Voltage Regulator


Regulator Voltage Setting Check

The regulator voltage test checks to see the high and low settings of the regulator. A regulator must be able to full field an alternator immediately. It must also be able keep the system at a predetermined voltage at normal operating temperature (usually about 13.5-14.5 volts). With lower temperatures, the regulator will raise the voltage.

With the engine running, the volt/ampere tester should show a gradual decrease in the charging current. the voltage should remain within normal operating limits. if the regulator is below the manufacturer's specifications, the regulator must be replaced. the battery will never be fully charged and it will become sulfated. older electromechanical regulators were adjustable. if the voltage is higher than specified, this might not be due to the alternator.

A voltage drop (resistance) on the ground side of the regulator will raise system voltage by the amount of the voltage drop.

Test the regulator ground with a voltmeter.

Checking the ground on a voltage regulator. Courtesy of Interstate Batteries.

If the ground side has no resistance, check the sensing voltage input to the regulator to see that it is the same as the battery voltage. If it is, the regulator should be able to keep voltage within specifications. It must be replaced if it does not.

Voltage Output Test

No-Load Test

This test is used to make a quick determination if the charging system is working properly. If the charging system is working correctly, then check for battery drain. The following procedure is for performing the test.

  • Connect a voltmeter across the battery terminals, observing polarity.
  • Connect the tachometer, following the manufacturer's procedure.
  • With the engine off, record the base voltage value across the battery.
  • Start the engine. Because most A/C generators do not produce maximum voltage output until 1,500 to 2,000 engine rpm, the engine speed needs to be brought up to this level.
  • Observe the voltmeter reading. It should read between 13.5 and 14.5 volts.

If the charging voltage was too high, there may be a problem in the following areas:

  • Defective voltage regulator.
  • Poor voltage regulator ground connection.
  • Defective wiring between the voltage regulator and the A/C generator.

If the charging voltage was too low, the fault might be:

  • Loose or glazed drive belt.
  • Defective voltage regulator.
  • Defective A/C generator.
  • Discharged battery.
  • Loose or corroded battery cable terminals.

Load Test

If the voltage reading was correct, perform a load test to check the voltage output under a load condition:

  • With the engine running at idle, turn on the headlights and the heater fan motor to high speed.
  • Increase the engine speed to approximately 2,000 rpm.
  • Check the voltmeter reading. It should increase a minimum of 0.5 volt over the base voltage reading taken previously.
  • If the voltage increases, the charging system is operating properly. If the voltage did not increase, perform the following test series to locate the fault.
if the charging system passes the no-load test but fails the load test, check the condition and tension of the Drive Belt closely.

Voltage Drop Testing

The voltage drop of all wires and connections combined should not exceed 3% of the system voltage. any particular wire or connection should not exceed 0.2 volt; total system drops should be less than 0.7 volt. the ground side voltage drop should be less than 0.2 volt. to perform the voltage drop test using the vat-40, follow these steps.

  • Connect the large red cable to the battery positive terminal.
  • Connect the large black cable to the battery negative terminal.
  • Select CHARGING.
  • Select EXT 3 V.
  • Zero the ammeter.
  • Clamp the inductive pickup around the A/C generator output wire.
  • Using the small red and black test leads, connect at the following locations:
    • Insulated circuit. Red lead to A/C generator output terminal. Black lead to the battery positive terminal.
    • Ground circuit: Red lead to the battery negative terminal. Black lead to the A/C generator housing.
  • Start the engine and hold the engine speed between 1,500 and 2,000 rpm.
  • Using the carbon pile knob, load the system to 9 to 20 amperes. Some manufacturers recommend measuring voltage drop when the generator is putting out its maximum; always follow the recommendations of the manufacturer. Remember if the A/C generator is not putting out any current, there will be no voltage drop even if the circuit is very corroded.
Turning on the headlights may be substituted for the carbon pile.
  • Read voltmeter.

If a high voltage drop is observed anywhere in the circuit, work up the circuit to find the fault. check every wire and connection. if a vat-40 is not available, the voltage drop test can be perform with just a voltmeter. simply follow the last three steps in the procedure.

Field Current Draw

The field current draw test determines if there is current available to the field windings. To perform the field current draw test using a starting/charging system tester, follow these steps:

  • Connect the large red and black cables across the battery, observing polarity.
  • Select CHARGING.
  • Select ANT 18 V.
  • Zero the ammeter.
  • Disconnect the field wire from either the A/C generator or the regulator.
  • Connect the multiplying coil to the field terminal. Make the connection toward the A/C generator.
  • Connect the field lead of the tester to the multiplying coil.
  • Move the toggle switch to the proper field-type position (A or B).
  • Read ammeter while toggle switch is depressed.
  • Compare results with manufacturer's specifications.

If the readings are within the specification limits, then the field circuit is good. If the readings are over specifications, a shorted field circuit or bad regulator may be the problem. If the readings were too low, then there is high electrical resistance that may be caused by worn brushes.

Alternator Field Test

There is a simple test to see if an alternator field is strong, done with the key on and engine off. check the rotor shaft at the outside of the housing of the rear bearing for magnetism. if there is enough magnetism to lightly hold a screwdriver, the rotor is energized. if the rear bearing is not magnetized, the problem could be related to the regulator, the brushes, or a defective rotor.

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