Aetna SM Versions - Manual (Page 2)

Wiring:
During wiring it Is recomnnended that the
negative battery cable be removed from the
battery or the master switch{es) turned off.
The tachometer has a grounded case and is for
use on negative ground systems only (an
isolated case version is available for isolated or
positive ground use.) Connect the "Ground"
terminal to a point that is connected to the
negative {-) battery terminal.
Connect the "+12V" terminal to a point that is
energized when the ignition is switched on;
typically a purple wire. In most cases, power
and ground are available by wiring to the
terminals on an adjacent temperature or oil
pressure gauge, A one amp fuse must be
installed in series with the "+" (Pos) wire in order
to enable the internal surge protector function.
To use the Model 8402 on 24 volts connect a 30
ohm, 25 watt resistor (our Model 8924) in series
with the “+12V" tachometer terminal For 32
volts use a 47 ohm, 25 watt resistor (our Model
8932). Failure to use the appropriate ballast
resistor
may damage the Model 8402
tachometer. The "+12V" terminal on the Model
8905 tach. may be safely connected to 12, 24 or
32 volt systems without a ballast resistor.
The "Points" (Sig.) terminal connects the RPM
information signal from the engine to the
tachometer. This connection should be made
using stranded two conductor twisted-shieided
wire. For multiple stations, two or more
tachometers may be connected in parallel
Diesel engines have no ignition and therefore
require a tachometer sender to generates the
digital impulses required by the tachometer.
Traditionally a mechanically driven sender was
mounted on the tachometer drive cable fitting on
the engine. Many newer engines do not have
such a fitting and instead use a magnetic pickup.
A “mag.*’ pickup is a device that senses the
passage of gear teeth in a non-contact manner,
it is screwed into a threaded hole in the gear
housing and adjusted so that the clearance is
approximately one-half turn back from contact
with the gear. The two sender wires are
connected to the signal and ground terminals at
the tachometer using a twisted/shielded cable.
Connect the two wires from the sender through
the two twisted inner conductors. Connect the
shield to the engine block. Do not connect the
tachometer end of the shield. If the signal is
continued on to another tachometer, carry the
shield connection through but do not connect it
to anything but the engine. Using a single wire
and the engine ground is not recommended and
will yield inferior results.
Diesel Sender Installation:
The Aetna Engineering Models 8910, 8912 and
8922 are magnetically coupled senders which
sense the passage of flywheel gear teeth or
similar items. They generates the electrical
RPM signal used by the tachometer. The 8910
and 8912 are housed in a diameter by 16
threads per inch case while the 8922 is
contained in a 5/8” by 18 threads per inch case.
Both the 8912 and 8922 have dual electrical
outputs.
Note that some engines built offshore have
metric sized sender mounting holes. Although
Aetna does not provide metric sized senders,
most metric “mag pickup” senders will
satisfactorily drive Aetna tachometers.
The sender is installed by screwing it into a
threaded hole on the flywheel bell housing or
other housing. It is screwed in until the end
contacts the gear then backed-off one-half turn
and secured in position with the jam nut. The
sender is connected to the tachometer using two
conductor twisted-shieided wire as previously
described.
This combination of tachometer and sender will
register the engine speed accurately to within +/-
one RPM when the sender is driven at exactly
the preset number of pulses-per-revolution rated
on the tachometer