Aetna SM Versions User Manual

PRECISION SENSITIVE DIGITAL TACHOMETER

INSTALLATION INSTRUCTIONS

Special Instructions for ‘-SM’ & ‘-SH’ Versions

Aetna Engineering Precision Sensitive Digital
Tachometers are premium quality instruments.
They use "LSI" digital circuitry to provide an
instrument guaranteed accurate to one RPM,
that, never requires calibration, and performs
reliably in the marine environment.

The ‘-S_’ version tachometers are supplied with
a special ratio setting designed to operate from
the signal provided from a “mag-pickup" or other
sensor which receives a particular number of
pu!ses-per-revoIuiion (PPR) from the rotating
device to be measured. The PPR ratio refers to

the preset number of pulses the tachometer
must detect for each revolution of the apparatus
to be measured. The -SM' version is rated for
use between 28 and 90 PPR and the '-SH'
version is rated for use between 90 and 255
PPR. The overall tachometer performance

involves functions beyond the PPR ratio, such as
filter constants and signal sensitivity. Therefore,
the correct version. '-SM' or '-SH' must be used
for the specific PPR ratio desired or satisfactory
performance will not be realized.

The tachometers are available with either Light
Emitting Diode (LED-Model 8402) or Liquid
Crystal Display (LCD-Model 8905) readouts.

The LED is best suited for use in shaded
sunlight while the LCD should be used in
applications where the tachometer will be
exposed to direct unshaded sunlight. Both are
self-iiluminated for night viewing.

The tachometers are supplied with a choice of
black or polished stainless steel bezel. The
black bezel tachometer includes a trim ring
which can be optionally used as required to
match existing instrumentation. Aetna

tachometers also fit perfectly inside both the

round and square VDO Blue Line cases (contact

us for detailed instructions).

Mounting:

Select a mounting location where the tachometer
may be easily read. The front of the tachometer
has a gasketed glass-to-metal seal making it
suitable for use in locations exposed to rain and
spray. It uses only non-ferrous materials so it

can be located in close proximity to a compass
without significantly affecting the compass
(nevertheless, the compass adjustment should

be checked after installation; particularly

because the removed tachometers may have
been influencing the compass). The unit may be
installed in an existing standard 3-3/8 inch
diameter hole. It will fit holes up to 4 inches

when used with the optional black trim ring. If

placing the tachometer under a protective glass

cover, be sure that it is adequately ventilated to
keep the maximum temperature of the case
below 175 degrees Fahrenheit. Exceeding this
case temperature may damage the tachometer.

Mount the tachometer using the clamp and
hardware provided. In exposed locations it
should be bedded down with a bead of sealant

behind the flange. Place the tach in the raised
trim ring if it is to be used, if the trim ring is
positioned so that it will collect water, drill a
3/16" drain hole at the lowest point in the ring.
The clamp legs may be bent over or cut off to
accommodate thicker panels. An O'ring is
included for use as a centering aid when
installing the unit in 3-3/8 inch holes. If the hole
is tight, roll the O'ring back from the flange then

insert the tach so that the O'ring rolls into the
hole as the tach is inserted. For larger holes an
Oaring type spacer may be made from a length

of appropriate diameter wire.

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