The 65 originally had a current pulse tach. This has a 12v and ground
supply on green and black wires, and the current pulse is obtained from a
white looped through an inductive pickup en route from the ignition switch
to the coil +ve or SW terminal. The earliest of these had a physical loop
of wire from the loom that was placed in the external pickup, later versions
had two separate whites with male and female bullets on the loom, the tach
having an internal pickup with flying leads or the bullets mounted on the
case. Later tachs (73 on) were voltage operated and had a single
white/black from the coil -ve to the tach in place of the white loop. You
need to determine what type of tach you have and if it is connected
correctly to whatever ignition system is on your engine. If electronic
ignition or fuel injected with an ECU you could well have problems with the
current pulse tach, this is not uncommon with aftermarket electronic
ignition systems on the standard engine. You stand a better chance with the
voltage operated tach, but it all depends on the ignition system for your
engine.
Since there are plenty of references around to converting standard tachs for
a V8 by adding a multi-turn pot in the appropriate place I see no reason why
this shouldn't work for a V6 with a suitable ignition system. V6? 3 firing
pulses per revolution? 1000 revolutions per minute? 50Hz? I've not done
it but again I have heard of using a simple battery charger with a simple
rectified (and not smoothed) AC output as a test signal for tachs, 50Hz if
single wave, 100Hz if full-wave rectified.
PaulH.
----- Original Message -----
From: "Richard Schultz" <schultz2146@earthlink.net>
To: "MG List (E-mail)" <mgs@autox.team.net>
Sent: Saturday, February 26, 2005 8:32 PM
Subject: MGB 65' v6 231 conversion alt/tach problem
> ... I recently acquired a 65' mgb that had the 231 v6 conversion.
>
> Like I said I have a Smith Tach. The needle does not deflect at all when
> engine is running at any speed.
>
> Checked the 12VDC source that comes to the tach. Question. since I
should
> see a pulse from this 12VDC, what AC source should I be seeing from same
> lead and at what freq for 1000 rpm?
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