World Class Catamarans 226SF Owner's Manual - page 32
TACHOMETERS
Operation
Electronic tachometers
work by counting pulses generated
by the ignition system or alternator.
The tach is
hooked up to
+
12VDC,
Ground, and one of the signal sources listed above.
By selecting the right tach and
setting the switch on the back to the correct position, you let the tachometer
know how many pulses are being
sent per engine revolution.
From this information,
the tach displays the correct engine speed.
Instrument part
numbers are located on a label attached to the outside of the case (i.e. TCOOOOA).
Application
4 cycle engines:
The tach signal terminal is connected
to the negative terminal on the ignition coil or to a
transistorized
tach driver circuit connected
to the ignition system.
This circuit will have a wire (usually gray) for
connection to the tach. The correct tachometer
will have a white label on the side indicating which switch position
is for each engine type. This label will include 4, 6, and 8 cylinder engines for positions 1, 2, and 3.
2 cycle engines:
The tach signal terminal is usually connected
to the unrectified AC output of the
alternatorllighting
coil. Sometimes
it is hooked directly to the stator output wire (usually yellow) other times a gray
tach output lead is provided.
The correct tach for this application will have a white label on the side with switch
positions for 4,6,8,10,12,
or 20 pole alternators.
Calibration
Set up a calibrated "shop tach" or "strobe tach" to monitor the engine's true RPM.
Start the engine and (after an
appropriate warm-up period and with the shift in neutral) increase it's speed to the boat's normal cruising RPM
read on the shop tach.
Set the coarse adjustment
switch to the proper position described on it's label.
Remove
the stop-plug or paper label corner (at the 8-o'clock
position on the rear of the case for most) and insert a 5/16"
Allen wrench into the "fine adjustment"
trim pot, rotating it CW or CCW as necessary to indicate the true RPM.
Troubleshooting
Symptom recognition is the first step in effective instrumentation
troubleshooting.
Tachometers
usually exhibit the
following symptoms:
A)
Dead - This is usually caused by: 1) No power applied,
2) No signal supplied, or 3) tach damaged by
electrical transients caused by disconnecting
the battery with the engine running.
1) Check to see if power is applied to tach by switching the instrument
supply switch on and off. As power is
applied, the pointer should jump slightly.
If it does not, check to see that the wires are installed on the correct
terminals and that 12 volts are actually applied to the terminals themselves.
2) If tach indicates that power is applied, check for the presence of a signal on the signal
terminal.
Measure
the signal between the signal and ground terminals.
This should read in excess of 2 volts DC.
3) If power and signal are present, then it is possible that the tach has been damaged by electrical transients.
B) Pegged - This condition occurs on tachs with internal mechanical
pointer stops.
It is caused by removing
power from the tach while it is running in excess of mid-scale
RPM's or by the switch on back of the tach being in
between positions.
When power is re-applied,
the tach pointer attempts to go clockwise to zero but cannot
because the internal stop is in the way.
Read "Marine Instrumentation
General Facts" on the previous page for
details on how to correct this condition.
C) Erratic - This symptom is caused 99% of the time by on intermittent connection
between the wire and the ring
or spade connector.
Often the wire's insulation is pushed into the crimp area and crimped.
The center conductor
casually touches the connector allowing the tach to work most of the lime but causing a nightmare for the
technician.
Electrical noise also can cause erratic readings.
See "Reading
High" below for further information.
D) Reading High - This is usually caused by the switch on the back of the tach being in the wrong position.
If the
number of cylinders or alternator poles selected by the switch is too low, the tach will read high. If a variable
alternator or magnetic pick-up tach is being used, then further calibration
may be necessary, as this calibration is
done by the end user See 'Calibration'.
Excessive electrical noise may also cause the tach to read high. These
noise spikes are counted by the tach as engine RPM's.
The wire affected by the noise can be identified by
connecting
one wire at a time to the tachometer
directly from the battery or the signal source on the engine.
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