
Ignition coil is located adjacent
to the expansion tank on the left side radiator panel.
388
V8 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.
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 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? |
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
|

The new replacement coil has bolted connectors
rather than the rivetted connectors on the original coils fitted to
the MGBGTV8.
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 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. |