Dan Masters wrote:
There are at least two type of tachometers used in British cars. One, the
"series type," was used in earlier MGBs, and maybe some other makes (I put
the term "series type" in quotation marks because I haven't a clue as to the
correct name for them). The other type - whatever they are called - was used
in all Triumphs that I am familiar with, and is the type used in most
american cars and aftermarket units as well.
The series type is wired, as the name implies, in series with the power lead
to the coil. Inside the tach, the wire makes a small loop through an
electromagnetic pickup on its way to the coil. Every time the coil fires, the
current pulse through the tach is detected by the pickup, and triggers the
tach. If you reverse battery polarity in an early negative ground MGB, this
loop of wire also has to be reversed for the tach to work. Basically, the
wire loop and the pickup act as the primary and secondary windings in a
transformer, or ignition coil.
The other type, by far the most common, is triggered by closure of the
points, the same as the coil. This type is wired to the negative post of the
coil. Closure of the points is detected by a transister circuit inside the
tach. As far as I know, all aftermarket tachometers are of this type.
To which I must add,
See, I told you my main 'lectical man was rarely if ever wrong. How
could I, a mere mortal, even contemplate THINKING about correcting him. I am
not worthy to hold your cicuit tester oh master of Masters. Thank you for the
clarification, and sorry if I led anyone else down the primrose path with my
spewing of little knowledge.
(sneaking out the back door, hoping to go unnoticed)
Morgan Hill, CA