190-01115-01 G3X™/G3X Touch™ Avionics Installation Manual
Rev. AV Page 21-28
21.3.5.3 P-Lead RPM Sensing
CAUTION
Only a MOD Level 2 GEA 24 or any GEA 24B can be connected to a P-Lead. Do not
connect MOD Level 1 or MOD Level 0 units to a P-Lead. The MOD Level status can be
determined by checking the serial tag on the unit. If a P-Lead is connected to a non-MOD
Level 2 GEA, an “Engine Interface 1 configuration error” System Message will be
generated, and configuration status will be reported to be in error in the system
information menu.
For a direct connection, the GEA 24 measures the electrical signal generated by the primary magneto coils
or “P-Leads” from both engine magnetos. The connections can be made to the magneto P-Lead stud or to
the P-Lead wire at the ignition switch as shown in Figure 27-4
. If the magneto does not have a ring
terminal stud, connect the GEA 24 to the ignition switch. Otherwise, connect it to whichever minimizes the
wire length to the GEA 24.
When making a connection directly to a P-Lead, a 400kΩ (±10%) resistor needs to be installed. To prevent
errant shorts to ground in case of a broken or shorted wire, (as outlined in Figure 22-14
) the wire length
between the P-Lead connection and the resistor must not exceed six inches. Shielded wires must be used.
Use a resistor that is 400K ohm (±10%), 0.5W, rated to maintain resistance and power rating at 150°C, and
qualified to MIL-R-10509.
Following the installation of the P-Lead signal wires, verify the continuity of each magneto P-Lead to
airframe ground while the ignition key is OFF. If there is evidence of discontinuity in the magneto P-Lead
grounding circuit, it must be corrected before further engine maintenance or checks. Continuity can only
be measured if the magneto points are open or the wire is disconnected from the magneto. Use a magneto
timing light to make sure the ohmmeter will not measure false continuity through the points or coil
windings.
WARNING
Do not turn the propeller and stay clear of the propeller arc when installing the P-Lead
signal wires.
21.3.5.4 Surefly TACH2 Module (P-Lead RPM Sensing)
The SureFly TACH2 module is used like a P-Lead input when a SureFly Ignition Module (SIM) is
installed. The TACH2 outputs a 0-5V rpm signal to the GEA for a SureFly Ignition Module (SIM). The
0-5V output from a SureFly TACH2 can replace a magneto P-Lead input to measure RPM if a SureFly
Ignition Module (SIM) is installed. This method does not require the resistor in-line with the signal.
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