[C38] Tachometer not working

[Les]

On Sun, 2007-07-15 at 06:30 -0400, jonbix at cox.net wrote:
> replaced
Check the wiring from the switch to the tach.  The tach requires 12V as
well as the tach signal from the alternator.  It sounds like the clip
for the 12v or for the ground is missing.  You should see 3 or 4
separate wires on the tach.
	1. +12 for the tach electronics and meter
	2. -(gnd) return for tach electronics, meter and AC signal
	3. AC signal from the alternator
	4 12V to the lamp (usually an internal connection)

If you trace the gnd back it should go to the black wire feeding from
the harness to the engine.  It can also either be broken or corroded in
one of the connections.  You can check this one with an ohmmeter back to
the block should read <1 ohm.

AC signal comes from either the AC signal on the alternator or from a
pin labeled TACH on the alternator.  Check your manual to see which
alternator you have and how it should be connected.

If you trace the 12V connection it should go back to the switch.  The
possibilities are: the wire is broken. the connection is dirty on one of
the ends or the aux switch contact is bad on the ignition switch.  You
can check this connection for 12V measured to the black lead with the
switch on. If you get something, but less than 12v (13.8 if the battery
is topped off), then you have a problem with the power from the switch.
Remove the connection to the instruments from the switch and check the
switch terminal.  It should read the 12V if nothing is connected.  If
not, the switch is definitely bad.  If it reads OK, and you connect the
panel and it drops again, you probably have either a bad switch or a
short somewhere in the panel.  Try removing all the lamps from the tach,
water temp, oil pressure, and fuel gauge (if you have one).  Sometimes a
lamp will short out when it burns out.  If the 12V returns with the
lamps out, check each one to see which is bad.  If not, then you have a
bigger problem and it's time to call an expert.

I hope this helps.

Regards,
Les H