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ELECTRICAL TESTS

Except as stated, make the following tests with an

ohmmeter on the low range scale.

ROTOR FIELD WINDING CHECKS

The rotor may be checked electrically with a self-pow

ered test lamp or an ohmmeter (figure 10).

Open Winding

To check for opens, connect the test lamp or ohmme

ter to each slip ring. If the lamp fails to light, or if the

ohmmeter reading is high (infinite), the winding is open.

Replace the rotor.

Grounded Winding

Connect a test lamp or ohmmeter from one slip ring

to the shaft. If the lamp lights, or if the reading is low,
the rotor winding is grounded. Replace the rotor.

Short Circuit or Resistance Check

Check the winding for short circuits or excessive

resistance by connecting a battery and ammeter in

series  with  the  edge  of  the  two  slip  rings.  Note  the
ammeter  reading  and  refer  to  “ Specifications”  later  in
this  section.  An  ammeter  reading  above  the  specified
value  indicates  shorted  windings.

An alternate method is to check the resistance of the

field  by  connecting  an  ohmmeter  to  the  two  slip  rings.  If
the  resistance  reading  is  above  the  specified  value,  the
winding  has  excessive  resistance;  if  below  the  specified
value,  the  winding  is  shorted.  The  specified  resistance
value  can  be  determined  by  dividing  the  voltage  by  the

current.

Remember that the winding resistance and ammeter

readings will vary slightly with winding temperature

changes. If the rotor is not defective, but the generator
fails to supply rated output, the problem is in the rectifi

er bridge, stator, or regulator.

If the rotor fails any of the above checks, replace it.

STATOR CHECKS

Check the stator with a self-powered test lamp or an

ohmmeter (figure 11). If the ohmmeter reads low, or if

the lamp lights when connected from any stator lead to

bare metal on the stator frame, the stator is grounded.

An ohmmeter can not be used to test the delta stator

for shorts or opens. Noticeable discoloration at any

place on the assembly usually indicates a problem in
the stator windings.

RECTIFIER BRIDGE CHECK

To check the rectifier bridge, connect an ohmmeter to

the grounded heat sink and one of the three terminals,

pressing down firmly on the flat metal clip (figure 12).

Observe the ohmmeter reading and reverse the lead
connectors to the grounded heat sink and the same

metal clip. If both readings are the same, replace the
rectifier bridge. A good rectifier bridge will give one high

and one low reading. Repeat this same test between
the grounded heat sink and the other two terminals, and

between  the  insulated  heat  sink  and  each  of  the  three
terminals.  This  makes  a  total  of  six  checks,  with  two
readings  taken  for  each  check.

Some digital ohmmeters cannot be used to check

diodes in the rectifier bridge. Consult the ohmmeter
manufacturer to determine your ohmmeter capabilities.

REGULATOR

The regulator cannot be checked. If the rotor, stator,

and  rectifier  bridge  are  OK,  but  the  generator  still  pro
duces  more  than  16  volts  or  does  not  produce  within  15
amperes  of  the  rated  output  during  a  generator  output
test,  replace  the  regulator.

chevrolet truck Light Duty Truck 1994 Series, Repair Manual

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