Not yet for normal car systems, but it will come.