John L Posted August 22, 2016 Report Share Posted August 22, 2016 I'm trying to help a friend with another well know sports cars, that is having problems with the ignition coil getting hot and a so eventually failing. Usually 2 a year! Its a non ballast system with the right sort of coil. Really 2 questions Is there a good reason for the std coil to get hot, and if so what is causing it, the points have been set at 60 degrees with a dwell meter its a 4 cylinder. I under stand that some of the Lucas branded ones are not that good, so what is a good one to get? Could the under bonnet heat also be a contributor to the problem even though the coil is mounted on the inner wing. Any help here would be much appreciated. John Quote Link to post Share on other sites
RobH Posted August 22, 2016 Report Share Posted August 22, 2016 (edited) Its normal for a coil to get warm John - that is why most are oil-filled to help dissipate the heat. It gets hot because all the time the points are closed there is a current flowing through the primary winding and hence power is dissipated in the coil resistance. It shouldn't get hotter than is just about bearable to the touch, which should be within the capability of the coil to cope, bearing in mind that the innards will be a bit hotter than that. The limitation is the enamel insulation on the wire as once that breaks down there will be shorted-turns. Are you certain the failures are due to heat or could it be something else ? What are the plug gaps set to? Is the condenser good? Edited August 22, 2016 by RobH Quote Link to post Share on other sites
harlequin Posted August 22, 2016 Report Share Posted August 22, 2016 John A friend was having a similar problem with a Morris, he resolved it by mounting the coil vertically. I suspect that the internals were not fully immersed in the oil and were overheating. I hope this helps and it may be a cheap fix. George Quote Link to post Share on other sites
stillp Posted August 22, 2016 Report Share Posted August 22, 2016 (edited) If the ignition is left on with the engine not running, and the points are closed, the coil will overheat. Pete Edited August 22, 2016 by stillp Quote Link to post Share on other sites
John L Posted August 22, 2016 Author Report Share Posted August 22, 2016 RobH What else could allow the coil to get hot, are the new Lucas coils filled with any oil these days, or perhaps its the coating on the wire that is not thick enough? We have run an air pipe up to the coil from the front which does seem to keep it cooler, but I'm sure this is not the real cure. It doesn't seem to run that hot, but it seems that the heat is killing it, as I'm sure you shouldn't have to change them twice a year! When it fails or the engine dies, just put on another and its fine again for a while, so wondering if there is something else in the system that is giving us the problem? Would the type of plug leads be an issue? It does have an alternator and the charge when running is just over 14 volts, but I don't suppose this should be the problem? George I will check the orientation, I think it is already down, but I will give it a look. John Quote Link to post Share on other sites
RogerH Posted August 22, 2016 Report Share Posted August 22, 2016 Hi John, what is the set up Coil type Voltage Electronic ignition Ballast supply etc etc Lucas coils are oil filled. I'm sure I have a seen an extruded heat sink that slips over the coil. Roger Quote Link to post Share on other sites
Andy Moltu Posted August 22, 2016 Report Share Posted August 22, 2016 If the ignition is left on with the engine not running, and the points are closed, the coil will overheat. Pete Not entirely convinced - when running the coil is drawing current for all of the time the points are closed they will be generating heat, proportionately not as much as when standing with points closed but still plenty and the reduced heat for when the points are open is probably offset by the reduction in heat loss by simply being in a hot engine bay next to a hot engine. I suppose if heat is the issue a heatsink could be attached to the coil. Quote Link to post Share on other sites
stillp Posted August 22, 2016 Report Share Posted August 22, 2016 But Andy, a heatsink will collect heat from the hot engine bay and transmit it to the coil! Pete Quote Link to post Share on other sites
RogerH Posted August 22, 2016 Report Share Posted August 22, 2016 Hi Pete/Andy, the under bonnet temp will be in the order of sub 80'c.. If the coil was only at that temp then it will live a long life. A heatsink on the coil body (if it could be fitted) would easily dissipate excess heat from the coil even if it was only down to 80'c. I've got an idea - pictures tomorrow Roger Quote Link to post Share on other sites
