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tial voltage is 2.5 volts. The VI-221 is used to drop the voltage when measuring
signals that switch from ground to bus voltage.
a) Action: Measure the voltage on the EDC “+” and “-” inputs for the function with the
problem.
7.2 Pressure Problem with one Function:
Pressure is measure on the aircraft with a pressure transducer. Power (+5 volts on the Red wire) and
ground (Black wire) is supplied to the transducer and a differential signal (proportional to the pressure) is
supplied to the EDC on the White and Green wires.
Symptom:
Symptom:
Symptom:
Symptom:
Symptom: Inaccurate, jumpy or zero reading.
1. Problem: One of the four wires from the Pressure Transducer to the EDC is open.
a) Action: Measure the voltage on each transducer wire at the Pressure Transducer. The
voltage on each wire is as follows: Red wire = +5 volts, Black wire = 0 volts,
Green wire = 2.5 volts and White wire = 2.5 volts.
Short the Green wire to the White wire and look for a zero pressure reading. If
you do not get a zero reading, you may have an open wire.
Ohm the Black, Green and White wires at the pressure transducer to ground.
The Black wire should be grounded, the Green and White wires should not.
7.3 Temperature Problem on all Channels:
Temperature is measured using a temperature probe with type K thermocouple (TC) wire. When the TC
junction in the tip of the probe is heated, a small signal (approx. 23 micro volts per ‘F) is produced and
monitored by the EDC on two extension wires. These extension wires must be made of type K TC mate-
rial.
When measuring temperature with TC material an ambient reference temperature must be provided. This
is done with a reference diode located on pins 10 and 28 of the EDC Middle Connector. The ambient
temperature is added to the temperature measured at the TC junction.
Symptom:
Symptom:
Symptom:
Symptom:
Symptom: Inaccurate, jumpy or zero readings on all channels.
1. Problem: The Reference Diode on pins 10 and 28 of the EDC Middle Connector is damaged.
a) Action: Measure the voltage on the Reference Diode. Pin 28 = 0 volts and pin 10 =
approx. 0.5 volts.
2. Problem: One of the temperature input channels is over voltaged. This can happen on a
channel that is used to monitor an Annunciator, Flap or Trim function. The max
input voltage for any temperature or pressure channel is +5 volts. The VI-221 is
used to drop the voltage when measuring signals that switch from ground to bus
voltage.
