Morris Minor Owners Club

Hi

Coil polarity is pretty important. When it's incorrect, as much as 40% higher voltage is required to fire the sparkplug, according to an article I read 40 years ago from Champion Sparplugs.

You can check the polarity with a coil tester or a high tension Voltmeter, but a simple way is to disconnect one plug lead while the engine's running and hold it about 1/4" from the plug.
Hold the point of a wooden pencil in the gap between - if the spark flares or flutters and has a slight orange tint on the plug side of the pencil lead the polarity is correct. If the spark flares on the opposite side reverse the coil connections.

Easy ain't it!

Cheers.

When you say coil polarity is important, does that mean simply that it is important not to muddle it up like I did so that you are driving the HT through the primary as well as the secondary, which is a stupid thing to do, or is actually important which is the anode and cathode on the plug, which would stretch credibility a bit more. I mean, a spark is a spark whether it flows uphill or downhill? Or am I missing a bit of physics somewhere?

Or is it that the earthed electrode is able to run a lot cooler than the centre electrode which has to be sheathed in insulation. There could be something in that, the hotter electrode would erode more easily, so you need to make it the.....um...er....my physics just ran out again. Just like the points pit one way on a positive earth car and the other way on a negative earth.

Those 42 volt systems, Cam.....why? Is it to do with less susceptibility to dirty contacts etc? Don't you start to run into problems of voltage insulation rather than current carrying capacity? Are these going to be based on a power bus with a separate signal cable so that devices can switch on/off locally rather than having loads of spaghetti power cables? Are they going to use 42 volt batteries or lower voltage and step it up and down with invertors?

Coil polarity is essential IF (like ours) the primary and secondary coils are electricaly linked to form the earth.

They both HAVE to be connected to the same common point otherwise the HT side will not complete it's circuit and there will be no spark!

A lot of people get confused with HT but it is still voltage, just higher.

If you increase the spark gap too much for the potential to overcome, then it will not spark. The same is true for LT but the gap is MUCH smaller. This is why 12v things spark when you are pulling the contacts away, it sparks when there is a small gap. The 'power' of the spark is determined by the amount of current flowing when you disconnect it.

It does not matter which way around the polarity of the spark is, it is still a spark, this is how positive earth cars converted to neg earth still work!!
BUT like you say, it will erode points, plugs etc, the opposite side more than the other as the electrons are streaming the opposite way.

The 42 volt system is being introduced because the 12volt system cannot provide enough power (volts x current) for new car requirements as more people want electric heating devices (seats, mirrors, steering wheels etc), but they also want instant electric heating without waiting for the engine to warm up!!!

increasing the voltage means that the power cables can be smaller for the same power (smaller current).

42 was chosen as under fault conditions it can rise to 60, which is the safe limit for human exposure.

The insulation at 42 volts has pretty much been perfected for a number of years and so is not a problem.

Yes, these are going to be based on a power bus with a separate signal cable so that devices can switch on/off locally rather than having loads of spaghetti power cables.

They will be using 42 volt batteries and just use 42 volt devices, so no need to reduce the voltage. But at the start there will be hybrid systems comprising of a 42v and a 12v battery with a common earth, until all the components have been redesigned to operate at 42 volts (a large task given the size of the motor industry).

Hi

I would not get too emotional about coil polarity, your car will still run even if the coil is wired incorrectly. A friend of mine has had an MGTD for over 35 years and it has always been a good starter and run very well. I rewired the car when he got it, and nothing has been changed since. Last year he decided he needed a radio/ CD player, so I wired the thing up and of course had to change the car to -ve earth. It was then that I discovered the Gold Lucas "Sports Coil" we had fitted in 1966 had the polarity incorrect!

You can take some things too seriously.

By the way, he found the radio a distraction, and couldn't hear it over the bark of the exhaust and wind noise, so it now sits on a shelf in the workshop - best place for a car radio in my opinion.


Cheers

Well, there you go!

That's the good thing about forums....everybody has got an opinion.

It would be interesting to see how the coil is working if the common earth is wired internally.

And don't worry I am not getting serious (I never get serious - that's just the problem!!), but if you can provide an explanation of the operation I would be genuinely interested.

It is always useful to add knowledge.

Thanks,

Cam wrote: ↑Tue Nov 19, 2002 10:05 pm Willie,

The simple ones are usually the hardest to find!!

And often the most satisfying to fix!!

Cam.

Spot on. This was the issue with my 1955 803.

Cutting out randomly, usually at a set of lights or as I slow down for a junction. Decided to take it off the road for a year or so and have decided to sell… managed to persuade the Mrs to give me a Saturday to give it an oil change and service before selling….. randomly came across a loose tightening nut on the coil. All sorted now, and much to her dismay i’m not selling!!

A response today to a 22 year old post, surely a record