GMC 4000 Series Service Manual page 437

Sec. 6Y
Page 426
A/C GENERATING SYSTEM
OUTPUT TRANSISTOR
(PART 15, Figs. 26 and 27)
In figure 26, if both readings in Step 1 are
zero, or if both readings are very low and identical,
the transistor is shorted. Similarly, if both read­
ings in Step 2 or Step 3 are zero or very low and
identical, the transistor is shorted.
In figure 27, if both readings in Step 1 are
infinite, or if both readings are very high and iden­
tical, the transistor is open. Similarly, if both
readings in Step 2 are infinite or very high and
identical, the transistor is open.
IMPORTANT: Replacing any of the above com­
ponents should not affect the voltage setting. If any
of the following components require replacement,
adjust voltage setting as explained under "Regula­
tor Voltage Adjustment" later in this section.
ZENER DIODE
(PART 16, Fig. 25)
If both readings are infinite or if both readings
are identical, replace the diode.
THERMISTOR
(PART 17, Fig. 25)
Connect the ohmmeter as shown in figure 25.
If the reading is infinite, the thermistor is open
and should be replaced.
VOLTAGE DIVIDER RESISTOR R -l
(PART 18, Fig. 25)
If one reading is infinite or nearly infinite,
or if both readings are infinite or nearly infinite,
the resistor is open and should be replaced.
R E L AY B A S E
Figure 2 8 — Field Relay Unit Check
G M C S E R V I C E M A N U A L
VOLTAGE DIVIDER RESISTOR R-3
(PART 19, Fig. 25)
if both readings are infinite or nearly infinite, the
resistor is open and should be replaced.
POTENTIOMETER
(PART 20 AND 20A, Fig. 25)
when connected as shown in Part 20, the potenti­
ometer is open. If one reading is infinite or nearly
infinite when connected as shown in Part 20A, the
potentiometer is open.
locate the adjusting lever in a vertical position
(fig. 23), turn the potentiometer resistance adjust­
ment to the middle position, then use a soldering
iron to melt the lever into potentiometer.
Refer to figure 28 for test connections.
shown in figure 28. An infinite reading indicates
an open shunt coil which will require replacing the
relay.
50 ohm variable resistor and voltmeter into circuit
as shown in figure 28. Slowly decrease resistance
and note voltage at which relay points close. If
voltage at which points close is not within lim its
listed in "Specifications" at end of this section,
replace the relay.
REGULATOR VOLTAGE ADJUSTMENT
through 20 previously; regulator voltage must be
calibrated to compensate for variations in operat­
ing characteristics between components. The volt­
age setting must be p recise to maintain system
voltage within lim its listed in "Specifications" at
end of this section.
potentiometer at point shown in figure 23, then dis­
connect lever from opposite side of panel board
while heat is being applied.
portion of potentiometer in middle position.
tom cover plate and potentiometer lever, connect
the generator, regulator, and battery into circuit
per figure 2. Connect a test ammeter between bat­
tery post and battery lead.
T-5594
tric load of 15 to 20 amperes.
across battery posts to obtain required amperage.
If one reading is infinite or nearly infinite, or
If one reading is infinite or nearly infinite
NOTE: When installing a new potentiometer,
REGULATOR UNIT RELAY
The relay unit may be checked as follows.
1. Connect an ohmmeter across terminals as
2. To check closing voltage, connect a battery,
After replacing any components in Part 16
1. Carefully apply soldering iron to lever of
2. Use a small screwdriver to position center
3. With regulator assembled, except for bot­
4. Drive generator at 6,000 rpm with an elec­
NOTE: A carbon pile resistor may be required