coil is located adjacent to the expansion tank on the left side radiator panel.
overheating - and now for something different
Bob Owen (Blaze 1625) 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.
a recent trip to Umbria in Italy (39C local temperature!)
the problem was still evident. "She who must be obeyed" cruelly remarked
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
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.