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MG MGA - Hot Coil

Hi,

If my memory serves me right, I had mentioned the hot coil problem on this forum before. My hot coil problem still persists to this day. When I say hot I mean so hot that it becomes untouchable with the human hand. I have tried about six different coils and they all get burning hot. My coil is mounted on top of the generator as original. To try to eliminate (at least partly) this problem, I was thinking of wrapping the coil in heat insulating material. I had bought some of this stuff to cover the carbs heatshield. and I have some left over. My question here is, would it be wise and effective to completely cover the coil body? Or would this make matters worse, in that the internal coils inside would heat up even more due to lack of air circulation reaching the coil body? not forgetting that the ambient air in the whole of the engine bay has no cooling effect while the engine is running. Has anyone tried this before? Any comments are most welcome.
F Camilleri

If my memory of heat transfer theory serves me correctly, wrapping it with insulation would be a bad idea. Yes, it would only make the internals hotter. You'd be better off attaching some cooling fins to help conduct heat away. In any event, a coil should not run that hot.
Regards,
GTF
G T Foster

Hi GT,

You say that "a coil should not run that hot". Agreed, many MG owners told me so. But my coil does get very hot, and as I mentioned I tried several different coils with the same result. The engine gets pretty hot after a run but the temperature gauge on the dash always remains within limits. No overheating problems were ever experienced since I bought the car. Only this morning I tried a Lucas coil HP12 which I borrowed from a friend's MGA 1500 (same engine as mine), and after a 4 or 5 mile run this coil also became scorching hot. Since the car runs perfectly well, I cannot figure out a reason for such an annoying problem.

Frank
F Camilleri

If you get the overheating with many different coils could it be that your dynamo is overheating? If the dynamo is working hard then you might find that this is too hot to touch after a fair run. It's not the ideal position to put the coil which itself is generating around 50 watts.
It could be that the factory had complaints in the early days about coil overheating and this is why they moved the coil away from the engine and dynamo for the 1600.
If you're not bothered about originality why not make up a 1600 coil bracket and move it?
Let me know if you want the dimensions of the bracket.....................Mike
m.j. moore

I don't know who you are talking to that gives you the impression it is abnormal. Ignition coils get HOT. So hot that you cannot hold your hand on it for more than a short moment. For me, that temp limit is around 150 deg F. The coil runs hotter than that, And I can't hold on to one. Especially on a 1500 where it can absorb heat from the generator it is mounted to. I don't know about the 50-Watt thing, My quick estimate would be between 15W and 30W while running. Certainly more if you turn on the ignition and don't start the engine. Either way, if it is 50W, then it is close to putting your hand on a 50W light bulb. Not something I would want to do intentionally.

My guess it that the reason it was moved to the fender on the 1600 was mostly vibration issues. Vibration can kill the coil and I have had a 1500 coil mount crack while mounted to the generator.

If your coil has a primary resistance of around 3 ohms, then there is nothing abnormal about your situation. Just let it be and don't put you hand on it. BTW the generator gets HOT too so stay away from that with your hands. OH..... and the exhaust manifold too. (insert smiley face)

Chuck Schaefer

Hi Frank,

This is pure speculation but makes some sense to me.

Could you have a very long dwell time which is heating up the coil too much?

When the points are closed the primary winding of the coil is basically a heating element. If the closed period is very long then I think the coil should get hotter.

If this makes any sense to you then have a look at the dwell setting - It should be between 50 & 60 degrees (depending on which distributer you have. I would try opening the points gap until you have a dwell of around 40 degrees and see if the coil runs any cooler.

Regards

Paddy
P Reardon

The coil on my A gets hot as well. It is a 'standard' coil mounted on the dynamo. Everything checked out, so I had a feel of the coil on my series III Land Rover, and although not quite as hot as the dynamo-mounted MGA coil (the Land Rover coil is bukhead-mounted), it too was so hot that you did not want to keep your hand there! I think it is normal, 45 watts is going to generate some heat, especially if your engine is stopped with the points closed.
Lindsay Sampford

Tks fellas for all your inputs so far. It's rather late now and I'm pretty tired, so I shall have to come back on the forum tomorrow morning. Good night to all, or good day depending which part of the world you live in. Very interesting posts, please keep them coming.

