Onan MJA Series Service Manual page 124

122
CONTROL SYSTEM
attempts to flow in the other direction. To test the relay, isolate it by
removing the generator connection (GEN). Check for continuity between
the battery and generator ter "ninals. Continuity here indicates that the
relay contacts are welded together. Measure the resistance from the
generator terminal to ground. This should be approximately 112 ohms.
6.2.3 AUTOMATIC EMERGENCY STOPPING. - There are 2 parts to
the emergency automatic stopping system, a high temperature cut-off
switch and an optional Jow oil pressure cut-off circuit.
NOTE: When the generating plant is used with Line Transfer or Auto ­
matic Demand Controls and one of the emergency stopping de­
vices. operates, the plant will stop and then crank until the con­
trol's cranking limiter opens.
6.2.3.1 High Water Temperature Cut-Off Switch. - This switch, located
on the rear of the cylinder head, is in the ignition circuit. Normally
closed it opens at 200 0 F
±
50, breaking the ignition circuit and stopping
the engine. The switch closes again at 160 0 F
±
7 0 .
6.2.3.2 Low Oil Pressure circuit (Optional) - This circuit includes a
non-adjustable low oil press12re switch and centrifugal switch located
on the engine and a latching relay (5MJB prior to Spec F only) in the
control box. The rvIJA and 5MJB (Begin Spec F) models have an emer ­
gency relay to perform the function of the latching relay on the earlier
MJB models. See Sect. 6.3.3.4
If
low oil pressure occurs, the pressure switch closes completing the
relay coil circuit. This operates the relay disconnecting engine ignition
system, which stops the plant. The centrifugal switch is required to
, prevent operation of the relay during the plant starting cycle, before oil
pressure builds up.
a. Low oil pressure switch - see Sect. 6.3.3.3
b. Latching relay 5MJB-R (Prior to Spec F) - This relay is designed so
that when energized, it latches in the energized position and can only
be released by pushing the T.eset button. Latching is accomplished in
the following manner. Coil A (Fig 6. C) energizes when the oil pres­
sure switch closes. This operates the relay, disconnecting engine
ignition system, which stops the plant. With the contacts energized,
current flows through Coil B. When Coil A de-energizes, Coil B,
supplied by the battery, holds the relay in the energized position.
When the reset button is pushed, it breaks the circuit through Coil
B and the relay assumes its normal position.
To test this relay in the circuit, operate the plant and short the oil
pressure switch to ground. This should energize the relay, shutting