Charging System: Testing and Inspection
Fig. 2 Checking voltage at alternator B+ terminal. XR4Ti:
Fig. 3 Checking voltage at alternator D+ terminal. XR4Ti:
Key Off Test
1. With ignition key in OFF position, check voltage at alternator B+ terminal, Fig. 2. If voltage is not approximately 12 volts, check for open in charging circuit between B+ and battery positive post.
2. Check for voltage at D+ terminal, Fig. 3. If voltage is present, diode(s) is shorted.
Fig. 2 Checking voltage at alternator B+ terminal. XR4Ti:
Output Voltage Test
1. With voltmeter connected as shown in Fig. 2, start engine and run at approximately 1500 RPM while observing output voltage.
2. Output voltage should quickly increase from the reduced start-up value as the starting load discharge is restored, then stabilize between 13.8 and 14.2 volts at approximately 77°F.
Diode Test
1. With engine running, note output voltage at B+ terminal.
2. Turn engine off, then measure voltage at D+ terminal.
3. If D+ voltage is more than .5 volts lower than B+ voltage, field diode(s) is shorted or open.
4. If output voltage was high but diode test was satisfactory, replace regulator.
5. If D+ voltage is more than .5 volt greater than B+ voltage, output diode(s) is defective.
Regulator Bypass Test
1. If output voltage is low and diodes test satisfactorily, start engine and run at idle.
2. While watching voltage at B+ terminal, ground connecting strip between rear brush and the regulator.
3. While observing voltmeter, gradually increase engine speed.
4. If output voltage increases to 16 volts, replace regulator. If not, replace alternator. Do not increase engine speed any more than enough to produce 16 volt output.
External Circuit Test
1. If output voltage is normal but undercharge symptoms are reported, there may be excessive voltage drops in circuit between alternator and battery.
2. Start engine and run at approximately 1500 RPM, then check output voltage at B+ terminal and the battery positive post.
3. If these two readings differ by more than .2 volt, check for loose or corroded connections at the B+ circuit.
4. Check battery ground cable for voltage between negative battery post and engine ground, using voltmeter's lowest voltage range.
5. If there is a drop of more than .2 volt, check for damage or poor connection in ground cable.
Excitation Voltage Test
1. If voltage does not increase above battery voltage with engine running, stop engine and check for voltage at D+ terminal with key in ON position.
2. If battery voltage is present, the field circuit is open.
3. If voltage is between 4 volts and battery voltage, excessive resistance is present in the field coil circuit, either at the brush holder coil spring in the alternator end frame, between the brushes and slip rings, in the field winding, in the regulator or at the regulator ground on the frame.
4. Release drive belt tension and spin rotor manually while measuring excitation voltage. If voltage fluctuates considerably but occasionally drops below 4 volts, clean or replace brushes and slip rings as necessary.
Fig. 4 Checking brush length. XR4Ti:
Brush Test
1. If spinning rotor resulted in considerable variation of excitation voltage, disconnect battery ground cable, remove regulator and brush holder assembly and check brushes for excessive wear or damage, bad connections or sticky action.
2. Brushes should project from the brush holder approximately {5/32} to {3/8} inch (5 to 10 mm), Fig. 4.
3. Clean and free up brushes or replace holder and regulator assembly as necessary.
4. While brush holder assembly is out, clean and polish slip rings with crocus cloth or fine abrasive, spinning the rotor by hand with drive belt loose. Check and clean the front brush contact on the brush holder and the contact spring inside the alternator end frame. Ensure that regulator ground terminal makes good contact with alternator end frame.
Field Test
1. With brush holder assembly removed, check field coil continuity and isolation with an ohmmeter.
2. Probe from slip ring to slip ring. If very low resistance is not indicated, field is partially or completely open.
3. Probe from either slip ring to alternator frame. If there is infinite ohms, field is grounded.
4. Install brush holder and regulator assembly, connect battery ground cable and repeat output voltage test.
Output Current Test
1. Disconnect battery ground cable and alternator B+ lead.
2. Connect a 100 amp ammeter between the B+ terminal and the end of the B+ lead and position B+ lead so that it cannot accidentally contact ground during test.
3. Connect voltmeter between B+ terminal and ground.
4. Connect battery ground cable and connect a carbon pile load rheostat across the battery terminals. Ensure that carbon pile is turned to OFF or NO LOAD before connecting it.
5. Connect tachometer, then start engine and run at idle.
6. Adjust carbon pile and engine speed until voltmeter reads 13.5 volts at idle, then note ammeter reading.
7. Repeat test at 1000 and 2000 RPM.
8. If engine speed output current at 13.5 volts is not 28-35 amps at idle, 75-85 amps at 1000 RPM and 89 amps at 2000 RPM, repair or replace alternator as necessary.