Frank
F Camilleri

Hi Paddy,

your post makes very interesting reading. As a matter of fact, I did check the dwell angle about two years ago. I remember that it was about 57 degrees then, so I left it at that angle. The recommended dwell is 60 degress +/- 3 degrees as you will know. Now my thoughts are on re-positioning the coil away from the engine block. Any good tips on where would be the best place for it? How about installing a sports coil? Would this withstand the heat any better?

Frank
F Camilleri

Hi Frank, I don't recommend the Lucas Sport Coil presently available, see picture! Personally, I don't see any advantage in "Super-Duper" coils, I certainly noticed no difference when I fitted a "Sport Coil", until it failed of course! All you need is a good distributor, electronic or otherwise, good timing and a "standard" coil, that is, one that is not designed to work with a ballast resistor (that would get very hot). The correct resistance for a standard coil should be just over 3 ohms.


Lindsay Sampford

Me again Frank, I would just like to add that if the heat isn't bothering your coil, there is no reason for it to bother you. If you are concerned, just carry a spare in the boot, they don't take up much room and I certainly have one in my tool kit.
Lindsay Sampford

Frank

I relocated mine (1500) to the 1600 position, primarily because I fitted an alternator and the strapping would not fit round (greater circumference). I used half of the original bracket and drilled a couple of holes in the inner wing.

I actually prefer this location. Slightly easier to read the + and - on the coil!

Steve


Steve Gyles

Lindsay, I've gone off the sports coil idea. I have never been too keen on them. My coil does get awefully hot, but never gave up on me yet. As per your advice I shall keep my original Lucas coil, which has so far served me well, and keep a spare in the boot. I have at least half a dozen good coils, four of which are original Lucas, so no problem if and when the one in place decides to fail completely.
Steve, tks for the pic, that seems a much cooler place for the coil, and as you say, it is easier to read the polarity signs. I've been tempted to fix it in that position before, but had been reluctant to drill additional holes in the body. Well, I suppose a couple of small holes in that location would hardly be noticeable. My trouble is that I am a fanatic on originality. We'll see.

Frank
F Camilleri

Frank

Just pretend you are a 1600 owner and match its attachment points on the inner wing. Not sure what that consists of but someone will inform us.

Steve
Steve Gyles

Frank, the whole engine compartment gets awfully hot. I'm sure you must have put your hand on top of the bonnet after a 20 mile trip in high summer and felt how hot it gets. It surprises me that paint doesn't burn off! Admittedly, the coil is going to get a bit hotter situated on the dynamo and in the sream of hot air blown over it by the fan, so moving to the inner wing as Steve suggests might be cooler and less subject to vibration. The only real outside influence on coil temperature is going to be, as Paddy suggests, the amount of time the points are closed for, and it sounds like yours are within the design limits. I'm sure you and I are not the only ones with hot coils!
Lindsay Sampford

On the 1600 and MKII, the coil is mounted to a bracket which is bolted to the chassis side of the engine mounts. If you get one of these, or can fab one, then there is no permanant modification required to the car.
Chuck Schaefer

I think the 1600 coil position has been nicely thought out. Not only is the coil as close as it can be to the cool inner wing but the side facing the engine and dynamo is protected by the bracket itself which acts as a radiation shield. Furthermore it is very close to the distributor. The brackets are available from the usual sources but they are simple to make yourself and are fixed to the top three holes of the engine mount.

Don't forget that the higher the coil operating temperature the quicker it will fail.................Mike
m.j. moore

I am not a concours sort of person because I do not like rusty bolts and all the issues brought about by trying to fix up a car as cheap as the factory did!

However mine is a 1600 and I have the 1500 coil position ie mounted on the dynamo and it is a yellow lucas sports coil. I wonder why in the last 8 years It has not failed? Guess if it ever does I will have to bite the bullet and mount it in its factory position! LOL.
Robert (Bob) Midget Turbo