John L Posted August 22, 2016 Author Report Share Posted August 22, 2016 RogerH Its a Lucas DLB 101 Distributor 25D4 Points Non ballast What should the resistance be from the + or - to the centre electrode on a good coil, I have a new Lucas here that's only got 9.14 ohms, but it does have 3 ohms + to - I thought this should be higher. I will try and get some of the failed ones and see what readings I get with those. John Quote Link to post Share on other sites
RobH Posted August 22, 2016 Report Share Posted August 22, 2016 (edited) "...It doesn't seem to run that hot, but it seems that the heat is killing it, ..." As I said John- that is just an assumption and it might be something else causing the failures if the heat is not excessive, though poorly-made coils may fail more readily of course. If your extra cooling isn't helping it does point to another cause. I am sure you will already have tried coils from different sources (?). There may be a number of factors contributing. A wide plug gap will mean a higher voltage is reached before it sparks - which might be problem for iffy coils over time. A failing condenser might cause excessive 'ringing' of the coil but you would probably see that as burned points. The Lucas DLB 101 is a 3-Ohm primary so your reading on the */- terminals is correct. I can't find a quoted value for the secondary winding for that coil but it would typically be in the range 8000 to 15000 Ohms. Bear in mind that the resistance on a coil with shorted turns may not be far different from a good one on a DC reading, so a fault may not be obvious. Edited August 22, 2016 by RobH Quote Link to post Share on other sites
stillp Posted August 22, 2016 Report Share Posted August 22, 2016 Hi Pete/Andy, the under bonnet temp will be in the order of sub 80'c.. If the coil was only at that temp then it will live a long life. A heatsink on the coil body (if it could be fitted) would easily dissipate excess heat from the coil even if it was only down to 80'c. I've got an idea - pictures tomorrow Roger We used to design auto electronics to withstand underbonnet temperatures of 120 C when I did that for a living. Apparently even that would be exceeded by a black car in Death Valley! Sure, the heat sink will improve the transfer of heat from the coil if it is hotter than the air under the bonnet, but also to the coil if it is cooler that the underbonnet air. Mounting it on the wheelarch would be more effective. The Lucas training manuals used to warn against leaving the ignition on with the engine stopped, and who can argue with the Prince of Darkness? Pete Quote Link to post Share on other sites
MikeF Posted August 22, 2016 Report Share Posted August 22, 2016 Not entirely convinced - when running the coil is drawing current for all of the time the points are closed they will be generating heat, proportionately not as much as when standing with points closed but still plenty and the reduced heat for when the points are open is probably offset by the reduction in heat loss by simply being in a hot engine bay next to a hot engine. I suppose if heat is the issue a heatsink could be attached to the coil. The young lad who did my MOT turned the ignition key to the right not left as I instructed and then forgot it .My coil boiled and did a good imitation of a volcano, squirting oil quite forcefully. Mike Quote Link to post Share on other sites
RobH Posted August 23, 2016 Report Share Posted August 23, 2016 (edited) Good point Mike - is the owner in question in the habit of leaving the ignition on for periods of time without the engine running ? If he is that would explain repeated failures. Edited August 23, 2016 by RobH Quote Link to post Share on other sites
Andy Moltu Posted August 23, 2016 Report Share Posted August 23, 2016 The point I was making is that the points are open for a limited part of the time when running and whilst less heat is generated when running that reduction in heat is likely to be offset by the fact that it will then be in a hot engine bay next to a hot block so unlikely to be significantly cooler. A decent coil should have the capacity to dissipate the heat generated without failing if subjected to a continuous current. So why is it failing? Internal short so drawing more current and this getting too hot and thus failing completely. Poor quality in the first place or insulation breaking down through age. Has some of the oil escaped allowing part of the coils to get too hot and fail? Are they 6 volt coils intended for use with a ballast resistor being subjected to 12 volts? Quote Link to post Share on other sites
peejay4A Posted August 23, 2016 Report Share Posted August 23, 2016 (edited) Could the coils be failing due to internal breakdown of the HT winding? If there's a problem with one or more HT leads or perhaps even too great a plug gap I wonder if that might cause the secondary voltage to break down the coil insulation. Check the HT leads for continuity. Check plug gaps. Try a new distributor cap and rotor arm. Edited August 23, 2016 by peejay4A Quote Link to post Share on other sites
