 Ignition coil is located adjacent to the expansion tank on the left side radiator panel. 188 V8 overheating - and now for something different Bob Owen was on holiday in Italy and discovered the cause of his overheating problem - and it was quite a surprise! (Oct.08) My MGBGTV8 has for some time run badly and eventually stopped when idling in traffic in very hot weather. The water temperature was high but not a problem (4 o'clock on the gauge rather than 7 o'clock). No water was being lost. The problem occurred after the fans had been running for some time and the underbonnet temperature was very high with the carburettor plenum chambers at 70C. If the car stalled, lifting the bonnet for five minutes would allow the car to start again. So I had assumed it was a carburettor/fuel vaporisation problem. Two new carburettors and 4" diameter fresh air ducts to the air cleaners plus fan forced draught onto the float bowls would sort that out I deduced . . . . . . wrongly. On a recent trip to Umbria in Italy (39C local temperature!) the problem was still evident. "She who must be obeyed" cruelly remarked that it | Why
did the faulty coil cause overheating? Bob Owen is an electronics engineer and, in response to a question "why did the faulty coil cause the overheating problem", he has produced a useful explanation by expanding on the physics of the fault. Remember the faulty coil had had rivetted connections whereas the new replacement coils have improved bolted connections. When the rivets on Bob's original coil got hot and expanded, they relaxed their grip on the connection tabs and the connection resistance went up, so causing the faulty ignition. When the coil cooled, the connection resistance went down again. The faulty coil gave reduced spark energy when it got hot because the increased contact resistance reduced the current flowing when the points were closed. Inductive energy is proportional to current squared - so if you halve the current, you only get a quarter of the energy. The energy in the primary of the coil is transferred to the secondary and then on to the sparking plug when the points open. So, the car was "overheating" in so far as it was running badly and finally stopping when the underbonnet temperature was high. The water was hotter than normal but not into the red on the gauge and not boiling, or even locally boiling in the block, as evidenced by the lack of water loss. When the car stopped and the bonnet was opened then the coil cooled, the rivets contracted and gave lower contact resistance, the available spark energy increased and so the car would run again. The coil was responding to it's ambient temperature but was not the cause of it. It was, however, the cause of the bad running at high temperatures and so in that sense the cause of the "overheating". Note that spark energy is crucial to the running of the engine. The spark has to have enough energy to cause a sufficient quantity of fuel and air to combine to produce enough local heat for the reaction to become self propagating - ie the presence of a spark is a necessary but not a sufficient condition for ignition . . . . . . hence the "feel" for a sufficiently "strong" spark when testing. On a separate but related topic, the spark energy issue is the main reason for the ballasted coil. The ballast resistor is by-passed during starting to boost the spark energy that would otherwise be reduced |  The new replacement coil has bolted connectors rather than the rivetted connectors on the original coils fitted to the MGBGTV8. due to the lower battery voltage while cranking. My V8 won't start if the ballast resistor is in circuit - see my earlier V8NOTE320 using a diode to carry out this function if the starter solenoid auxilliary contacts fail. My cranking voltage at the starter was 8V. If the running voltage of the batteries is taken as 14V, and the ballast resistor were not by-passed, the current through the coil when cranking would be only 0.57 of that when running - ie the spark energy would only be a third of normal, just when you need it to be high. I have recently installed twin 12 volt batteries in parallel following the useful V8NOTE357 contributed by Kai Knickmann. That set up still results in 12V but with twice the current delivering capability; the original 6V batteries which were in series so making 12V but with the same current delivering capacity as one battery. With the twin 12 volt batteries it is possible the higher cranking voltage with that new battery arrangement would allow starting even with the ballast in-situ - an interesting thought. You could run a higher current through the coil at all times and obviate the need for a ballast, but that would mean running the points at twice the current with a consequent acceleration in the contact erosion. This is not relevant with electronic ignition and so they don't use ballasted coils. |
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seemed worse for all my efforts in making those modifications. Then something
told me to feel the ignition coil. It was very hot. Could this be the problem?
Pierro Fusaroli took me to a garage next door to his factory and a spare ballasted
coil was found in a scrap bin. The car ran with this so it was jury rigged on.
Off we went with the group up into medieval villages, crawling around steep streets
at 35C; all stayed sweet as a nut! So the overheating problem was the coil all along - manufacture date 1977. Paul Hunt on his website cautions against the early coils with rivetted connections; later ones have screws and nuts. Mine had rivetted connections. When the rivets got hot and expanded they relaxed their grip on the connection tabs and the connection resistance went up, so causing the dodgy ignition. The total coil resistance is only about 1.5ohms, so the contact needs to be good. Moral: Beware of dodgy coils and beware of false diagnoses based on conventional wisdom! Coil GCL111 - see page 35 of the Moss MGBGTV8 Parts Supplement MGL001B. More | ||
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