TED20 Engine Refurb.

[Mursal]

Does this help?
Hopefully just a coil ........
Good carbon in the middle of the cap to run on the rotor?

 

[DrWazzock]

That's very useful thanks. Those are the resistance values I was looking for. At 5k I am slightly low but not far off.

I have ordered a new coil, a new set of HT leads and connectors, as the old ones look a bit perished. They aren't that expensive if they save hours of messing about and at least eliminate that part of the system. The old lead to the coil is particularly bad, especially where it connects to the coil.

 

[DrWazzock]

Now I am wondering if the coil is connected the right way round. The tractor was converted to negative earth at some stage, and the starter and Dynamo work fine. But the markings on the coil would be wrong? If the coil was originally supplied for positive earth, then the contact breaker would be connected to the + terminal of the coil (?) and the battery supply to the negative terminal. Changing the battery connections to negative earth wouldn't affect the coil as long as nobody changed the connections on the coil over (?) It's a very old coil. I might have got the connections wrong. Can't read any markings at all next to the terminals. What matters more is which is the common terminal and which isn't, not whether it's + or - ve. How did they deal with this when earthing was gradually changed from positive to negative???

 

[Mursal]

From memory the coil will work in both directions, but if the contact breaker is on the positive single wire LV terminal, the spark will not be as strong and it will be jumping in the opposite direction across the plug. So the earth electrode will wear on the plug, rather than the center electrode. But nothing to stress over ..............
Someone else might know more about the changeover .........

 

[DrWazzock]

I think I tried it both ways round and it still worked, but if it's the wrong way round then the HT circuit won't be interrupted by the breaker. This must weaken the spark a fair bit. Still learning.

And I still think there is a thermally activated fault in the coil.

 

[Mursal]

No the HT circuit doesn't need to be broken, its the switching off (breaking) of the primary winding that induces a high voltage in the secondary. If that's any help.
Keep in mind some coils have an air gap in the secondary to force the voltage to jump the gap, this ensures a high voltage spark at the plugs, especially useful when on the TVO.
Some had top coat buttons on the plug leads with the copper through adjacent holes again forcing the spark to jump an air gap, before it reaches the plugs.

 

[Ley253]

Disturbing the cobwebs in my brain once more, but I think ,if you run it till it stops, then check for a spark by putting a plug in the end of the king lead, and flicking the points apart you should be able to narrow down the problem areas. If no spark, but a large one( small one is normal) on the points, the condenser is u/s, no spark with no points arc, then coil and LT supply circuit needs looking at. Leads etc will not cause your problems, it wouldnt run at all if they were the cause.
Ht circuit is never broken by the contact breaker, its induced by the collapse of the LT circuit, which is the domain of the cb points.

 

[Ley253]

No the HT circuit doesn't need to be broken, its the switching off (breaking) of the primary winding that induces a high voltage in the secondary. If that's any help.
Keep in mind some coils have an air gap in the secondary to force the voltage to jump the gap, this ensures a high voltage spark at the plugs, especially useful when on the TVO.
Some had top coat buttons on the plug leads with the copper through adjacent holes again forcing the spark to jump an air gap, before it reaches the plugs.

Oh yes, the shirt button answer to plug oiling! Thats how all those "Ignition supercharges" you could buy worked, but they cost a bit more !

 

[DrWazzock]

So when my new coil arrives and it's marked positive and negative, how do I know which terminal is the common terminal to the LV and HV coil?

Does the supplier of the coil anticipate a negative earthed vehicle nowadays, in which case the distributor should be connected to the -ve terminal, or does he anticipate a positively earthed vehicle in which case the distributor should be connected to the +ve terminal.

I know it doesn't make a lot of difference but is there a convention or a way of finding out?

Why don't they just use SW and CB?

 

[Mursal]

As you say -ve terminal to points .................

Why don't they just use SW and CB?

I d'no ..........

 

[DrWazzock]

And then there is the argument that the spark plug central electrode would be better if it was negative as it is hotter and the electrons would move off it better as in the days of valves in radios that had a heated negative electrode.

I'm well out of my depth now so I'll just connect the negative to the distributor.

 

[DrWazzock]

Here we go.

http://www.mossmotors.com/graphics/products/PDF/Coil Polarity.pdf

 

[Ley253]

So when my new coil arrives and it's marked positive and negative, how do I know which terminal is the common terminal to the LV and HV coil?

Does the supplier of the coil anticipate a negative earthed vehicle nowadays, in which case the distributor should be connected to the -ve terminal, or does he anticipate a positively earthed vehicle in which case the distributor should be connected to the +ve terminal.

I know it doesn't make a lot of difference but is there a convention or a way of finding out?

Why don't they just use SW and CB?

The contact breaker is always on the earth side, so just connect the coil the same as the battery.

 

[DrWazzock]

The contact breaker is always on the earth side, so just connect the coil the same as the battery.

If you do this then the coils will be in series for negative earth and in parallel for positive earth but his might have the beneficial effect of making the spark plug central electrode negative in both cases, ( if the coils are wound the right way).

I'm struggling to get my head round the idea that there is a direct electrical connection between the LV and HV coils. I had been under the mistaken impression that they worked like an isolation transformer.

 

[Mursal]

Yes it does work like an insulation transformer.
But some bright spark decided that joining the winding's would be beneficial to the output.
I would hazard a guess that the old coils found on the tractors when new, were not connected together and worked as you describe.

 

[Ley253]

There will be no LT current flow in the HT coil, the spark gap will act as an open switch.The internal link is just a convenient place to connect the HT return, otherwise you would need an extra terminal. Over the years I have changed many systems to neg earth in order to fit stereos, alternators etc, and all that was done with the coil, was to change the LT connections over.

 

[Mursal]

But if it was connected just for convenience ...................
How is the HT circuit completed with the contact points just after opening?

 

[Ley253]

I suspect that the points will have closed again by the time the HT jumps the plug gap.In fact, if they didnt, the HT return, would jump the points which would lead to a very short life I think! I seem to remember that the HT, earthing through that connection also helps to speed up the flux decay, and so improves the coil output. I dont think its all that new an idea, I had some Runbaken oil filled coils from the early 1930s which must have been wired that way, as they only had the two LT terminations.

 

[DrWazzock]

The points and the condenser are in the HT circuit by virtue of the common connection, but

I'll just connect the -ve to the distributor!

 

[Mursal]

But when the HT spike (the spark) is induced the condenser/capacitor is only in the circuit, so the points are safe.