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MG TD TF 1500 - coil trouble

I was out today with my mgtf today testing the car after the water pump was replaced. I idled the car for about 10 minutes then got in for a run. I went around the block a few times and then over to the next neighborhood. I was going around the corner when the engine died. After a couple of minutes it started and I was off again. Within a few seconds it died again. I just happened to be in the neighborhood of a friend who is a mechanic. We checked the fuel and it was OK, we checked the spark and it was OK so I started the car again. It ran for a few moments and died again. He pulled off the spark plug wire to check the spark and when I cranked it there was not spark. It was also noticed that the coil was very hot to the touch. He happened to have a digital temperature gage and the temp of the coil was 132 degrees F. I needed to get back so we cooled the coil down to 98 degrees using a fan and the car started and I was able to get it home. I had replaced the coil a year ago after having a similar issue. It cured the problem at that time. Any thoughts on the coil problem? Two Moss coils in a year are two coincidental. Could some component be damaging the coil over time? I had intended on attending the GOF in New Hampshire with the car, but now am insure. Thanks!
Andre de la Chevrotiere

I did a search for coil failure and found this,

Maybe you just had 2 bad ones ?? Call Martin at AS,, Maybe he is open on tuesday ? I would think you could still get one at yout local parts store?
Steve Wincze

Is a coil polarity sensitive ??
Steve Wincze

"Is a coil polarity sensitive ??"
Not really, but it is slightly more efficient if the primary is connected correctly. The very old coils were marked CB and SW (CB - contact breaker or points and SW - ignition switch). Later coils were marked with a very straight forward + and -. This later markings are very specific, where as with the CB/SW markings you have to know which vehicle polarity the coil was manufactured for. In the real world, there is not much difference in the way the coil is hooked up - if it hooked up backwards, the output voltage will be marginally lower (by less than 50 volts). Cheers - Dave
DW DuBois

On my TFs I have never had trouble with the original Lucas Q12 Coils
for over forty years !

Fortunate also more recently to get a correct NOS LA12 Coil with 'scalloped' bracket ready for the TF1500.

Long live the 'real' Lucas.

Rob Grantham
Rob Grantham

I had a similar problem with a new Moss coil on my car. As a matter of course, I had replaced the original 1952 Lucas coil with a new one when I rebuilt my engine. It would run perfectly for 20 minutes or so until the engine was fully warmed up, and then it would begin to miss badly. It never completely stopped or stranded me, but it certainly took the fun out of driving back home and it was driving me crazy trying to find the problem. The distributor had been rebuilt by Jeff Schlemmer, so I was sure it was ok, but even so I replaced the condenser more than once, to no avail. I checked, re-checked, and then checked again all the usual suspects: plugs, timing, carbs, valves, compression, fuel pump, and so on but found nothing amiss. I finally thought I
Joe Olson

Once had a Lucas coil fail when the clamp bolt holding it in place loosed up and fell out and over time the vibration wore through the case and the insulating oil leaked out slowly and somewhat unnoticeably and the coil would fail after a period of running but recover for a while after cooling down. New coil fixed the issue.
John Quilter

I had this trouble with three successive coils. Turned out to be a float valve sticking shut.
So I've got a new phrase - if you've got what appears to be a fuel problem, it probably is.

David Wardell

Ignition coils do get hot and this is influenced by dwell time of the points. You need to check that you have the right points gap. It shouldn't cause a good coil to fail, but it will make a poor coil fail earlier. The best inexpensive coil is the Bosch blue (3ohm), its no longer blue but comes as aluminium finish that can easily be painted black to look more or less authentic.
Dave H
Dave Hill

As D Hill points out a hot coil can be caused by dwell time. In points ignition the dwell time is the amount of time the ignition coil primary has current running through it, or points closed. Spark at a plug occurs when points open. Too narrow a points gap will give more dwell, or excessive charge time, and heat the coil and pit the points, and can cause poor low speed engine running. Though a narrow gap (high dwell)can improve very high RPM spark delivery, that is the only plus. Nothing's perfect.

D mckellar

I had same problem with a new coil with Moss reference but bought from a local dealer
The coil was oily when I bought it but the dealer told me there was one leaking in the lot.
Actually, I don't know if mine was leaking because I used it for a very short period before it died. But it was not full with oil. One can hear the oil inside and the coil was very light.
The physical phenomena is :
If the coil leaks, air is going inside and oxydes the oil. The oil looses its dielectric feature. Especially when hot.
Even if the coil does not leak but is not full of oil. The coil is no more cooled by oil convection and gets very hot. And the dielectric feature is no more ensured.

The windings are OK but the spark remains inside between windings or between winding and casing.

Conclusion : buy a heavy coil that do not allow to hear the oil bubling inside.


LC Laurent31

I should add to my above comment the fact that if ignition points are adjusted too far apart (lower dwell) the coil may run cooler, but it will also probably cause the engine to run very poorly at idle, and lower RPM.
D mckellar

I have had an overheated coil on a TD and a TF. Both times I replaced the grounding strap from the battery and the problem was solved.
tom scott

This thread was discussed between 04/09/2016 and 07/09/2016

MG TD TF 1500 index

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