Installation Guide User Manual

Wiring Overview
particular aircraft. Ensure that the power lines include a circuit breaker or an appropriately sized
fuse for the wire you select.
The EMS-D120 system-wide power requirement is 12 watts typical and 14 watts maximum. On
a 12-volt system, this translates to about 1 amp of maximum current draw. On a 24-volt system,
this translates to about 0.5 amps maximum current draw. Normally, a 2-amp circuit breaker or
fuse is sufficient.
Grounding
Many of the engine sensors require a connection to a
shared electrical ground with the EMS-D120. There are
many places on an aircraft where you could connect these
sensors. However, the ideal location to ground these
sensors is near the EMS-D120 to minimize voltage
differences between the sensor and instrument grounds.
Some sensors (e.g., oil pressure and oil temperature)
connect to ground via their cases’ contact with the engine or aircraft body. There must be a solid
connection between this “case ground” and the EMS-D120 ground. The oil temperature sensor is
very susceptible to voltage differences between the engine case and the negative terminal of the
battery. Ensure that solid, thick electrical connections exist between the engine and battery
ground. Other sensors (e.g., fuel pressure) do not have a grounded case and have two leads
instead. One lead must be connected to ground, the other to the sensing input of the EMS-D120.
The EMS-D120 has 3 pins on the 37-pin harness which may be used for connecting such sensors
to ground. More than one sensor’s ground may be connected to any of these three grounds using
a splice.
Pin Color Function
5 Black Ground
16 Black Ground
17 Black Ground
The case of the EMS-D120 is connected to its supply ground. If your panel is connected to
aircraft ground, the connection between the instrument’s case and the panel dramatically helps
minimize voltage differences between the instrument and sensor grounds. If your panel is not
metal, or is otherwise isolated from engine ground, connect a 14 AWG or larger wire to the
instrument case. The most convenient place to do this is at the back of the mounting tray.
Additionally, connect any unused EMS ground leads to a convenient ground. Keep all ground
leads as short as possible.
Because of the current drawn by the EMS-D120, even very small resistances between battery
ground and instrument ground can cause voltage differences which adversely affect engine
sensor readings. An easy way to test the quality of the instrument’s ground is to measure voltage
between the ground pin at the EMS-D120 and the ground lead at your aircraft’s battery. With the
EMS-D120 powered on, connect one lead of your voltmeter to a free ground lead coming from
the EMS-D120. Connect the other lead of your voltmeter to the ground terminal of your battery.
The voltage between these two points should measure very close to 0 mV (within 5 mV). If it
does not, you must improve the ground connection between the ground of your battery and that
of your avionics bus.
2-2 EMS-D120 Installation Guide