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MG Midget and Sprite Technical - what kills the secondary winding in a coil?

Evening all

NMC, but most have the same coil!

The big brown P6 has thrown a(nother) wobbler - eaten another coil. Ballasted or non-ballasted it makes no difference, it eats them all. Om nom nom. This one lasted 4 months.

They're all dead on the secondary side - the bit that fires the big voltage. Is there any reason why that might be happening a lot? Plugs are the right ones, gapped properly and also only 4 months old, leads are new-ish expensive magnecore jobbies.

It has a dual points distributor which apparently gives more dwell time - is this a likely culprit?

Any light that could be shed would be handy :)
Rob Armstrong

Rob. Dwell is a function of how long the primary windings are energized. When they are no longer energized, the magnetic field collapses which induces the voltage in the secondary windings. So, the dual points dizzy should have no effect on the coil other than giving the primary windings more time to charge up. Some of the points replacement systems offer long dwell time than the points type distributors and people are not reporting coil problems as a result.

The most common cause of shorted secondary windings is over tightening of the coil's mounting bracket to the coil. This is most often seen when the coil nose is mounted downwards. Assuming that you have a good quality coil (sometimes a hit or miss thing today) the first thing I would do would be to examine the outside bodies of the failed coils and see if you are seeing any deformation where the clamp holds it. If you do, that is the most probable cause of your problem.

If you continue to have problems, it might be worth checking on what the output voltage of the coil is at idle or steady speed operation. I find that my cars require about 11K volts and the standard coil easily provides this. If you have a significantly higher operating voltage, it may tend to wear out a coil more rapidly. But, not having had that problem I cannot say for sure.

Les
Les Bengtson

I'm a bit confused when you say ballasted or non ballasted Rob, why are you using both types?

The coil should be loaded/fed by whichever circuit it's designed for surely :)

What resistance are you getting at the secondary coil, whichever type of system the resistance should be between 3,000 and 14,000 ohms measured between the centre electrode and the +/SW terminal at the primary circuit. You will get a reading at either terminal but the - terminal will have the extra 3 ohms or so added

The primary circuit resistance will vary depending on what system the car has, a 12 volt basic system should have between 3 and 6 ohms at the LT terminals (+/SW and -/CB)
A 12 volt ballasted system should have between 1.2 and 3 ohms and an electronic ignition system coil should have between 0.3 and 1.2 ohms

The scondary windings are of very fine copper wire and as Les says can be easily damaged if mishandled or misfitted somehow.

I read an AA study that suggested that many coil failures occurred at the junction points inside the terminal turrets as simple mechanical joint misconnection.

(Maybe badly crimped wire junctions, I never fancied taking a failed one apart to find out)
Bill1

nothing at all on the secondary coils - they have become disconnected and show '1' on all settings of the meter (not quite infinite but let's not split hairs)

Good spot Bill. It originally was ballasted, but after the first coil died I took it all away (the PO had made the biggest rats nest of messed up wiring - worse than my K!) and started again with an unballasted arrangement, as that was the spare coil I had at the time (a new spare I had while the midget was A series)

Rob Armstrong

OK Rob, just a thought (thinking in ex-AA hat here) have you tried to tighten the inner contact in the HT turret? I know some coils have a screw head inside there, might be worth a try.

Otherwise, looks like you are "unlucky in coils", there are likely to be far worse things to be unlucky in.

it has to be worth measuring the voltages the coil is switching, maybe the failed coil could be an indicator that there is trouble brewing elsewhere.

Good luck sorting it though.
Bill1

Rob,
there's an explanation of dual points - and more info that could be useful to you - on this vid - http://www.youtube.com/watch?v=18KYBLKPVXE

a mate of a mate had a RV8 engined car running on dual points and my mate swapped it to a 123 and it ran so much better straight out of the box

I had a 3500S to which I fitted an electronic igniter head in the existing dissy and it went very well on that

I know you wont like it but I'd suggest ditching the dual, I remember on a tour an Elan Sprint came flying by us but a couple of miles later we were waved passed as he had the bonnet up, confirmed at pub lunch he'd had to stop and reset the dual
Nigel Atkins

Cheers Bill, the new one's in and behaving beautifully now.

Nigel, I'm starting to hate the dual points nightmare hateful device of devilness. It's better now I've taken the load off it with a DIY maplins transistor box but it still needs taking out and readjusting every 3000 miles. The points cost a fortune for it as well.

