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Oildyne Pump Problem & Fix

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Avidflyer2

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This thread is a result of our findings relating to a Post on alternator problems by Matt.Lehtinen. His problem started at #105 with a short video showing wild current fluctuations during a landing gear retract cycle. We and the previous shop went down the proverbial rabbit hole chasing a non-existent electrical problem!

Symptoms:
1. Load meter showing 10-15 Amps until landing gear is selected UP. During gear retraction the load meter would swing to 60-70 Amps then drop to zero, repeating two or three times in quick succession.
2. Landing gear DOWN operation completely normal.
3. Alternator would sometimes drop offline requiring a manual reset.
4. Problem could not be duplicated with aircraft on jacks.

Tests performed:
1. Previously a shop had tested the hydraulic pressure switch and found it to be set “a little low” and replaced it with a new pressure switch.
2. Previously a shop had inserted a pressure gauge in the retract pressure line at the pump to check pump pressure.
3. Before we did anything, I had hooked up a test lamp at the pump motor’s “blue” retract power wire to check the control wiring and UP relay, then flew the airplane to check pump operation while hoping to duplicate the original problem. The problem reappeared with a simultaneous load meter fluctuation as the hyd pump unit cycled on and off as the UP relay opened and closed. An “Ah-Ha!” moment for sure.
4. Aircraft placed on jacks and gear retracted with no discernible problem. What the heck? Repeat test a few times and problem reappeared once. Later we were able to duplicate problem fairly regularly during each retract cycle.
5. Hydraulic pressure gauges installed in both the UP pressure line and the RETURN line. During retract test the UP high side pressure would rise to about 1200 psig before the pump would shut off and then cycle back on under load at approx 900 psig. (Starting under load explained the sudden high amperage draw on the load meter). The RETURN low side pressure had risen to 250 psig! “Why is the return pressure so high?” The normal return pressure value Is not mentioned in any manual that I could find but was thought to be excessive at anything over 50 psig, but why?

At this point I was convinced that the UP pressure was being opposed by the RETURN pressure and that the landing gear actuators could not move until the hydraulic fluid displaced by the UP pistons in the actuators returned back to the pump reservoir. The load meter swings were being caused by a build up of pressure in the UP line by the failure of the fluid returning back to the reservoir and eventual opening of the pressure switch turning the pump off. As the RETURN line pressure decreased and the gear actuators moved, the UP pressure would drop and the pressure switch would turn the pump back on. The pump was having to start under load, momentarily doubling the normal 18 amp current draw.



Fix:
The Oildyne pump unit was removed from the aircraft and partially disassembled to inspect both the shuttle valve (Prestolite systems have this valve in the pump mounting base, Oildyne units have this shuttle valve in the pump unit) and the RETURN restrictor orifice inside the fluid reservoir.

With the pump unit upright and the hydraulic lines facing you, the shuttle valve is located under the external left side 7/8” size hexagonal plug. This plug was incredibly tight requiring the use of an air impact wrench partly because it is difficult to hold the unit while applying torque to the hex plug. The spring and shuttle valve fell out with no discernible defects. Using the Parker (Oildyne) Series 108 Pump Unit’s Operation & Maintenance Manual I then removed the reservoir cup by removing the one bolt at the bottom. A quick inspection of the exposed pressure relief valves, suction screens, and restrictor orifice plug didn’t show anything obviously wrong. It was not until I removed the restrictor orifice plug to check it’s 0.020” diameter orifice did I find it completely plugged! It took a small diameter wire from a wire brush to poke & push out the debris, a quick blast of shop air confirmed the passage was now clear and the restrictor orifice plug was reinstalled. The pump reservoir was reinstalled and serviced with 5606 hyd fluid.

After reinstalling the pump unit back in the airplane another retract test was performed. Everything worked flawlessly! The RETURN pressure during the retract cycle did not exceed 50 psig. The UP pressure during gear movement averaged about 900 psig, with a high of 1200 as the mains moved and sagged as the nose gear came up. Just before the gear was completely UP the hyd pressure increased to a max of 2200 psig as the pump shut off.

E04C3D39-1C6E-4936-8D43-72E1C2CFA672.jpegReservoir cup removed.8F9060EA-DC1B-40C2-9152-EFD41C47C4A4.jpeg End of restrictor orifice, note .020 hole in center.

27DE3A88-DC09-478E-8588-CCDA23348695.jpeg
 

Attachments

  • IMG_7554.MOV
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  • 761-639 - ARROW III_TURBO ARROW III SERVICE MANUAL - PA-28R-201_201T, page 177.pdf
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