Piper PA-34-200T Seneca II, Service Manual

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b. Service Replacement (Oildyne).
The Oildyne hydraulic pump is a gear type unit driven by a 12 volt reversible motor designed to
operate in a pressure range of 2,400 ± 200 psi. To prevent excessive buildup of pressure in the
hydraulic system due to expansion, a primary thermal relief valve is incorporated in the pump body
which will open at 3,000 +300/-200 psi. Other valves in the pump channel fluid to the proper outlet
during retraction or extension of the gear. A shuttle valve located in the base of the pump allows
fluid displaced by the cylinder pistons to return to the reservoir without back pressure. This shuttle
valve has a delivery pressure of 400-800 psi during the extension cycle.
A bypass free-fall valve assembly is incorporated in the system to permit extension of the landing gear
should a malfunction in the system occur. This valve is manually operated by means of an emergency
gear knob located on the instrument panel. This knob must be fully extended to permit emergency
extension. Restrictions in the system prevent the gear from extending too fast.
For a description of the landing gear and electrical switches, refer to Section VII, Landing Gear and
Brake System.
6-3. TROUBLESHOOTING. (See Table VI-III.)
CAUTION: PRIOR TO STARTING ANY INVESTIGATION OF THE HYDRAULIC SYSTEM,
PLACE THE AIRPLANE ON JACKS. WITH THE AIRPLANE ON JACKS, PULL THE
FREE-FALL VALVE KNOB FULL OUT THUS PREVENTING THE BUILDUP OF
UNNECESSARY PRESSURE ON THE ACTUATING CYLINDERS AND
CONNECTING HYDRAULIC LINES WHEN THE GEAR IS RAISED OR LOWERED
MANUALLY. FAILURE TO COMPLY WITH THESE INSTRUCTIONS COULD
RESULT IN THE BUILDUP OF SUFFICIENT PRESSURE TO UNLOCK THE
DOWNLOCK MECHANISM ALLOWING THE GEAR TO COLLAPSE WHEN THE
WING JACKS ARE REMOVED. PRIOR TO REMOVING THE AIRPLANE FROM
JACKS, PUSH THE FREE-FALL VALVE KNOB IN, TURN ON THE MASTER SWITCH
AND SELECT GEAR DOWN, OBSERVE THAT ALL THREE GREEN LIGHTS
INDICATING THE LANDING GEAR IS DOWN AND LOCKED ARE ENERGIZED.
TURN MASTER SWITCH OFF.
Malfunctions in the hydraulic system will result in failure of the landing gear to operate properly. When
trouble develops, jack up the airplane (refer to Jacking, Section II) and then proceed to determine the
extent of the trouble. Generally, hydraulic system troubles fall into two types; troubles involving the
hydraulic supplying system and troubles in the landing gear hydraulic system. Table VI-III at the back of
this section lists the troubles which may be encountered and their probable cause, and suggests a remedy
for the trouble involved. A hydraulic system operational check may be conducted using Figures 6-1 or 6-
2. When the trouble has been recognized, the first step in troubleshooting is isolating the cause.
Hydraulic system troubles are not always traceable to one cause. It is possible that a malfunction may be
the result of more than one difficulty within the system. Starting first with the most obvious and most
probable reasons for the trouble, check each possibility and, in turn, by process of elimination, isolate
the troubles.
NOTE: If the Prestolite hydraulic pump is at fault and requires disassembly, it should be overhauled by
an accredited overhaul facility. Pressure checks with adjustments may be accomplished in
accordance with instructions given in paragraphs 6-6 thru 6-9. Replacement brushes for the
Prestolite pumps are no longer available. Accordingly, when the brushes wear out, the pump
must be replaced.
PIPER SENECA II SERVICE MANUAL
VI - HYDRAULIC SYSTEM 01/01/09
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