Peter Cobbold Posted August 23, 2016 Report Share Posted August 23, 2016 Luc*s are now owned by ZF Freidrichshafen who also own TRW - of brake master cylinder infamy. I suspect Luc*s parts are manufactured well to the east of germany...and if quality control is as good as on the master cylinders, thats why the coils are failing. Peter Quote Link to post Share on other sites
TriumphV8 Posted August 23, 2016 Report Share Posted August 23, 2016 Simply add a electronic system that limits the dwell time. It is not correct that the coil acts like a resistor. When points are closed a magnetic field is generated and during that time the coil has high resistance and will not heat up. Simply cut the loading of the coil when this point of full load is reached. Electronic systems can do so and start 2ms before coil fires with the loading and than fire the coil by cutting off the current...... 123ignition does so. All modern cars work that way, I have a coil in use with 0.4 ohms that will be killed immedeately with points and coil on plugs are similar in resistance. After that time of 2ms the coil resistance reduces to the quoted 3 Ohms and the coil is boiled with 14Volts x 4Amps = 60 watts what is quite a lot. To have a sufficient dwell time availiable to "load" the coil under high revs the resistance of the coil must be low what makes it vulnerable at standing still with points closed and with ignition on and low revs are bad, too. The other thing that helps is the ballast resistor that limits the current when engine is used under these bad conditions but does not play that role at high revs. The reason is simple: When standing the coil has 3 ohms and another 3 ohms of the ballast resistor will limit the total current by 50%. Positive is that the heat is separated to coil and resistor, leaving 25% on the coil what is 15 watts in our example what is much more friendly than before! Under high revs when the coil is loaded it has a high resistance for lets say 2 milliseconds and the additional 3 ohms dont play a role for the dwell time compared to the high resistance of the coil. Simply physics when fully understood to save a lot of trouble! All that repositioning of the coil is more or less curing at the symptoms than solving the problems. Heat in the coil also has nothing to do with the condenser or the cables. Quote Link to post Share on other sites
RobH Posted August 23, 2016 Report Share Posted August 23, 2016 As you say Andreas, if the ignition is left switched on with the engine not running and the points happen to be closed, the coil does act like a pure resistor and will overheat because (at a nominal 3 Ohms and 12 volts) it will be continually dissipating 48 watts or so. That could be one possible reason for a coil being damaged over time if done repeatedly, particularly if the materials used in its construction are not of the best quality, because the heat may incrementally degrade the insulation on the wire. However, It is not clear yet whether heat is really anything to do with the failures. From the OP's posts we do know that the fault is on a conventional non-ballasted ignition system with the correctly-specified parts and that additional cooling has been tried but has not helped. I don't think anyone suggested that the condenser or the cables might causing overheating, they were suggested as another source of coil failure by causing possible over-voltage transients which a poor quality coil may not withstand. Quote Link to post Share on other sites
TriumphV8 Posted August 23, 2016 Report Share Posted August 23, 2016 So what will you do? 123ignition electronic ignition like Megajolt red BOSCH coil with a ballast resistor Quote Link to post Share on other sites
RobH Posted August 23, 2016 Report Share Posted August 23, 2016 Or perhaps just find the fault and leave the standard system alone... There are thousands of them out there working perfectly well. Quote Link to post Share on other sites
ChrisR-4A Posted August 23, 2016 Report Share Posted August 23, 2016 Check the voltage at the coil with engine running, is the alternator output too high? Quote Link to post Share on other sites
TorontoTim Posted August 23, 2016 Report Share Posted August 23, 2016 Tell your friend this is a standard fault on "another well known sports car" and get him to buy a TR instead!!! Quote Link to post Share on other sites
RogerH Posted August 23, 2016 Report Share Posted August 23, 2016 Quote Link to post Share on other sites
TriumphV8 Posted August 24, 2016 Report Share Posted August 24, 2016 Or perhaps just find the fault and leave the standard system alone... There are thousands of them out there working perfectly well. These thousands do not employ cheap Chinese parts. Had a clubmate who used three condensers on his TR4 at one trip to a meeting at Nürburgring. If you have these low quality parts you will have to make life easier for them or buy better parts like BOSCH and upgrade the existing system. Quote Link to post Share on other sites
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