65
Symptom:Symptom:
Symptom:Symptom:
Symptom: “REF” is displayed for the digital value for all temperature functions.
1. Problem: The Middle Connector on the EDC is loose or not connected. The Reference Diode on
pins 10 and 28 of the EDC Middle Connector is damaged. Note: Pin 28 should measure
0 volts and pin 10 should measure around .6 volts.
a) Action: Inspect the EDC Middle Connector and measure the voltage on pins 10 and 28.
Symptom:Symptom:
Symptom:Symptom:
Symptom: “OVR” is displayed for the digital value for a function.
1. Problem: If a channel on the EDC is over ranged (>2.5 volts between the “+” and “-” inputs), an
“OVR” will be displayed for that function. This can happen on a channel that is used to
monitor an Annunciator, Flap or Trim function. The max input voltage for any tempera-
ture or pressure channel is +5 volts to ground. The maximum differential 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 prob-
lem.
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 = 1.65 volts and White wire = 1.65 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 moni-
tored by the EDC on two extension wires. These extension wires must be made of type K TC material.
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 tempera-
ture is added to the differential temperature measured by the TC junction.
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