TM 1-1520-238-T-4
6–20
Change 5
6–7.
SYSTEM DESCRIPTION (cont)
6–7
(7) Transducers and Switches.
Transducers and switches are used in the transmission and nose
gearbox oil systems to monitor pressures and temperatures. Transducers send a signal to the multiplex
subsystem for a visual indication of pressures and temperatures on the CPGs HOD. Switches send ground
signals to the pilot and the CPGs caution/warning panels that turn on caution lights.
(a) Pressure Transducers.
All four pressure transducers (NGB 1, XMSN 1, XMSN 2, and NGB 2) are
identical. Each has four wires but only three are used; the green is never connected. The black wire is connected
to a –5 VDC signal and the white is connected to a +5 VDC signal. Both signals come from the appropriate
MRTU in the multiplex subsystem. The red wire sends a signal to the MRTU that is –5 VDC at a 0 PSI state and
is +5 VDC at 100 PSI. The OVDC state is defaulted to 50 PSI.
1. The characteristics of these transducers result in three typical failures displayed on the HOD
when there is an open in the circuit. A 0 LB reading on the HOD is caused by an open in the white wire (–5 VDC
signal from the black wire thru the red wire to the MRTU), a 50 LB reading by an open in the red wire (no signal is
interpreted by the MRTU as OVDC), and a 99 LB reading is caused by interpreted by an open in the black wire
(+5 VDC from the white wire thru the red wire to the MRTU). The highest reading on the HOD is 99 LB (read as
greater than 99 LB).
(b) Temperature Transducers.
All four temperature transducers (NGB 1, XMSN 1, XMSN 2, and
NGB 2) are also identical. Each has four wires; two unshielded wires for power and two shielded wires for the
output signal. The red wire is always the 28 VDC power signal from the engine start circuit breaker (CB58). The
yellow wire is the output signal (0 – 10 VDC) to the appropriate MRTU. The other two wires are both blue; one
inside the shield with the yellow wire and the other unshielded. Both are connected in the inside of the
transducer.
1. The characteristics of these transducers result in one typical failure displayed on the HOD; a –32
C reading that is caused by no signal to the MRTU (interpreted as OVDC). This can be the result of an open in
the red wire (no power to the transducer), the yellow wire (no signal to the MRTU) or an open in both blue wires
(no ground for the transducer). There is no failure noted as long as one of the blue wires terminates in a ground.
(c) Pressure Switches.
There are five pressure switches; (NGB 1, XMSN 1, XMSN 2, NGB 2, and
ACC GRBX) in the drivetrain system and all are identical. Each has two wires and all are wired the same; white to
ground and red to the appropriate caution/warning indicator.
1. With no pressure applied (28 2 PSI decrease) to the system, the pressure switch should be
closed completing the circuit ground. If either of the wires has an open the caution light will not be illuminated.
With pressure applied (45 PSI INCR) the switch should be open thus removing the ground and extinguishing the
caution light.
(d) Temperature Switches.
All four temperature switches (NGB 1, XMSN 1, XMSN 2, and NGB 2)
are identical and are a part of the system chip detectors. The temperature switch function of the chip detectors
used the white wire for the signal to the caution/warning system and the black wire for the ground. The red wire is
for the chip detector function.
(e) Troubleshooting.
Troubleshooting methodology should always start by attempting to divide the
system between an actual oil system, transducer or switch electrical problem.
1. If two or more sensors agree then the problem is probable with the appropriate oil system. An
example is the OIL PSI NOSE GRBX 1 caution/warning light illuminates and the HOD shows NGB 1 at less than
28 LB oil pressure and the NGB 1 temperature is higher than NGB 2 temperature. These indications are mutually
supportive of a problem with the NGB 1 oil system.
2. If only one sensor is indicating a fault then the problem is probably with that sensor or its
associated wiring. An example is the OIL TEMP MAIN XMSN 2 caution/warning light illuminated by the HOD
shows XMSN 2 at normal system temperature and closely matching XMSN 1 temperature and the XMSN 2 oil
pressure is normal. These indications support a problem with the temperature switch in the XMSN 2 chip
detector.
