I have an '85 fxwg.It was converted over to points before I got this......My questions are, With the ignition switch on should I have power into the coil? Second, The wire running from the coil to the points should this be hot with the ignition on, if so with the points open or closed?I suspect that it should pulsemy test light as icrank the ignition....right?I have no spark at the plugs and trying figure out a game plan to fix this darn thing...I have replaced the points and condensor, The old points looked ok but don"t know how to check a condensor....so coil or ignition switch is next from what I see so far..... Anyone got any other ideas?


Thanks Ben

 

you should have power on both sides of the coil when the ign is on and the points wire disconnected...... if you replaced the condenser, assume its good. if you have all that, start moving back from the coil, to see where the power stopped.

 

The points are wired to the negative side of the coil. Check and see what the voltage to the + side of the coil is. Should be battery voltage with very little voltage drop.
All the points do is turn the coil on and off rapidly to allow the magnetic field in the coil to collapse. When you enegize a coil of wire you get a magnet. When you take either the voltage away or the ground which is the case with a points type ignition,the magnetic field in the coil collapses. When this happens the voltage is increased to thousands of volts and this voltage is routed out of the coil through the plug wire to the plug. This is how the ignition coil works on your bike ,car, lawnmower etc...
With the points open (air gap)there is no path to ground (through the points)so you will read voltage at the wire from the coil to the points. Close the points (contacts touching) the voltage should go away. Once again energizing the coil of wire in the coil itself making the electromagnet. The reason the voltage goes away is because the coil of wire(electromagnet) consumesthe voltage. This is normal.
All the condensor does is help to prevent the points from burning by reducing the spark you see when the points open.
And yes your test light should pulse as long as you ground the test light clip and touch the probe to the post at the points. Open the points light comes on, close the points light goes out.
Hope this helps. Keep us posted and if you need more help you can PM me too.

 

That was all well-enough said except the part about why there's no voltage present at the wire from the points to the coil when the points are closed. It's not because the coil is consuming the voltage, it's because you're trying to measure voltage across a closed switch. I'm sure if your volt meter had high enough resolution you'd read some voltage drop across the closed points. But you'd have to have a lab-quality meter for that.

When the switch is open and the coil is connected, you'll be reading the potential available for use when the switch goes closed.

 

As long as the circuit is healthy, the Load (in this case the coil) consumes the voltage.

 

Beep, beep...back up the truck.
The coil is just a step-up transformer. The ignition supplies 12 V to the primary circuit of the coil (from the battery) and the biiiiig voltage from the secondary circuit of the coil goes to ground across the sparkplug gap everytime the points close. (making a complete circuit of the coil's secondary through the points to the plug wire, to the plug, across the plug gap to ground.) The timing is set up to have the points close every time the appropriate, corresponding cylinder has been charged with the air/fuel mix and is looking for it's spark.

CN

 

MixedNuts,

I really didn't want to pick apart what you'd said. But there are a couple of things...

Agreed.

Maybe too brief, but agreed.

First sentence: agreed. Second sentence, well, okay. The voltage measured across the points does "go away" when they close to complete the primary coil winding circuit. Third sentence (and fourth): negative. The reason the voltage, as being measured across the points contacts, "goes away" is not because it's being consumed, but because you've merely shorted your test leads together with the closed points.

Actually, the (in modern terms) "capacitor" does much more than this in the circuit. How well will the ignition work without the cap? Not very. But it is true that a cap across switch contacts will function to suppress noisy switching harmonics and to soften the blow, so to speak, of the current start/stop as the switch contacts operate.

Yes, but for the reason given earlier. With the ignition on and the points open, there will be the full battery voltage potential across the contacts. With the points closed there will be (effectively) zero potential across the contacts. Potential is what lights your test light. It has nothing to do with the voltage being "consumed" by the coil primary winding.

I'm more used to working with large power transformers (if your three-car garage was three stories tall it might contain one) and engine ignition transformers share only the most basic operating principle with them. That is, when the magnetic field surrounding the primary winding collapses, a voltage is induced in the secondary winding by that collapsing magnetic field. The same actually happens in reverse as the magnetic field rises but for the type of transformer used as the ignition coil the way the field collapses is the useful action.

Refer to http://en.wikipedia.org/wiki/Ignitio...timed_ignition at your leisure for a fair overview.

 

http://autorepair.about.com/cs/gener.../aa052502a.htm
Check this out.

 

Anyhow, Ben did you get your bike running?

 

That was a fair description of the system. The wikipedia article is much more accurate, however. Especially in terms of the part the capacitor plays in the circuit.