Service manual
TACHOMETER
TACHOMETER/HOUR
METER
The tachometer/hour meter used
in
propulsion engine instru-
ment panels contains
two
separate electrical circuits
with
a
common
ground.
One
circuit operates the hour meter and the
other
the
tachometer.
The hour meter circuit operates
on
12
volts alternator charging voltage supplied
to
the (
+)
terminal
on
the
back of
the
instrument.
The tachometer circuit operates
on
AC
voltage
6-8
volts,
fed
from
~ne
of the diodes
in
the alternator
and
supplied
to
the
tachometer input terminal while
the
engine is running,
and
the alternator producing battery charging voltage 13.0-14.8
volts
DC.
The following are procedures
to
follow
when
troubleshooting
a fault
in
either of
the
two
circuits
in
a tachometer/hour
meter.
Hour
meter
Inoperative
Check for the proper
DC
voltage between (
+)
and
(-)
terminals.
1.
Voltage
present - meter is defective - repair
or
replace.
2.
Voltage
not
present - trace (
+)
and
(-)
electrical con-
nections for
fault.
(Jump
12
volts
DC
to
meter(+)
terminal
to
verify the operation.)
Tachometer
Inoperative
Check for the proper
AC
voltage between tachometer input
terminal
and
(-)
terminal with the engine running.
1.
Voltage
present - attempt adjusting meter through calibra-
tion
access
hole.
No
results, repair or replace
meter.
2.
AC
voltage not present - check for proper alternator
DC
output
voltage.
3. Check for
AC
voltage at tach terminal on alternator
to
ground.
4. Check electrical connections
from
tachometer input ter-
minal
to
alternator connection.
Tachometer
Sticking
1.
Check
for
proper
AC
voltage between "tach
inp."
terminal
and
(-)
terminal.
2.
Check
for
good ground connection between meter
(-)
terminal
and
alternator.
3. Check that alternator
is
well grounded
to
engine block at
alternator
pivot
bolt.
Tachometer
Inaccurate
-a~
With
a hand-held tach on the front
of
the
crankshaft
pulley retaining nut or with a strobe-type tach, read the
front crankshaft pulley rpm
at
idle.
b. Adjust the tachometer using
a 5/64 or 2mm Allen wrench
through the CAL. (calibration) access hole in
the
rear of
the
tachometer. Zero the tachometer and then adjust it to
the rpm indicated by the strobe or hand tachometer.
(Verify
the rpm at idle and
high
rpm and adjust the
tachometer
as
needed)
31
NOTE:
Current
model
tachometers
use
a
coarse
adjustment·
·
dial
to
set
the
tachometer
to
the
crankshaft
pulley
rprils.
The
calibrating
screw
is
then
used for
fine
tuning.
TACHOMETER
CHECK
(New
Installation)
NOTE:
In
a
new
installation
having
new
instrument
panels,
the
tachometer
may
not
always
be
correctly
calibrated
to
the
engine's
rpm.
This
calibration
should
be
checked
in
all
new
installations.
1.
Warm
up
the
engine
to
normal
operating
temperature.
Remove
any
specks
on
the
crankshaft pulley
with
a
clean
cloth
and
place a piece of suitable reflecting
tape
on
the
pulley
to
facilitate
use
of a photoelectric type
tachometer.
2.
Start
and
idle
the
engine.
3.
Aim
the
light of
the
tachometer onto the reflecting
tape
to
confirm
the
engine speed. Check
the
instrument
panel
tachometer reading. Adjust the tachometer
in
the
panel
by
using
the
instrument coarse adjustment
to
calibrate
the
instrument reading
to
the closest
R.P.M.
that
the
photo
tach
is
showing.
Then
use
the
fine
calibration adjustment
to
bring
the
instrument
to
the exact
r~ading
as
the
photo
tach.
4.
Set
the
tachometer
to
the
idle speed
(the
engine
idle
speed
has
been
factory
adjusted
and
the
idle screws
and
high
speed
screws
have
been locked
in
place).
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