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B.  The engine exhaust and inlet pressures are applied to the bellows assembly of the transmitter. A change in either of these pressures cause differential bellows movement. The bellows movement affects the sensing mechanism, which, with the aid of the amplifier and the motor-gear train, causes the (synchro transmitter) rotor to rotate and generate three-phase electrical signals.
5.  Engine Pressure Ratio Indicator
A.  The engine pressure ratio indicator provides pointer and digital readout of the engine exhaust pressure ratio (Pt7/Pt2) and on some airplanes pointer and digital readout of the Command EPR input signal. The indicator consists of a synchro receiver, servomotor, geartrains, coarse indicator pointer and numerical counter, command index and numerical counter, command set knob, graduated dial face, integral lights, and case. The indicator is not hermetically sealed. The dial face is graduated from 1.0 to 2.6 EPR. Each counter consists of three rotating drums with numerals, readable from 0.50 to 2.00 EPR. Downward rotation of drums indicates pressure ratio increase. The indicator incorporates a failure warning flag, which drops in front of coarse counter numerals when power is not received, when voltage is too low, or when sustained mechanical malfunction of indicator occurs. The indicator displays Command EPR over the range of 1.0 to 2.6 EPR by means of a moving index against a fixed scale and a three wheel numerical counter. Increasing EPR will be clockwise rotation of the index and downward rotation of the counter wheels and is set by clockwise rotation of the set knob located on the lower right front face.
6.  Operation
A.  The system operates on ac power (Fig. 2).
B.  The engine exhaust and inlet pressures are sensed by the pressure sensing probes. These pressures act on the bellows assembly of the pressure ratio transmitter, causing differential bellows movement whenever either of the pressures change. The relative bellows movement effects the sensing mechanism of the EPR transmitter which, with the aid of the amplifier and motor-gear train, cause the sychro transmitter rotor to rotate and generate three-phase electrical signals. The generated electrical signals are transmitted to a respective pressure ratio indicator over a three-wire system. The indicator converts the electrical signals into the pointer shaft rotation or indicator pointer movement corresponding to the pressure change in the engine (Fig. 2).
C.  On airplanes incorporating PDCS, an EPR-driven command bug is used to indicate the optimum EPR setting for maximum economy of operation in various flight modes.
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Engine Pressure Ratio Indicating System Schematic  567 
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ENGINE PRESSURE RATIO INDICATING SYSTEM - TROUBLE SHOOTING

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ENGINE PRESSURE RATIO INDICATING SYSTEM - ADJUSTMENT/TEST
1.  Engine Pressure Ratio Indicating System Test (Preferred Procedure)
A.  General
(1)  
Testing the engine pressure ratio (EPR) indicating system consists of two phases. The transmitter inlet (Pt2) and exhaust (Pt7) pressure lines are tested for leakage, and an operational test on the entire system is performed. The components included in this systems test are the pressure ratio transmitter, indicator, and the inlet and exhaust pressure lines.

NOTE:  The order of the two test phases should not be reversed. If systems operational test is performed prior to the leakage test, erroneous results may be obtained because of possible leaking lines or transmitter.

(2)  
Testing the engine pressure ratio indicating system will not include the inlet pressure sensing probe and its connecting line through nose dome because of the difficulties in closing the vent hole in the nose dome for the testing purposes. Also, the exhaust pressure probes and manifold will not be included in the test due to the difficulties involved in obtaining access. Therefore, it should be kept in mind, that if the first phase of the test does not reveal any leakage, and the second phase indicates faulty systems operation, the cause may be assumed to be in the areas mentioned above.

(3)  
Refer to step 2 for an alternate EPR test procedure.
　
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