Frank, A mentioned, coils do run hot anyway, especially with a points inductive system. A good portion of the time the charging current through the coil is wasted even if the dwell is within reasonable limits. I would check the points gap and make sure it is not too narrow. If your experience is with a car that is running a CD ignition, you will find the same coil only runs warm as there is very little wasted current going through it as the capacitor in the CD ignition discharges through it only. This is the main reason why a properly designed CD ignition is easier on ignition coils (providing the CD doesn't give more voltage to the coil primary than necessary, otherwise the insulation can break down from internal arcing inside the coil) Chuck Schaefer has a really good write up in the archives on dwell from this year. I read in this thread a reference to Sports coils. I read recently in a publication that on a high compression engine that the compression would 'blow out' the spark if a high voltage sports coil wasn't used. This is incorrect. The spark doesn't blow out, it never formed in the first place. The resistance across the plug gap increases in proportion to the density of the fuel air charge in an almost linear relationship. Double the compression means double the density and thus (roughly) double the resistance, and if the voltage is too low, the spark will not bridge the gap. Actually that statement is incorrect too. The spark never forms because the threshold voltage to overcome the resistance and cause ionisation(which will allow a lower voltage to sustain the arc) isn't reached. It is a complete misfire. A sports coil has enough voltage to make up for high resistive gaps (either wide or due to compression). However, if you don't need one, you shouldn't use one. It is spark current that lights fuel, and a high voltage coil will deliver lower spark current leading to possible misfires or only partial combustion. They can also run really hot being low resistance in the primary (someone correct me here if I am wrong), and be hard on points. Fred Winterburn
FC Winterburn

Fred, I believe it is the ionization of the air that normally initializes the spark event. The higher the pressure, the higher energy necessary to break a few if those ions free and allow for the start the arc. As you say, it is virtually a linear function relative to pressure. I'm not sure how wide a pressure range that one can use that rule. I do know that it works that way under lower pressures; the higher the altitude (lower air pressure), the lower the dielectric breakdown voltage of a given space in air. In addition, there are other things that can affect ionization voltage. You are, of course, correct in that the spark never forms vs the spark blowing out in high compression engines.

Regarding the coil resistance, I can personally attest that if you have a coil with ~1.6 ohm primary resistance AND run points with plastic rubbing block, you will quickly melt the plastic and soon have little to no point gap and no spark. I ran this test unintentionally on my first MGA. I managed to crawl home by re-adjusting the points by eye. It quickly started to misfire a mile or two later as I was pulling into my drive.

Cheers!
Chuck
Chuck Schaefer

Thanks Chuck, I really should have described it as a threshold voltage that doubles as compression doubles more or less linearly. Thanks for the info on plastic rubbing blocks. I'll bear that in mind and warn others not to use them with low resistance coils. Take care, Fred
FC Winterburn

Chuck, is this what you mean by a melted rubbing block (see picture)? This was caused by running with a coil (fitted by a previous owner) that should have had a series ballast resistor!


Lindsay Sampford

Lindsay, It has been 20+ years since that happened to me. That photo looks exactly like what I remember. Same scenario too. The DPO had a low ohm coil and a resistor wired hap-hazardly in series with it. Wires strung all over the place. I was new to MGAs at the time but knew that the MGA didn't require a ballast resistor. So I removed it, not realizing that the coil was wrong too. Presto. Melted plastic.

Great photo by the way.
Chuck Schaefer

Chuck, there is a story behind my coil. I bought my car as a runner, but it had a misfire. I changed the coil for a sport coil but the misfire was still there and eventually traced to incorrect heavy felt air filters causing bad engine breathing and oiling #2 plug. That was fixed and the engine ran perfectly until the sport coil exploded (see post #10). I re-fitted the coil that came with the car and it was OK for a long time, but then a misfire developed which gradually got worse (but not caused by the points closing, as in your case). I checked the coil resistance and found it to be too low. Assuming the coil was faulty, I replaced it, (which cured the misfire). It wasn't until I 'Googled' the number on the 'faulty' coil, that I dicovered it was the wrong type! Shortly after that, I changed the points and found the melting.
Lindsay Sampford

Frank, if you do decide to move the coil away from the generator onto the bulkhead, make sure that the HT lead from it is long enough.

I recently changed my distributor cap and leads as part of a change over to an electronic ignition system and I didnt realise that the HT lead was on the short side.
I was driving through some really enjoyable country roads last summer and caught up with a little Mazda MX5 who was just cruising along at just 40mph.
After 5 minutes following him I began to get a little impatient and started to look to overtake.

The first straight I came to looked ok so I changed down, floored the pedal and got past fairly briskly
( well as briskly as my MGA can anyway!) Unfortunately, the Right handed corner at the end of the straight was quite a lot tighter than I expected and I ended up opposite-locking around most of it!

Once I had gotten both the car and my heart-rate back under control I realised that the engine was misfiring quite badly and I pulled over to see what my panic braking and sideways driving had done to the engine. I checked everything over and all looked ok until I noticed that the HT lead from the coil had been pulled out probably due excessive engine movement.
The Mazda driver gave me a friendly wave as he went by but his grin showed what he was really thinking!

