Blue Bird Coachworks 450 LXi, Owner'S Manual

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4-2 Generator
450 LXi Owner Manual
~ 4-2-14 ~
Rev. “C”
4-2.6.11 Rectifi er Checking
Each armature full wave bridge rectifi er has 5 terminals and 6 rectifying junctions. Rectifi ers may be readily checked
on the low range of an ohmmeter. From the “+” tab to the “AC” tab the ohmmeter should show a high resistance when
polarity of the ohmmeter leads are reversed. The same conditions should be found from the “+” tab to any other “AC”
tab and from “-“ tab to “AC” tabs. If a ZERO resistance reading is found, this junction of the rectifi er is shorted and the
rectifi er must be replaced. If a HIGH resistance is found with both polarities of the ohmmeter, this junction of the rectifi er
is “OPEN” and the rectifi er must be replaced.
Armature with 3 phase FULL-WAVE bridge rectifi er. The three phase full-wave rectifi er is now standard on most armatures
used in generators. This 3 phase (full-wave rectifi er) is a single unit with 6 diodes in a special case. The (+) terminal
is identifi ed by a red dot on the case and is connected by a short lead to the (+) terminal of the armature to which the
(+) rotor lead and suppressor leads are connected. The other 3 terminals at the top of the rectifi er are AC connections
to each of the armature phase leads. The case is the (grounded) (-) lead to the rotor. To test the diodes disconnect the
rectifi er positive lead at the armature (+) terminal. Test between the rectifi er (+) and any AC terminal. Make the test
also between the rectifi er (-) lead (ground to case) to any AC lead. The test determines that all diodes are good or that
one or more is defective. Since a ground armature winding gives the same test results as a bad diode, it is necessary
to disconnect all AC rectifi er connections and test the armature windings for a short to ground before a fault can be
positively identifi ed. Also test each diode separately (-) to each AC terminal, and case to each AC terminal to positively
identify which diode is bad.
The armature with two full-wave bridge rectifi ers. On some generator models an armature will be supplied with two
full-wave bridge rectifi ers very similar to the rectifi ers used on armatures with one 3 phase FW rectifi er. To test these
rectifi ers, remove the two rectifi er (-) leads at the R (rotor) (+) terminal and test each rectifi er separately, fi rst from + lead
(rectifi er #10) to any AC terminal. Then go to rectifi er #2 and test + to any AC terminal, then case to any AC terminal. A
failure in the test “case to any AC terminal” could be a bad diode in either FW bridge rectifi er. To determine which diode
is bad, all AC terminals must be disconnected and each AC terminal of the rectifi er tested to both + and to - (case) of that
bridge rectifi er.
4-2.6.12 Voltage Suppressor
Voltage suppressors are similar to rectifi ers in that they contain in effect a single semiconductor one way junction. A
suppressor should have a high resistance with one polarity of test leads and low but not zero resistance in the opposite
direction. Resistance measurements sometimes fail to identify a defective suppressor. The best test is to remove the
suppressor from the circuit. If an obvious improvement in the generator is observed, the suppressor is bad. Some suppressors
have a high resistance in either polarity of applied voltage (if applied voltage is low), but have a low resistance of voltages of
300 to 450 volts.
4-2.6.13 Resistance of Windings
Frequently in troubleshooting a generator, a defective component can be identifi ed by measuring the resistance of a winding.
The Exciter fi eld, armature, rotor and stator should withstand 1500 volts between winding and ground with less than 0.002
ampere of current between winding and ground. All electronic components such as rectifi ers, suppressors, and resistors must
be disconnected.
A.
B.
C.
If 12 volt excitation causes the engine to growl and load the engine with no or very low generator output voltage, the
stator could be grounded or shorted. Or, a short or ground in the wiring of the generator power circuit could be the main
fault. In either case, the stator will develop hot spots or could even smoke after running a few minutes. Run the generator
until a hot smell is detected, or stop in 5 minutes (whichever occurs fi rst). Feel the stator winding. If it is not, the stator
or power wiring contains a short circuit. Examine the stator for burned (black) insulation which indicates a defective or
damaged stator. Measure the stator resistance T1 to T2 and T3 to T4 if possible. With 12 volt excitation measure the
voltage of T1 to T2 and T3 to T4. If one of these voltages is very low while the other is close to normal, the low voltage
winding has shorted turns.
If 12 volt excitation causes an increase in voltage but the output voltage is less than 60% of the rated voltage, the rectifi er
(see 4) in the exciter armature could be defective or the exciter armature could have an open circuit. Also, one pole of the
main fi eld (rotor) could be shorted or grounded. If any of these effects exist, failure of the electronic regulator will occur.
Replacement of the regulator alone will be followed by failure of the new regulator. If the electronic regulator has failed,
it is wise to check the exciter current by placing a DC Ammeter in the F+ lead to the exciter fi eld. Normal exciter current
at no load rated voltage is 0.65 to 0.95 ampere. A higher current is another indication of a generator defect (described
above), which could cause a new voltage regulator to fail.
C.
D.