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
one
of
the diodes in the alternator and supplied
m.
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." termi-
nal
and (-) terminal.
2.
Check for good ground connection between meter(-) ter-
minal and alternator.
3.
Check that alternator is well grounded to engine block at
alternator pivot bolt.
Tachometer
Inaccurate
1.
With a hand-held tach on the front
of
the crankshaft
pulley retaining nut or using a strobe type tach, read the
front crankshaft pulley rpm at idle.
2.
Adjust the tachometer using an 2mm Allen head wrench
5/64 (2mm) through the calibration access hole in the rear
of
the tachometer. Zero the tach and bring it to the rpm
indicated by the strobe or hand tach (verify rpm at idle
and at high speed). Adjust the tach as needed.
NOTE:
Current model tac_hometers use a coarse adjustment
dial to set the tachometer to the crankshaft pulley
rpms.
The
calibrating screw
is
then used
for
fine tuning.
(+)
TERMINAL(l)
CALIBRATION
IDLE
SPEED
ADJUSTMENT
&
TACHOMETER
CHECK
(New
Installation)
Checking
the
idle
speed
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 reading as the photo tach.
4. Adjust the idle speed
if
the engine speed is not within the
specified value.
NORMAL
IDLE
SPEED:
RECOMMENDED
RPM
RANGES
MODEL
IDLE
CRUISE
MAXIMUM
RPM
RPM
RPM
M3·20B
1000
-1200
2500-3000
3500-3600
M25XPB
1000-1200
2000-2500
2900-3000
M35B
800-1000
2000-2500
2900-3000
M40B
800-1000
2000-2500
2900-3000
MSOB
800-1000
2000-2500
2700-2800
NOTE:
Attempting to reduce idle speed below the minimum
shown
may
produce unstable engine operation and stalling.
4
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32
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