Real steel do an electronic insert for the Mallory, so I might well be tempted to invest in that.
Rob Armstrong

Rob,
every 3,000 miles sounds long lasting you must be doing something very right

I'm not a fan of one set so two would be the devil's twins

so Real Steel are still going then, IIRC they used to be out Bedford way (for Santa Pod I expect)

if the Mallory dissy is fairly new then the electronic kit would be the answer (but boy did the standard 123 transform the RV8 engine when swapped against the dual-point dissy in that other car)
Nigel Atkins

Rob
IMHO, heat may be a contributing factor
R W Bowers

I had a mate at work with Mallory dual points in his car, (might have been a Dolly Sprint it was a very longtime ago), he could never rely on the settings of it.
He was always having to fettle with it.

Seems the most unreliable way of opening and shutting the flow to the coil to me.

Guess why I like lecktronics...

An insert for the Mallorys sounds pretty good to me.
Bill1

I'd change to igniter head (or 123 :) dissy) then check/reset timing and possibly carbs
Nigel Atkins

found a price for a 123 distributor - it's about 450 quid!

Mallory insert is 60 quid - a bit more like it, (but still a bit much for mr tight...)

Rob Armstrong

Isn't there a compatible Lucas or Delco dizzy with a suitable advance curve (As with some 1850 Dollies) that could be used and take the twenty quid red unit that you can get for some Land Rovers?

450 squids would get the car mothballed if t'were mine.


Ouch!
Bill1

In regards to an insert for the Mallory dual points: if I recall correctly, the insert is only doable in either the non-vacuum unit or the vacuum unit--can't recall which. I think it's the non-vacuum unit which has enough room for the insert.
JM Morris

SimonBBC does a full electronic distributor for 127 quid. Which is a lot cheaper. Any distributor from a V8 should work, not sure if the advance curve's the same on all of em though. Some were electronic even, in later land rovers.

There are 2 types of mallory insert on the real steel website, one for vac advance and one without. mine has a vac 'for economy' lol.

It's working again on a new coil now. So there's a bit of breathing space. 450 quid is about 5 tanks of fuel for the monster, which will get me some 1600 miles!
Rob Armstrong

I have a Mallory dizzy on mine, originally dual points, and now using a pertronix insert. It's been absolutely trouble-free since I installed it.

Scuttlebutt has it that dual points are quite unnecessary on road-going four cylinders, as the coil has plenty of time to charge anyway. I get the idea they make better sense with more cylinders and higher speeds.

-:G:-
Gryf Ketcherside

Rob,
IIRC 64 lite tank and can still do simple maths (although I have failed to do so here before) that's about an average of 22/23mpg

assuming I've got that right and depending on your driving style and how much is town driving that doesn't sound very economical

mine was 17 around town and 26-28 on a long run (manual and old fella driving), I didn't bother checking mpg when I was on fast runs I just collected all the loyalty points and gifts :)

I thought the 123 price might hurt you but it does make the Mallory conversion seem better :)

apart from possibly better mpg to get your money back you might also get better starting and idle and smoother running

a mate was around yesterday having just put his P5b back together, he's a honorary Yorkshire as he's so tight he doesn't like to breath out, but he's finally going to cough up for a rolling road set up as he on around 12mpg now

he'd booked in with an expensive not so efficient person but I've persuaded him to go to a excellent value and effective RR tuner call Peter Burgess :)

I reckon if he's lucky he could have his money back in less than three tankfuls even allowing for the petrol it'll take him to get there
Nigel Atkins

it isn't very economical. Mine's a 68 litre tank, and a 3 speed auto, which sucks the power and economy out like nobody's business. It also has the high (and is higher because of an overbore) compression engine so is run a little rich to avoid pinking, plus it has to have shell sooperdooper or tesco momentum and an additive to get it near 100 octane... It'll do 26mpg on a long run if I've got helium shoes on, but all my shoes seem to be made of lead - it gets driven as hard as the K. It's expensive but the noise is just amazing.

PO put the Mallory on as part of a £3000 RPI engine rebuild 20 odd thousand miles ago.

Though it currently idles happily at about 400 rpm, and will stand a pound coin on the rocker cover while doing it.
Rob Armstrong

64 instead of 68 is very close for my memory

it must have been high compression if it needs 100, was it set for racing!

26mpg doesn't sound bad in the circumstances

my mate with his 3.9 (TVR) engined P5B I think got his engine from RPI and I've just convinced him a visit to Peter Burgess's rolling road will easily pay for itself

pity I haven't got a photo of my mate's (now belonging to his brother again) engine, (the one formerly with the dual) as IIRC that was built as a project for young race engineers so is very special, it's housed in a 90s Westfeild
Nigel Atkins

This thread was discussed between 07/08/2013 and 10/08/2013

MG Midget and Sprite Technical index

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