Cessna 170, Service Manual

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Cessna 170 Service Manual
Electrical System
1
Twelve-volt electrical systems are used on all models. An engine-driven generator supplies the normal
source of power during flight and maintains a battery charge controlled by a voltage regulator. An external
power source receptacle is offered as optional equipment on all models to supplement the battery-generator
system for ground operation. See Appdx. EL, Fig. EL-01.
BATTERY AND EXTERNAL POWER SYSTEM.
The battery and external power system consists of a battery, a battery solenoid, a master switch and an
external power receptacle (optional equipment). The battery and solenoid are mounted on the fire- wall. No
electrical power is supplied to the aircraft bus until the master switch is turned on. The master switch closes
the battery solenoid, connecting the battery to the bus.
BATTERY
. Multi-cell, wet-type batteries, which have non-spill type filler caps, are used on all models. The
battery capacity is 25 ampere-hours (originally 24 ampere-hours). All batteries are 12-volt and are housed in
a battery box attached to the firewall. The battery on the 170 is accessible by removing the cowl, or on
certain models thru an access door on the cowl.
BATTERY BOX.
The battery is contained in an aluminum box, which is riveted to the aircraft firewall
structure. The battery is vented by a tube which attaches to the bottom of the battery box and extends
downward thru the cowling exhaust opening. A battery box lid completely encloses the battery to reduce
any spillage of electrolyte or accumulation of battery gases inside the cowl. Internal metal parts of the
battery boxes are best coated with an acid resistant paint.
BATTERY SOLENOID.
The battery solenoid is bolted to the forward side of the battery box. It is a plunger
type contactor, which is actuated by turning the master switch on. When the master switch is off, the battery
is disconnected from the electrical system. A silicon diode has been added to some aircraft to eliminate
spiking of transistorized radio equipment. The large terminal of the diode connects to the battery terminal of
the battery solenoid and the small terminal of the diode connects to the minus terminal of the solenoid coil.
The minus terminal of the solenoid coil is the small terminal that the master switch wire connects to. Several
models of battery solenoid with differing appearance have been utilized over the life of the airplane, but their
operation is essentially identical. The original solenoid was contained within a metal box, while subsequent
models resemble a metal cylinder with two large threaded terminals for main battery cable connections (one
from positive battery terminal, the second connected to the rest of the aircraft electrical system at the starter
switch mounted upon the starter motor) and either one or two smaller terminals. One of these smaller
terminals is utilized to connect the solenoid to the cockpit master switch (where it provides a pathway to
ground so the normally open solenoid will close, thereby completing the connection from battery to the
previously described aircraft electrical system. The other smaller terminal, if present, completes an external
solenoid circuit function providing battery power to activate the solenoid. On single terminal solenoid models
that circuit is internally provided for and therefore hidden/not required.) Most modern replacement solenoids
are of the single, small-terminal type. See Appdx. EL, Fig EL-02.
MASTER SWITCH
. Operation of the battery and generator power system for all models is controlled by a
switch located on the instrument panel. Master switches are double pole, single throw. (In other words,
there are two circuits actuated by the Master switch. When the master switch is turned on, one circuit
provides a ground connection for the battery solenoid (small terminal) and the normally open solenoid
closes, connecting the battery to the electrical system. The switch also closes a second circuit, completing
the generator field connection to the voltage regulator. This allows the voltage regulator to regulate the
generator when it comes online after engine start. See Appdx. EL, Fig EL-03.
GROUND SERVICE RECEPTACLE.
The AN-2552-2 ground service receptacle is optional equipment and
may be located on the lower firewall in the engine cowl. A 12-volt battery cart or ground service generator
may be plugged into the unit for starting and operation of the electrical and radio equipment on the ground.
If the operator has specially constructed battery “jumper” cables, the ground service receptacle may be used
to provide electrical power to the aircraft from a ground vehicle or other source. Caution: No reverse polarity
protection exists for the aircraft. One must be careful to connect jumper cables in the correct polarity. The
center contactor pin of the AN-2552-2 is connected to the positive terminal of the battery solenoid at the
same location as the battery positive terminal connection. Therefore, any connection to the AN-2552-2
receptacle is, in effect, a direct connection to the battery. The end contactor pin of the AN-2552-2 is
directly grounded to the airframe.