A 74 tach is voltage pulse triggered from the points via the white/black,
which is continually switching between a nominal 12v and 0v when the engine
is running. A 74 CB should have a 45D4 distributor, which has a nominal 51
degree dwell. This means that it is closed for 51 degrees of the 90
(distributor) degrees for each cylinder, or about 57% of the time. Since
the actual system voltage is usually closer to 14v than 12v, 57% of 14v
gives 7.9v. If you connect an *analogue* meter to the points wire even
though it is trying to move between 0v and 14v it cannot react fast enough
and so displays an average of the two based on the ratio of the time the
points are open to the time they are closed (ratio is another way of
expressing dwell angle), so your reading of 7.6v is pretty close to the
theoretical. Digital meters usually take an instantaneous voltage reading
several times a second, so generally never give a reading stable enough to
be useful in a circuit such as this.
But if you don't see system voltage between the green and black wires at the
tach then that is the cause of its non-operation. According to my diagrams
on a 74 CB the feed to the tach comes through a multi-way dash connector
plug. On the loom side of that plug there are two green wires - one going
on to the voltage stabiliser for the fuel *and* temp gauges, the other
coming from the fusebox via a 4-way bullet connector behind the dash. If
the temp gauge works OK, and it has the feed from the stabiliser (i.e. it
hasn't been frigged by a PO with a direct 12v connection) then it follows
that the stabiliser and the 12v feed to that must be OK. As this comes via
the loom side of the dash connector either those pins aren't making
connection (has one been pushed out the back of the insulator block?) or
there is a break in the wire from there to the tach. If none of the tach,
fuel or temp gauges work then the 12v feed to the dash connector (and from
there to the stabiliser) must be missing, and a likely place for this will
be at the aforementioned 4-way bullet connector. If the temp gauge is
working and does have the 'stabilised' (a Lucas joke, since it is a voltage
switching between 0v and 12v about once per second) voltage, then you have
two (at least) faults :o)
PaulH.
----- Original Message -----
From: <BarrMark262@aol.com>
To: <mgs@Autox.Team.Net>
Sent: Saturday, January 29, 2005 1:07 AM
Subject: Help with electrical
> I am stumped. !974 B cb, tach stopped working. The voltage I measure from
> the coil is about 7.6 to the tach. What should it be? This b/w wire is
what
> makes the tach work right?
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