Basic Electrics - How Do Car Coils Work?

[malcolm]

Quick question. I should know this but I'm terrible with electrics. I'm setting up car points ignition timing. Does the spark goes off at the moment the points open or at the moment the points close?

And for a bonus point could anyone offer any simple science to help me remember. I remember much better when I understand what's going on.

 

[Paul.]

The spark occurs when the points open, as the magnetic field in the coil collapses when the power is removed,

 

[brightspark]

coil sparks when the points open

 

[shenion]

Yes, points open. Will be very short and weak if the condenser is bad. The condenser and coil make up a tuned circuit causing the current to oscillate.

 

[rtbcomp]

Points open.

The output voltage depends on

1) Primary Current
2) Turns Ratio (primary : secondary)
3) Speed of change of magnetic flux

When the points close the current in the primary builds up slowly due to the self inductance of the primary. A voltage will be induced in the secondary, but not enough to produce a spark.

When the points open the current stops dead - or it would if the points opened quickly enough to prevent an arc. The capacitor prevents such as arc by being a short accross the points, until it starts charging. BUT the capacitor charging prevents the current from stopping as quickly. Thus the value of the cap has to be a compromise between preventing an arc and allow the current to stop quickly.

The capacitor is actually in both the primary and secondary circuits of the coil. In the secondary it forms a tuned circuit which produces a longer spark (timewise).

 

[brightspark]

what would we do without google

 

[rtbcomp]

what would we do without google

Cheeky sod that was from my school days

 

[brightspark]

Cheeky sod that was from my school days

christ u have a good memory

 

[malcolm]

Many thanks - I was sure it was when the points opened. Thanks for the explanations too. I'll try to remember the spark is when the field collapses. If there were no current to start with there would be nothing to collapse.

The thing that's confusing me is the rotor arm is halfway between two posts on the distributor cap when the spark is supposed to go off. That's never going to work. Haven't worked through all the possibilities yet. I've turned the engine over on the starter so I've a good idea of the direction of distributor rotation. I've checked the drive gear is in the right orientation (though that shouldn't influence anything apart from where the distributor sits). I'm just confused.

 

[eil]

Can remember things from 40 yrs ago as if it were yesterday, remembering things from yesterday thats the hard one.

 

[eil]

Plugs out and turn it by hand the rotor arm should match up to your firing order on the distributor cap.

 

[shenion]

Do you have a dwell meter? Really useful when setting up points.

The gap will have a consideral effect on the timing. Using a feeler gauge will get it close but a dwell meter is best.

Normally when the points open it is near the center of the post on the cap. Is this new points? Could be wrong one.

When you say it is centered between posts, is that at the exact point the points open or when the points are centered on the flat?

 

[Charred fingers]

Simple static timing, turn the motor till No1 is at top dead centre, then assuming it hasn't been fitted with an exotic camshaft back it off to 5 degrees BEFORE TDC should be a mark on the bottom pulley. Slacken distributor pinch bolt and turn it with the ignition on and the cap off till the points just open , you'll hear them fizz, lock off pinch bolt . Put No1 lead in the cap socket the rotor arm is facing then insert the rest in the correct firing order according to the direction of rotation

 

[malcolm]

I've got a strobe and a dwell meter. Usually I'll mark the spot where the distributor used to sit, put it back in roughly the same place, then set things up once it's running.

This one is a new build that's been completely apart and I haven't tried to get it running yet. I'm confused that it doesn't seem to set up with static timing in roughly the right place. The rotor arm seems to point halfway between two posts on the distributor when set on static timing, and the distributor is close to it's maximum adjustment. Neither seemed right.

The points could well be wrong - for the new points I bought the part mounted to the base plate was a mirror image of what it should have been. I put that down to the Chinese and ground an enormous pit out of the original and fitted that to get running. That shouldn't affect timing.

It's a good engine so should start whatever. Old fashioned way of twisting the dizzy around until it goes and then doing the timing with a strobe will probably work.

 

[shenion]

That will not really work right. the timing may be right but the dwell will likely be way off. Set the gap using a feeler gauge or dwell meter then rotate distributor to set the timing.

Set the gap. Put engine at 5 BTC, then rotate distributor until the points just open.

Edit" sounds like the gear is one tooth off.

 

[Paul.]

Easy way to set the static timing is with a 12volt test lamp, gap the points with a feeler gauge first, connect the lamp between the contact breaker terminal and earth, CB terminal on the coil is a good place to pick it up, turn the engine by had until no 1 cylinder is approaching TDC on the compression stroke, about 5 deg will do most motors to get them running, turn on the ignition, if the light is on the points are open, so turn the distributor until it goes out, then back until it just comes on, clamp it up job done, I use a 21watt bulb in my test light, with a gentle touch you can get it so that the lamp is just glowing, that's the exact point to lock it up, then check it with a strobe.

 

[R Kraft]

Many thanks - I was sure it was when the points opened. Thanks for the explanations too. I'll try to remember the spark is when the field collapses. If there were no current to start with there would be nothing to collapse.

The thing that's confusing me is the rotor arm is halfway between two posts on the distributor cap when the spark is supposed to go off. That's never going to work. Haven't worked through all the possibilities yet. I've turned the engine over on the starter so I've a good idea of the direction of distributor rotation. I've checked the drive gear is in the right orientation (though that shouldn't influence anything apart from where the distributor sits). I'm just confused.

Do you have the distributor installed a tooth or so off.
Robert

 

[rtbcomp]

Can remember things from 40 yrs ago as if it were yesterday, remembering things from yesterday thats the hard one.

True true true

 

[rtbcomp]

........
The thing that's confusing me is the rotor arm is halfway between two posts on the distributor cap when the spark is supposed to go off. That's never going to work. Haven't worked through all the possibilities yet. I've turned the engine over on the starter so I've a good idea of the direction of distributor rotation. I've checked the drive gear is in the right orientation (though that shouldn't influence anything apart from where the distributor sits). I'm just confused.

The gear & shaft rotate relative to the dizzy body, so if you're a tooth out it's possible to have the points opening at the right time relative to TDC, but the rotor out of position relative to the cap.

Can you remove the dizzy, set the rotor to point at No1 with the points just opening, set the engine to No1 firing position and refit the dizzy without letting the shaft rotate relative to the body?

 

[malcolm]

You're right - popped out this morning and decided the shaft was one tooth out. Corrected it and everything lines up perfectly.

Still maintain that shouldn't make a difference. Must have been having a brain dead moment last night.