Return-Path: <microsoft!timke@beaver.cs.washington.edu>
To: british-cars@Alliant.COM
Subject: Speedo on TR6 (and E-Type)
Date: Thu Feb 15 12:32:32 1990
...
Which brings me to question 1: what
would make the tach read proportionately high (like reading 1400 for
1000
and 2800 for 2000 when checked with a diagnostic tach)? Question 2: has
anybody out there ever hooked an E-Type's speedo up?
well, on my friends bugeye'd sprite, the tachometer was electrical, and
measured the RPM by means of an inductive pickup off the wire bringing
12 volts (originally ground) to the coil. when the points closed, and
the coil charged, a current would flow in the wire which would induce a
voltage inside the tach.
there was a little capacitve circut that counted the voltage spikes/second
to determine the RPM with a transistor to amplify the signal to drive
the needle. there was also a potentiometer that could adjust
the RPM that the dial read--since the tach only read to 6000 rpm, and
the engine would redline at 8000 rpm, i adjusted the tach to read 6000 rpm
when the engine was revving 8000 rpm (the tach always read 3/4 of the
actual rpm).
i had the thing apart in the first place to convert it from positive ground
to negative ground, since it hadn't worked since he'd converted the car.
i took the simple wait out, and simply isolated the electronics from the
case and switched the power to them. they didn't seem to care about the
polarity of the voltage spike (or perhaps i reversed the wires on the
inductive pickup--i don't remember).
i guess the tach's really didn't work that well, since they soon changed
to a gear-drive tach, with the pickup off the generator (not alternator),
of all places!
so, if you have an inductive pickup tach, there may well be an adjustment
inside the case. it was recognizable because the wire to the coil had
to pass by it, and was wound several times around a funny D-shaped metal
arrangement that went into the tach body (where there was another wire
wound around it).
craig
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