So I learnt two things,
1st : on a lovely summers day there really isnt any sensible reason to overtake every car that is just cruising along even if they are holding me up.

2nd : Now that I have fitted a longer HT lead to the coil I can catch up and overtake the smiling MX5 driver the next time I see him!!

Colyn
Colyn Firth

Colyn, vehicles will always look for a way to embarrass you. Many years ago I was riding a mildly tuned 650cc BSA Gold Flash "cafe racer" through our village at the legal 30mph when a lunatic in an open-topped sportscar (prbably an MG!) came hurtling past me. I kept my patience until I got to the end of the 30 limit and then dropped into 2nd and opened the tap wide. I was thinking "i'll show this guy what acceleration is all about" but just as I got by him, something shot past my left knee, the engine fell onto one cylinder and the sportscar sailed past as I came to a halt. The whole centre electrode and porcelain insulator had blown out of the plug leaving the threaded bit still screwed into the head. I have never had that happen since!
Lindsay Sampford

Frank, just did a 20 mile round trip in the MGA and at the end, the standard coil is un-touchably hot. Despite this, the engine is running as sweet as a nut, so I reckon this is perfectly normal; until such a time as it stops working anyway!
Lindsay Sampford

Lindsay,

I fully agree with you, it seems that it's quite normal for the coil in an MGA to run hot. I have never yet experienced any problems with the running of the engine as far as the coil is concerned. But, I do believe that too much heat will shorten the life of the coil. Keeping a spare in the boot is the answer. Just to keep all posted, I have fabricated a bracket out of a brass plate and mounted the coil under the front wing. The bracket is attached with two nuts & washers to the visble threaded end of two wing attachment bolts. The coil is steady and hardly visible from above, although I had to lenghten the wires a little. I haven't tested it's temperature after a run yet, but I'll be back with the result.

Frank
F Camilleri

After a 6 mile run, I opened the bonnet (hood) to do a touch test of the coil in its new position right under the front wing. It is just opposite the engine bay air vent. To my surprise & disappointment the coil became almost as hot as it was getting when installed on the generator. So, hardly any improvement there. Would anyone with a 1600 possibly be so kind to post a pic of their coil position? I may try it there. If no improvement here, then my coil will go back to its original positon on top of the generator.

Frank
F Camilleri

This is mine . No wires but plenty of dust! There are five screws on the bracket, the two for the coil and the three normally used to fix the engine mount ( you can just see one nut under the dynamo) ..........Mike


m.j. moore

I see you are using one of the new Wi-Fi coils Mike!
Lindsay Sampford

Linsay, One must match use a Wi-Fi coil when installing a Wi-Fi Disty. I see there is a Wi-Fi generator too. Pity he had to use the wired starter.

Frank, you may have taken 10 deg out of the coil by relocating it and then again maybe not. If you are using your hand as a judge, Hot is Hot and 10Deg difference is not discernable by touch once something is too hot to hold on to. The only proper way to know what temp it is is to use a IR thermometer or a thermocouple. There is another accepted way using timed resitance readings after running and doing some mathematiucal calcualtions. But that is beyond the scope of this discussion.

Your original post sums it up fairly well. You tried 6 different coils and they "all get burning hot" As you put it. Why do you insist that this is a real problem? If you want it to "feel" cooler, get a plastic cased coil. They don't really run cooler, they just feel like it. In fact, they may actually run hotter internally. I think you are searching for an unrealistic solution.

I am not an automotive coil designer but have professionally designed and worked with magnetic devices for over 40 years. It is not uncommon for well designed magnetic devices to normally run 100, 110, or even 120 deg C or more. I would not be surprised to see these temps internal to the coil under the hood of an automobile.
Chuck Schaefer

Tks for the pic Mike, it looks quite straight forward to mount the coil in that position.
Chuck, OK your comments make a lot of sense. Maybe fantasy is running away with me. I do strongly believe though that no vehicle ignition coils should run under such extreme temperatures. Their life will be drastically short if they do. However, I firmly believe that it is the engine bay heat that's contributing to such conditions, and therefore, my conclusion is that there has to be shortcomings in the engine cooling system.

Frank
F Camilleri

Frank, "I do strongly believe though that no vehicle ignition coils should run under such extreme temperatures." sounds like you are starting a society for the prevention of cruelty to ignition coils!
Lindsay Sampford

This thread was discussed between 27/03/2011 and 05/04/2011

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