Tachometer Troubleshooting; Tachometer/Hourmeter; Hourmeter Inoperative; Tachometer Inoperative - Universal M3-20A Operator's Manual

TACHOMETER TROUBLESHOOTING
TACHOMETER/HOURMETER
The tachometerlhounneter used in propulsion engine
instru~
ment panels contains two separate electrical circuits with a
common ground. One circuit operates the hounneter and the
other the tachometer. The hounneter 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 Yolts, fed
from one of the diodes in the alternator and supplied to the
"tach iop." terminal while the engine is running, and the
alternator producing battery charging voltage 13.0 - 14.8
volt, DC.
The following are procedures to follow when troubleshooting
a fault in either of the two circuits in a tachometerlhounneter.
Hourmeter Inoperative
Check for the proper DC voltage between (+) and (-) termi-
nals.
a. Voltage present - meter is defective - repair or replace.
b. Voltage not present - trace (+) and (-) electrical connec-
tions for fault. (Jump 12 volts DC to meter (+) terminal to
verify the operation.)
Tachometer Inoperative
a. Voltage present - attempt adjusting meter through cali-
bration access hole. No results, repair or replace meter.
h. AC voltage not present - check for proper alternator DC
output voltage.
c. Check for AC voltage at tach terminal on alternator to
ground.
d. Check electrical connections from "tach inp." terminal to
alternator connection.
Tachometer Sticking
a. Check for proper AC voltage between "tach inp." tenninal
and (-) terminal.
h. Check for good ground connection between meter (-) ter-
minal and alternator.
C.
Check that alternator is well grounded to engine block at
alternator pivot bolt.
Calibration
a. With a hand-held tach on the front of the crankshaft pul-
ley retaining nut or using a strohe type lach, read the front
crankshaft pulley rpm.
b. Adjust the tachometer with a sma\! Phillips type screw-
driver through the calibration access hole in the rear of the
tachometer. Zero the tach and bring
it
to rpm scI by using
a strobe or hand tach (verify rpm at idle and at high speed
1500 - 2500 - 3000 rpm). Adjust the tach as needed.
NOTE:
Current modellachometers use a coarse adjustment
dial to set the tachometer
[0
the crankshaft pulley rpms_ The
calibrating screw is [hen used for fine luning.
LIGHT
TACH INP
CALIBRATE
EARLY MODEL TACHOMETERS
(-) TERMINAL
1-)
TERMINAL
~~
COARSE AOJIJST"ENT--f:jj~
1-)
TERMINAL
CURRENT TACHOMETER
~"ZlIt----I+)
TERMINAL
FINE
CAI.lBR'ATING.-~~~:"~
INP
29