EPU-3 Cat in the case trimmedEPU-3 FacePlate


(33) Post | LinkedIn - V2500 Engine Preservation Demo.

Engine Preservation Video 1 

Engine Preservation Video 2

Engine Preservation Video 3

FAA Opinion on Engine Preservation Unit 


 The Engine Preservation Unit is a battery powered service kit for opening the airframe fuel Shut Off Valve (SOV) and Fuel Metering Valve (FMV) on an "off-wing" gas turbine engine while monitoring the fuel Pressurizing Valve Indicator (PVI) switch.

An "Air Start" switch has been provided to provide 24 VDC (limited by 2 Amp fuse and a 5 Ohm resistor placed in series) for remote operation of the Air Start Relay.


Connector Catalogs

Pyle DTL - 26500 Catalog

Pyle MIL-DTL-83723 Circular Connectors

Aero-Electric - 83723 Catalog

This is a good search tool of multiple vendors for parts

Electronic Component Search of Authorized Distributors | TrustedParts.com

An assortment of engine-specific cables have been designed and used for a wide variety of engines at a low initial investment. 

The current list is:

  1. CFM56-5 (A and B) plus CFM56-7
  2. CF6-80C2 FADEC and PMC Schematic
  3. CF6-80C2 FADEC 1
  4. CF6-80C2 FADEC 2
  5. CF6-80C2  PMC
  6. GE90
  7. GE90-115
  8. CF34-10E
  9. CF34-8C
  10. V2500
  11. PW305

Cable sets and procedures are currently In development for:

  1. PW206
  2. V2500
  3. PW4000
  4. LEAP engines

Description of Equipment

The EPU-3 is a portable high-impact plastic case housing a face-panel with controls and indicators to provide 2 precisely controllable current sources.

These currents engergize the Shut Off Valve (SOV) and Fuel Metering Valve (FMV) coils to open these engine functions for the flushing the kerosene out of the fuel system.

A rotary switch selection one of 12 inputs, either a pair of large 10-turn potentiometers to set current levels manually or one of 11 sets of trimpots that store the electrical current levels as pre-sets for normal operation. 

The 10-Turn pots are used to determine the minimum current level required to reliably operate the SOV and FMV. 

Once these current levels are determined, a position of the rotary switch is dedicated to this engine and the two trimpots are set to the same current levels that were set on the 10-turn pots.

Two rocker-switches that turn the SOV and FMV currents on an off are located on the front panel. 

The SOV switch is a momentary contact to open and close the fuel Shut Off Valve (SOV).

The FMV switch is a continuous contacting on/off typetpo open the Fuel Metering Valve (FMV).


A novel cable configuration has an array of aircraft connectors splayed out from the EPU-3 Connector.


The Cat Cable

This cable set is a combination of an end-user selected mix of cables to make a single cable set.

The basic cable set includes the wire, protective sheaths, labling supplies and the breakout box, all -pre-assembled and ready to install the connectors.

The EPU has 5 basic functions:

  1. Battery Test
  2. Air Start Solenoid
  3. Shut Off Valve Current
  4. Pressurizing Valve Indicator
  5. Fuel Metering Valve Current and Polarity

New functions to be added in next revision

  1. RPM on core engine rotor for pump-RPM threshold to open FMV for automatic control
  2. RPM for CF6-80C2 N1 Sensor
  3. Variable Stator Vane control
  4. Variable Bleed Valve control

The EPU-3 has 4 electrical outputs to the engine:

Air Start Solenoid is fused and current limited by a 5 Ohm series resistor
 Pressurizing Valve Position indicator supply current limited to 20mA
Shut Off Valve current limited at 500mA.
 Fuel Metering Valve current up to 200mA.


Additional features include:

Battery charger input

Battery check switch


Schauer Battery Charger - JAC0224 manual.pdf


The POWER ON/OFF switch is also a 3A circuit breaker.

The PRESS VALVE LED shows the position of the Pressuring Valve Indicator (PVI) switch.

The Rotary Selector Switch connects a pair of preset potentiometers for SOV and FMV current SetPoints at each switch position to the input of the current regulators.

The CAL POTS position on the Rotary Switch connects the two large MA ADJ potentiometers on the right side as the SetPoints to the current regulator circuits.

An external 24VDC BATTERY CHARGER is connected using a screw-coupled connector.

