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.
a) Action: Measure the voltage on each of the temperature channels.
7.4 Temperature Problem with one Function:
Temperature is measured on the aircraft 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
o
F) is produce and monitored 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 temperature is added to the
differential temperature measured by the TC junction.
A. Symptom: Inaccurate, jumpy or zero reading.
1. Problem: One of the two wires from the Temperature Probe to the EDC is open or shorted to ground.
a) Action: Remove the extension cable from the probe. Ohm both wires to ground. Each wire
should measure 12.1K ohms to ground.
2. Problem: The temperature probe has a problem.
a) Action: Ohm between the two leads of the temperature probe. You should get a short (<10 ohms)
Ohm one lead of the probe to ground. You should get an open (> 100K ohms).
7.5 N1 or N2 RPM Problem:
An N1 or N2 Tach Generator produces a sine wave at a frequency equal to the RPM of the engine. The voltage level
of the sine wave is very low at low RPM and can be above 20 volts at high RPM. The signal is generated from a coil
inside the unit. Most Tach Generators have three coils and one end of each of the coils is tied together as a common.
On some Tach Generators the common ends of the coils are grounded. The EDC only needs a signal from one coil. In
this case, one pin on the Tach Gen should be routed to the RPM input on the EDC.
Some Tach Generators have the common ends of the coils open. In this case, one end of a coil should be grounded and
the other end routed to the RPM input on the EDC.
A. Symptom: Inaccurate, jumpy or zero reading.
1. Problem: Improper connection to the coil.
a) Action: Ohm the Tach Gen. pin connected to the EDC with respect to ground. It should measure
between 10 and 50 ohms. If this pin is open, you may not be grounding the other end of the coil.
Also, it's not uncommon to have a bad coil. In this case you can select another pin (coil) on the Tach
Gen. to connect to the EDC.
b) Action: The Tach Gen. can be removed from the engine and spun with a drill (be sure the Tach
Gen is grounded to the same ground as the EDC. You should see a reading around 10 to 20%.
Normally, 4200 RPM = 100% reading.
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