Charging System Service
Chapter 31
© 2012 Delmar, Cengage Learning
Objectives
• Measure and interpret voltage drops on the
positive side of the charging circuit
• Measure and interpret voltage drops on the
ground side of the charging circuit
• Test and repair an alternating current (AC)
generator
• Diagnose voltage regulator problems
© 2012 Delmar, Cengage Learning
Introduction
• This chapter discusses testing and repair of
common charging system problems
– Principles of operation and electrical
fundamentals are useful
– Diagnosis of failures before replacing parts is
important
• Most parts stores will not accept returns of
electrical items
© 2012 Delmar, Cengage Learning
Charging System Service
• Common charging system complaints
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Dead battery
Battery water low
Indicator light glows
Noises
• Perform complete charging system diagnosis
– Before replacing parts
• Check battery charge
– At least 80%: perform 15-second load test
– Fails load test: might be sulfated
© 2012 Delmar, Cengage Learning
Testing the Charging System
• Perform visual inspection
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Corroded or broken wire connections
Indicator lights and gauges functioning correctly
Listen for noises
Loose or damaged alternator drive belt
• Loose drive belt causes two problems:
– Low charge rate
– Rotor shaft overheating can cause drive end
bearing failure
© 2012 Delmar, Cengage Learning
Testing the Charging System
(cont'd.)
• Connect volt-amp tester to battery
– Set meter range to highest scales during starting
– Adjust to lower ranges as needed
• Four charging tests
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Charging system output test
Regulator voltage
AC generator full-field test
Charging circuit resistance tests
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
Charging System Output Test
• Procedure
– Start the engine
– Raise idle speed to 2,000 rpm while lowering
battery voltage
– Read amount of current put into battery
• Maximum output will be more than gauge shows
– Move amp probe to B+ wire
• More accurate estimate of output
• Testing using an amp clamp
– Place it around all of wires from battery terminal
© 2012 Delmar, Cengage Learning
Regulator Voltage Check
• Regulator must be able to:
– Full-field an alternator immediately
– Keep the system at predetermined voltage
• With engine running, meter should show
gradual decrease in charging amperage
– Voltage should remain within normal operating
limits
© 2012 Delmar, Cengage Learning
Full-Field Test
• Low output of AC generator
– Could be due to generator or regulator
• Full-fielding the alternator eliminates the
regulator from the circuit
– Energizes the rotor fully
– Causes the alternator to produce full output
• Full-field test results
– If alternator puts out more at same voltage with
regulator bypassed: regulator is faulty
– When current output remains low: bad alternator
or drive belt are probable causes
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
Full-Field Test (cont’d.)
• PCM controlled voltage regulation service
– Warning message may or may not be displayed
on the instrument panel for the driver
• If warning light does not come on when the key is
first turned on, look for a problem in that circuit
• If the warning message comes on and stays on as
the engine runs, check for diagnostic trouble
codes
© 2012 Delmar, Cengage Learning
Diode Tests
• Leaking diodes decrease alternator output
• Diodes:
– Rectify AC output to DC
– Prevent AC from leaving in the B+ output wire
– Are defective if more than 0.5 AC volt in B+
output
– With engine off, there should be less than 0.5
milliamp of current flow in B+ output wire
© 2012 Delmar, Cengage Learning
Diode Tests (cont’d.)
• Testing diodes with a power probe
– Peak-to-peak mode: tester measures differences
between positive and negative peak voltage
levels
• Scope testing
– Lab scope can be used to look at AC generator
patterns
• Diode noises
– When the sound is loud enough for a customer
complaint, a bad diode is probably the cause
© 2012 Delmar, Cengage Learning
Charging System Voltage
Drops
• Check voltage drops as preventive maintenance
– Combined voltage should be less than 0.5 volts
– Positive circuit with 10 amps flowing: no
connection should have more than 0.3-volt drop
– System with an ammeter: 0.7-volt drop limit
– Small amount of voltage drop through the fuse
link is normal
© 2012 Delmar, Cengage Learning
AC Generator Service and
Repair
• Most shops do not repair AC generators
• Easy to disassemble
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Remove the bolts
Separate the rotor from its housing
Brushes may be serviced without disassembling
Stator is discarded if it has signs of burned or
overheated insulation
• Test the continuity of the rotor winding for
opens, shorts, or grounds
– Resistance is checked with ohmmeter
© 2012 Delmar, Cengage Learning
Diode Service
• Diodes must be tested under load
– Diode tester
– Ohmmeter
• Good diode
– Shows high resistance in one direction
– Low resistance in the other
• Shorted diode
– Low readings in both directions
• Open diode
– High resistance in both directions
© 2012 Delmar, Cengage Learning
Alternator Reassembly
• Reassembling the alternator is not difficult
– Brushes inside the frame must be raised against
their springs
• Slip rings positioned under them
– Brushes mount from outside are compressed
against springs as holding bracket is installed
© 2012 Delmar, Cengage Learning