Generator Warning Light; Removal And Replacement Of Generator; Polarizing The Generator/Regulator - Cessna 170 Service Manual

Cessna 170 Service Manual
Electrical System
CAUTION: If the generator system has a filter capacitor connected for suppression of radio noise, be
certain that it is connected ONLY to the armature terminal of the generator or regulator. If it is connected to
the field terminal it will cause burning of regulator points and possible damage to the battery and/or fire due
to overcharging.
GENERATOR WARNING LIGHT. Cessna 170's were not factory equipped with generator warning lights.
However a few owners desired this feature and adapted it from other Cessna models. The following
description of this system is provided for those aircraft so modified. The generator warning light is designed
to indicate when the generator is not charging the battery. The light is electrically connected across the
cutout relay contacts of the voltage regulator. Whenever the cutout contacts are open the light will measure
the potential difference between the battery and the generator armature. When the battery voltage is in
excess of the generator output (such as an idling condition) the light will come on. As the generator speed
(output) is increased, the potential difference across the light will diminish and the light will grow dim. When
the cutout relay connects the generator output to the battery the light circuit is bypassed through the voltage
regulator and the light will go out.
AMMETER. The ammeter is connected between the battery and the aircraft bus. The meter indicates the
amount of current flowing either to or from the battery. With a low battery and the engine operating at cruise
speed the ammeter will show the full generator output when all electrical equipment is turned off. When the
battery is fully charged and cruise RPM is maintained with all electrical equipment off, the ammeter will show
a minimum charging rate. Important: Any time the ammeter is replaced, a gauge of sufficient generator
capacity must be installed that also includes an external or internal shunt amperage-rated at LEAST as great
as the generator.

REMOVAL AND REPLACEMENT OF GENERATOR.

a. Remove the cowl from the aircraft and drain the oil.
b. Loosen the clamp securing the blast tube and pull the tube clear of the generator.
c. Disconnect the generator wiring and pull it clear of the generator area.
d. Remove the three mounting nuts and washers attaching the generator to the engine accessory
case.
e. To replace the generator, reverse this procedure.
It is recommended that anytime the generator be replaced that a new oil seal (PN 352068) be installed on
the front of the generator. Remember that the oil seal open end faces TOWARD the engine to keep oil
inside the engine. Examine the generator drive shaft to detect interference between the woodruff key and
the oil seal. Any interference will damage the oil seal with resultant loss of engine oil. Generators are
usually received less drive gear and hub coupling. After installing the oil seal, drive the hub coupling to
where it bottoms on the generator shaft. Make certain the woodruff key remains in place. Assemble the
rubber disc with its groove side up. Assemble the drive gear on shaft, fitting the lug on the gear with the
rubber groove. Insert the special 5/16" washer over the generator shaft, and screw on the shear nut and
secure with a cotter pin.
A light coating of Permatex No. 2 Aviation gasket sealant may be used on both sides of the generator mount
gasket.
CAUTION: It is recommended that anytime the generator be removed that a completely new generator-
mounting gasket (PN 652072) be utilized upon reinstallation. It is VERY IMPORTANT that the gasket NOT
be cut or modified in any way. This requires that the tach-drive housing also be removed and reinstalled, as
the generator mount gasket includes provision for that tach-drive housing. Cutting the generator mount
gasket at this area as a "short-cut" method is highly discouraged as the potential for a high-volume oil leak to
develop in-flight can occur without warning to the pilot with the potential of complete oil pressure loss.
Whenever the tach-drive housing is removed/replaced, it is also highly recommended that the tach-drive
housing oil seal (PN 642714) be replaced to prevent oil from being carried up the tachometer cable into the
instrument and into the cockpit. Do not violate this procedure regardless of how innocent cutting that gasket
may appear.
POLARIZING THE GENERATOR/REGULATOR. A generator of the type used on aircraft utilizes a residual
magnetism in the field pole shoes in order to produce a charge. Whenever any work is performed on the
basic electrical system the generator should be polarized to make sure a charge will be produced. A
common defect after installing a battery, replacing a regulator, or replacing a generator is a NO CHARGE
condition due to lack of proper polarization. To polarize a generator/regulator system, connect a jumper
momentarily between the ARMature and BATtery terminals of the regulator BEFORE starting the engine, but
with the battery connected and the master switch ON. A momentary surge through the generator is enough
to correctly polarize both it and the regulator.
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