**ON A/C ALL
LOW OIL PRESSURE WARNING SYSTEM − DESCRIPTION AND OPERATION
1.
General
Refer to Figure 1 and Figure 2.
The low oil pressure warning system for each engine consists of an engine oil pressure low
switch, which is mounted on the engine oil tank on the left side of the engine (refer to
INDICATING − GENERAL, Figure 1). The switch is connected to the appropriate ENG OIL
PRESS caution light circuit in the caution lights panel (Pre Mod 8/0235 aircraft) and to similar
circuits in a warning lights panel (Mod 8/0235 aircraft).
The caution lights panel is located in the flight compartment. The warning lights panel
supplied on modified aircraft is mounted on the lower edge of the caution lights panel (refer to
Chapter 33).
The oil pressure low switch of each engine is connected into engine intake lip heater circuits
of the corresponding engine. On aircraft incorporating Mod 8/0446 engine No. 2 low pressure
switch is connected into hydraulic system inter−system power transfer circuits.
2.
Description and Operation
Each pressure switch has two ports, one marked ‘ENG’ and the other ‘REF’. The ‘ENG’ port
is connected by rigid piping to a port on the downstream side of the engine oil pressure
regulating valve. The ‘REF’ port is similarly connected to a vent connection sensing the
turbomachinery accessory gearbox cavity pressure.
The switches are each provided with two contact sets, the normally closed contact of one set
in each switch is connected to and controls the operation of the appropriate ENG OIL PRESS
caution light (Pre−Mod 8/0235) or ENG OIL PRESS warning light (Mod 8/0235). The normally
open contact in the other contact set in each switch is connected to the corresponding engine
intake heater circuit (refer to Chapter 30). On Mod 8/0446 aircraft the normally closed contact
in engine No. 2 low pressure switch is connected into hydraulic system inter−system power
transfer circuits (refer to Chapter 29).
Switch mechanisms are calibrated to operate their contacts from N.C. to N.O. on increasing
engine oil pressure at 59 psid (Pre Mod 8/0965) or 49 psid (Mod 8/0965) and from N.O. to
N.C. at 52 + or −2 psig decreasing pressure (Pre Mod 8/0965) or 42 + or −2 psig Mod 8/0965.
With the engine oil pressure above 59 psid, (Pre Mod 8/0965) or 49 psid (Mod 8/0965), both
switch contacts are open and the ground for the appropriate ENG OIL PRESS light is
removed and the light is turned off.
Should the oil pressure of an engine fall below 52 + or −2 psid (Pre Mod 8/0965) 42 + or −2
psid Mod 8/0965, for any reason both contact sets of the switch close. The connected contact
set establishes a ground to initiate lighting of the appropriate ENG OIL PRESS light.
D
SERIES 100
AIRCRAFT MAINTENANCE MANUAL
MASTER
EFFECTIVITY: See Pageblock 79−36−00 page 1
79−36−00
Page 1
Dec 20/2004
