Figure 4-41.1. Alternator testing.(Sheet 1 of 8)

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SECTION XIII. ELECTRICAL SYSTEM INSTRUMENTS AND GAGES

TM-55-1905-217-12 Landing Craft Mechanized; Steel; DED; Overait Length 74 Feet Mod 1 Marl VIII Navy Design LCM-8 Hull Numbers 8500 and 8580 thru 8618 Manual

Figure 4-41.2. Alternator testing. (Sheet 2 of 8)

TM 55-1905-217-12

auxiliary terminal is higher than specified, field circuit is

(6) Stop engine and remove jumper wire

across auxiliary and field terminals. Connect a dc

defective-check brushes. If voltage reads 0 volts at

auxiliary terminal, check field excitation device and

ammeter across field and positive output terminals (fig.

associated circuit. If voltage is not correct, perform test

4-41.1) to measure field current (regulator

in (6) below.

disconnected). Current should be 1.5 to 2.0 amperes.

Disassemble and repair alternator if current is excessive

or low.

NOTE

c. In-Vessel Testing (Hull numbers 8540 thru 8560

Disconnect jumper wire after testing.

and 8580 thru 8618.)

(1) Hull numbers 8540 thru 8560 and 8580

(3) With propulsion unit or engine running,

thru 8618 contain a revised model of the 70-amp, 24-volt

check voltage across auxiliary and negative output

terminals (fig. 4-41.1) with dc voltmeter. Correct

alternator. This model differs from the previous model

voltage is 29.4 0.2 volts. If voltage is low, proceed with

due to the replacement of the isolation diode with a field

diode assembly. The assembly consists of three rectifier

tests.

diodes, mounted on a common heat sink, electrically

(4) With engine running, check voltage across

connected in parallel with the conventional positive

positive and negative output terminals (fig. 4-41.1) with

rectifier diodes. After initial alternator excitation utilizing

dc voltmeter. Correct voltage at positive output terminal

battery energy, the field diode assembly provides the

is 1.0 volt less than voltage in (3) above. If voltage

continuing energy requirement. A new voltage regulator

difference exceeds 1.0 volt, isolation diode is defective.

is used in conjunction with the new alternator. The new

(5) Stop engine and disconnect voltage

regulator is similar to the previous model but operates

regulator (fig. 4-41.2). Place jumper wire across

the field at a one-volt lower value. Figure 4-41.4 is a

auxiliary and field terminals. With engine-running at idle,

schematic of the alternator system. The following test

check voltage across auxiliary and negative output

terminals. Correct voltage is 29.4 0.2 volts. If voltage

equipment is required to conduct the in-vessel alternator

system checks:

was low in (3) above and now rises to correct voltage,

regulator is defective and shall be replaced. If voltage

remains low, alternator is defective.

Figure 4-41.1. Alternator testing.(Sheet 1 of 8)

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