Back to Electrical Problems

(Overheating Coils)

It’s that time of year again; we are planning Christmas festivities and looking ahead to 2011.  Things don’t look too good on the economic front but let’s hope we weather the storm and pull through.  On this gloomy note I will take the opportunity to wish you all a happy and prosperous 2011.


After many articles covering Car Electrical Systems and Minor Body Repairs the feedback to both myself and the Editor would suggest that the electrical articles are by far the more popular.  I still regularly receive questions relating to electrical problems from perplexed owners.  It always amazes me how, clearly, very competent mechanics panic over electrical issues!  This panic, in many cases, leads to confusion, a mental block, and an exaggeration of the specific problem confronting them. Over-heating coils is one such

aggravation and causes more concern to owners than almost any other problem reported.  An owner, at the end of his tether, called me recently concerned that his coil was too hot to touch   He had surfed the net searching for info to point him to a solution but had ended up more confused!  His question was very basic – “Why does my coil get so hot?” This month I’ve decided to put my Body Work Articles on ‘the back burner’ and focus on resolving the question ‘Why does my coil overheat’?



An ignition coil, to all intents and purposes, is a transformer and like any transformer has primary and secondary coil, both wound on a soft iron magnetic core.  The primary coil is connected to earth via the points. When the points are closed current passes through the primary coil and the metal core becomes magnetized.  As the points open the magnetism in the core collapses and as it does a voltage is induced in the secondary coil.  This voltage is fed to the spark plugs via the rotor arm.  The level of this induced voltage depends on the ratio of turns of wire on the primary coil to the turns on the secondary coil (in the order of 2,000 to 1).   ‘Chrome’ cars (see Fig.1) have 12-volt coils and ‘Rubber’ cars ( see Fig. 2) have a

9-volt coil with a ballast resistor connected in series.  Both coils generate heat under normal operating conditions.  Coils are designed such that they dissipate this heat and do not, under normal operation, become unduly hot.  However, if they do there is clearly something wrong, begging the question what?  Back to the original question, ‘why does my coil get too hot to touch’?  Many of you have spent hours searching the net for a plausible answer – subsequently I have done the same without finding a valid explanation, I therefore understand your frustration!  You will not thank me but if I am to point you to a solution I must first bore you with some technical ‘waffle’.  Back to first principles.  Coils convert 12-volt DC to approximately 24,000 volts DC and in the process become hot, but not too hot as they are designed such that the heat generated is dissipated via the coil external surface.  Coils overheat simply because the heat generated is not being dissipated – why not?  The answer is simply, that due to an internal fault, such as a short circuit, the current through the primary coil will have increased, consequently the heat generated will have increased.  Referring to Fig 1 (‘Chrome’ cars) you will see that a standard 12-volt coil has a resistance of 3.4 ohms and when connected to a 12-volt battery 3.5 amps flow.  This in turn produces 42 watts of energy.  Under these conditions the coil will operate as designed and not overheat.  Now, say, due to a fault the resistance of the primary coil drops to 3 ohms,  the current will increase to (12 volts divided by 3 ohms) which equates to 4 amps, resulting in an energy increase to 48 watts (12 volts x 4 amps = 48 watts). With an increase in energy of 12.5%, the coil surface will not dissipate this extra energy and will therefore get very hot.  See Fig 2 (‘Rubber’ cars) showing the set up for a 9-volt coil and a series ballast resistor (See also Fig 3 for circuit diagram.).  For this configuration a reduction in either the coil resistance or the ballast resistance will cause the current, and hence the power, to increase and the coil to overheat. Study Figs 1 & 2 and all will be obvious!


I have explained the theory but what you want to know is how to resolve the problem.  Firstly, you must carry out tests to establish the resistance of the coil and in the case of  ‘Rubber’ cars the resistance of the ballast resistor (See Figs.1&2).  More about ‘rubber’ cars later.  Referring to Fig 4, disconnect the coil (‘Chrome’ cars) and using a multimeter test the resistance between the two terminals.  It should not be less than 3.4 ohms.  Now test the resistance between the negative connection and the high tension connection.  It should be between 8,000 and 10,000 ohms.  If you obtain significantly lower readings then you have located your fault – buy yourself a new coil!  In the case of ‘Rubber’ cars the most sensible solution is to replace the 9-volt coil with a 12-volt coil.  Tony (at Watford Classics 01727 768752) a well-established authority on all things MG, endorses this course of action.  It is very simple to do.  Firstly, replace the original 9-volt coil with a 12-volt coil (see Fig 3) and connect a new wire from the positive terminal on the new 12-volt coil to the brown/white wire connection on fuse 2. This will by-pass the ballast resistor and apply 12 volts directly to the new coil.  I strongly recommend this solution as the ballast resistor in the

9 volt system (‘Rubber’ cars) is an inherent source of problems.


Many of you who have enquired about my after-market accessories, i.e. Theft Deterrent, Hazard Warning kit and ‘Tiptro’, and will fit them this winter.  They are inexpensive ‘essentials’ I designed initially to enhance my MG driving experience.  Due to word of mouth and demand I decided to produce them ‘commercially.

 I have intentionally kept the price to a minimum.  For example, a branded Hazard Warning kit costs in the order of £50 (against the cost of my kit £19.50!).  I designed this kit after being stuck on the M3 with a failed fuel pump – not uncommon!  I designed my Theft Deterrent kit to give me peace of mind and protect my ‘C’ Roadster from theft.  I made this decision after a colleague had his immaculate ‘B’ Roadster stolen from a Club gathering at Duxford some years ago.  It is so easy to purchase a derelict car, ‘nick’ a concourse example, and simply exchange the ID plates.  This is more common than you think and MG events are the ideal territory for the ‘professional thief’. My laminated circuits and voltage tester have helped many owners trace and resolve electrical problems. For £4 they are a must for owners with limited electrical knowledge – keep one in your boot!


Next month I will endeavour to answer another regularly asked question relating to a common electrical problem and also continue my series of articles on Body Repairs. Keep your questions coming, if I can’t help I will know a man who can.  Dec 2010  Andy Capy


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