Quantcast Alternator  Testing

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Never  operate  an  alternator  on  an  open  circuit. With  no  battery  or  electrical  load  in  the  circuit, alternators are capable of building high voltage (50 to over  110  volts)  which  may  damage  diodes  and endanger  anyone  who  touches  the  alternator  output terminal. Alternator maintenance is minimized by the use of prelubricated bearings and longer lasting brushes. If a problem  exists  in  the  charging  circuit,  check  for  a complete field circuit by placing a large screwdriver on the alternator rear-bearing surface. If the field circuit is complete, there will be a strong magnetic pull on the blade of the screwdriver, which indicates that the field is energized. If there is no field current, the alternator will not charge because it is excited by battery voltage. Should  you  suspect  troubles  within  the  charging system   after   checking   the   wiring   connections   and battery,  connect  a  voltmeter  across  the  battery terminals. If the voltage reading, with the engine speed increased,  is  within  the  manufacturer’s  recommended specification,  the  charging  system  is  functioning properly. Should the alternator tests fail, the alternator should  be  removed  for  repairs  or  replacement.  Do NOT  forget,  you  must  ALWAYS  disconnect  the cables from the battery first. Alternator  Testing To  determine  what  component(s)  has  caused  the problem, you will be required to disassemble and test the  alternator. ROTOR TESTING.—To     test     the     rotor     for grounds, shorts, and opens, perform the following: To  check  for  grounds,  connect  a  test  lamp  or ohmmeter from one of the slip rings to the rotor shaft (fig. 2-29). A low ohmmeter reading or the Figure 2-29.—Testing rotor for grounds. lighting of the test lamp indicates that the rotor winding is grounded. To check the rotor for shorts and opens, connect the  ohmmeter  to  both  slip  rings,  as  shown  in figure  2-30.  An  ohmmeter  reading  below  the manufacturer’s   specified   resistance   value indicates a short. A reading above the specified resistance value indicates an open. If a test lamp does not light when connected to both slip rings, the winding is open. STATOR TESTING.—The   stator   winding   can be  tested  for  opens  and  grounds  after  it  has  been disconnected from the alternator end frame. If  the  ohmmeter  reading  is  low  or  the  test  lamp lights  when  connected  between  each  pair  of  stator leads  (fig.  2-31),  the  stator  winding  is  electrically good. A high ohmmeter reading or failure of the test lamp to light when connected from any one of the leads to Figure 2-30.—Testing the rotor for opens and shorts. Figure 2-31 .-Testing  a  stator  for  opens. 2-21

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