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The Engine Preservation Unit is a battery powered service kit for opening the airframe fuel Shut Off Valve (SOV) and Fuel Metering Valve (FMV) and monitoring the position of the fuel Pressurizing Valve Indicator (PVI) switch.


An assortment of engine-specific cables enables a single EPU-3 to service a wide variety of engines at a low initial investment.


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

1.       Airframe fuel shutoff valve current up to 650mA.

2.       Pressurizing Valve Position indicator supply current

3.       Fuel Metering Valve current up to 350mA.


Additional features include:

Battery check switch

Battery charger input

Fused output to SOV and FMV



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


How the Current Regulators Work

When the SOV switch is pressed either up or down, the SetPoint voltage selected by the rotary switch is input to the current driver circuit.


The second contact of the SOV DPDT momentary (On)-Off-(0n) switch routs the output current to either the OPEN or CLOSE coil of the SOV solenoid. Current flows through a front-accessible fuse socket/holdercapable 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 of the Rotary Switch


Decide what engines are going to be inserted


Prepare artwork for any manner of printed label. Options include silkscreen on the facepanel if we know what itís going to be when the silkscreen legends go on.

Printed adhesive backed vinyl labels can be prepared and installed at any time.

For quicker temporary labels, any PC with a printer can be used to draw up a lable.

Dymo dial-a-label maker

Magic marker


Familiarization Exercise

To get started with familiarizing yourself with the EPU-3, start by pressing the Batt Check switch up to observe the green LED illuminates, which means the 2 12V Jell Cell batteries connected in series have more than 21 volts output. If one or the other battery is discharged all the way, its output voltage quickly goes down a few volts so the LED won't illuminate.

To connect the battery charger, plug the cord into the connector below the Batt Check switch and tighten the screw collar on the connector to make the electrical connection solid. Plug the battery charger into 110VAC house-power and leave the charger on for a few hours or overnight. Don't leave the battery charger on it all the time because it's not good to over charge the batteries.

To get familiar with the EPU-3, use a DMM to check current coming out of it.

Connect the DMM across the FMV wires at the end of the universal cable.

Set the rotary switch to Cal Pots and then press the FMC switch up to turn it on. Adjust the rotary knob to adjust the current from 0 to 350 mA (or so, depending on the resistance on the load).

The trimpots accessible thru the holes next to every other switch position are presets for specific engine types. Once you figure out what engines you want at each switch position labels can be made up to stick onto the front of the panel.

The DMM will indicate the FMV current that agrees with the current meter on the EPU-3.

If you get a scrap torquemotor or a 30 ~ 150 Ohm resistor and put 0.038" diameter pins on it to use as a simulated, or dummy-load a more realistic indication will be seen.

Move the DMM across either the SOV open or close contacts and check the operation of this circuit.

The EPU-3 controls are laid out such that anyone familiar with engine preservation should be able to intuitively operate it in a few minutes...

I'll get started building another of these for you this weekend.

When will you ship the CF34 connector you have?





The difference being that the SOV can output up to 600mA, where the FMV circuit is limited to 350mA.


It is likely that the maximum current needed for all of the FMVs on all of the controls will be less - perhaps 100 or 150mA. In that case, the current drive circuitry can easily be adjusted to further snub-down on the maximum current flow possible by increasing the internal series current limiting resistor values.


Voltmeter Displays

The two Voltage display digital panel meters have a 0-20V span, and the output of the current regulators with no current flow gets up to about 22 volts, which is slightly over-range for the 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 an indication that the coil circuit is incomplete and something needs to be plugged in.


The regulated SOV and FMV currents flow through fuses to the output connector. For ultimate over-current safety, 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.


If the Rotary Switch is setup properly and the operator sets it at the right position, the current limiting resistor, high-side current sense and fused output will provide adequate protection against any over current situations.







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.


For greater safety the current regulator circuit has its sensing resistor in the high-side, so if there is a short between the coilís low side and ground, the current regulated output will not go to its maximum on loss of feedback.

This is why the volt meters flash when the switches are on with the cables disconnected from the load. If thereís no current flow, the EPU current regulator will continue to increase output voltage until it gets current to the SetPoint or the output gets up to the battery supply voltage.



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

These Datelģ 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 wonít 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.


When the maximum current levels ever needed by any of the engines to be serviced by this EPU-3 and the SOV and FMV coil resistances are all known, the current limiting resistor values can be increased to further reduce the maximum possible output current for the two regulators. The initial setting of 350 mA is likely too high, and perhaps 250mA would be better. This is a decision to be made after we get all of the data together.


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.

Connect the calibration tool

Set the rotary switch to the desired position.