just a few lines to add to the overdrive discussion. (well a lot really)
A- types usually operate on 2nd, 3rd and fourth
whereas j-types usually operate on 3rd and fourth.
the reason is that the j-type has smaller gears and is more vulnerable to
the extra torque you get from second gear. there is no mechanical reason
that an overdrive cannot operate on any forward gear. the factory healys
used to race with overdrive on all forward gears (but someone else paid for
the rebuilds).
operating an overdrive in reverse will break lots of expensive metal bits.
the switching is really very simple; you have two choices
1) you can disable (lockout) the overdrive in certain gears (eg reverse and
first)
2)you can enable (switch in) the overdrive in certain gears (eg 3 and 4).
the tr6 and 2500 cars use option 2 (as may all triumphs but i haven't seen
them all)
the factory tr6 manual shows quite clearly how the switching works (diagram
86.00.06 and others)
there are two gearbox switches (for A-types)one for second and one for third
/ fourth. these form a mechanical 'OR' gate which is to say the overdrive
will be enabled if you are in second or third or fourth. (leave out the
second gear option for J-type.)
the overdrive will not operate yet unless the column / gearknob switch is
operated.
the complete logic is;
overdrive on = columnswitch AND (second OR third OR fourth).
the three switches involved to achieve this cause the overdrive relay to
operate which switches the actual solenoid current (a few amps) and engages
the hydraulics.
personally i think this system works fine and any extra electronics, as
suggested on the list, to drop out the overdrive when passing through
neutral would be a problem.
imagine you are driving quite fast in overdrive fourth and you change down
to third, normally you would get over drive third, but if the unit drops out
you have suddenly changed down two gears and you may get more revs than you
want.
hope this helps explain a few basics.
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