When pressed, the BATTERY TEST switch compares the battery voltage against an internal 20V Zener diode. If battery voltage is higher than 20 volts the BATTERY OK LED illuminates.

The dual-position, momentary contact SOV rocker switch energizes either the Open or Close coils of the fuel valve.

The  FMV switch ergizes the coil in the fuel metering valve.


The current regulator circuits are arranged in two columns for SHUTOFF VALVE and FUEL METERING VALVE.


The SOV DPDT momentary (On)-Off-(0n) switch routes the output current to either the OPEN or CLOSE coil of the SOV solenoid. Current flows through a front-accessible fuse socket/holder  capable of holding a wide variety of English and Metric sized slow-blo and fast-blo fuses.


The FMV switch turns the FMV current on and off. The SOV and FMV current regulator circuits are electronically identical, but configured with different current and voltage ranges.


Setup Procedure for the Rotary Switch

Getting the right connectors

The engine manufacturers 

Determining the turquemotor pins

Set SOV switch tarting with


Voltmeter Displays

The two Voltage display digital panel meters have a 0-20V span. When over-voltage is present, the meter display blinks at about 1 Hz.

This is a handy continuity indicator for the SOV and FMV coil circuits.

The output voltage of the current regulators when no current can flow because there's not coil present gets up to over 22 volts, which is slightly over-range for these meters.  

If the panel meter get more than 20V on the inputs all of their display segments will flash, indicating an over-range condition. 

The over-range in this case is not great enough to be a problem, and actually serves as a handy indication that the coil circuit is incomplete and something needs to be plugged in.


Current Limiting

The regulated SOV and FMV currents flow through fuses to the output connector. 

The ultimate over-current safety is a fast-acting fuse of appropriate amperage. 

Select the fuse amperage to be the next- higher value above the maximum the solenoid or torquemotor coils should ever see, and use fast-blow fuses for maximum protection.



Block Diagram

This simplified block diagram will help to understand how the EPU works.



Figure 1 EPU-3 Block Diagram. (Click Image for .pdf file download.)

Maximum current output of FMV circuit is 350mA and SOV circuit maximum current is 500mA.

The LED next to the fuse illuminates when voltage is applied when the fuse is blown.

The SOV Zero trimpot adjusts the setpoint at which the output current will be held at 0 mA.



The Voltage (V) meter makes a differential voltage measurement directly across the SOV and FMV coils.

These Datel® DMS-40PC-2-RS panel meters have +/- 1.9999 displays to indicate measured voltage range of +/- 19.999 volts.


When the input voltage exceeds this maximum*, the display blinks all of its segments on and off indicating an over-range. This wee bit of overvoltage won’t hurt anything. In our case it just means the cables are disconnected.

The Current (mA) meter measures return current as the differential voltage drop across a 1 Ohm resistor in the return path.


Safety Features

Output current is held to the SetPoint current levels regardless of output impedance by a high-side sense current regulator circuit. Failure mode analysis showed that if torquemotor drive current is sensed on the low side (like the majority of test stand PMC simulators are set up), if the return-path conductor wire is shorted to ground, the current regulator loses its feedback signal and output current goes to its maximum value. Using high side regulator avoids this potential over-current situation.


The EPU-3 has current limiting resistors in series with the output current to prevent potential harm by simply limiting the maximum possible current with a 50 Ohm coil load to 350 mA, the maximum current rating for a large torquemotor.


Testing Battery Voltage

To test the battery voltage before heading out to perform a preservation procedure, press the Battery Test Switch up, and observe the green Battery OK LED illuminates.

Setting Up the Rotary Switch

Set the Rotary Seitch to its Big Pots position

Set the engine up for the preservation by replacing the fuel inlet hose to the main fuel pump with a hose with preservation oil pressurized at 50 psig or so...

Start the huffer, open the Shut Off Valve with the EPU-3, open the air start valve either electrically or mechanically to get the engine spinning, when the pressurizing valve opens rotage the FMV Big Pot  clockwise slowly, observing the currentmeter to make sure  you dn't over curren the engine's fuel metering valve torquemotor.

Typical range is +100mA. Null Currents are typically between 20 and 40 mA. 

Fuel metering valves are integrators, in that if the FMV current null is 20mA, and the current is 21 mA, the Fuel Metering Vavle will move slowlly in the opening direction until it is fully open.

If the current is  19mA, then the FMV will move slowly in the

Connect the calibration tool

Set the rotary switch to the desired position.