Alternators off car testing and on car testing

OFF CAR TESTING/BENCH TESTING

The task that had to be done was to strip down an alternator, and test the individual components to make sure that everything is still working on the alternator, then rebuild it again to a condition that it can be put back on a car.

The first thing that must be done is to remove the back cover of the alternator along with the brush holder and voltage regulator then remove the rectifier then the test are ready to be performed on these components.

The first test to test whether there was grounding on the rotor widing this test is done by setting the multimeter to ohm's 2k and putting the red probe on the slip ring and the black probe on the rotor shaft there should be no reading if there is a reading this means there has been a short circuit and the rotor would not produce a magnetic field and the alternator would have no output. The next test was for rotor winding internal resistance this test is done by setting the multimeter to 200 ohm's and by placing the probes on each end of the slip ring. If the test had come up less than two ohm's which is the minimum specification then this would mean there is an internal short or a break in the insulation of the rotor windings and the current will be missing some of the windings. This would mean less windings for the current to pass through and this would produce a weaker magnetic field and the alternator would have a reduced output. If the reading was higher than six ohm's which is the maximum specification this could mean that there corrosion on the winding connections causing higher resistance this higher resistance will reduce current flow and this would mean a weaker magnetic field from the rotor winding and this would mean that the alternator would have reduced output and the battery would get less charge.

The next test was to test the resistance of the stator windings which should be very low if the resistance is above specifications then this would mean that the current being induced into the stator from the rotor would be reduced and this would give the alternator less output. The other test on the stator windings was test whether there was grounding, this test is done using a multimeter setting it to ohm's 2k and then placing the red probe on the common stator winding terminal the terminal that has the most wires and the black probe goes on the alternator body, the reading should come up open circuit if there was a reading on the meter this would indicate that a circuit is being produced and the current being induced into the stator would be grounding on the alternator body rather than going to ground at the battery this will result in less output from the alternator and the stator would need to be replaced.

The next test was to test the voltage drops across the positive diodes of the rectifier, if the readings came up below the specification this would indicate that the diodes are not consuming the power and this could result in a higher output from the alternator, if the reading was higher than specification this would indicate that there are poor or dirty contacts at the diodes which would cause resistance and more voltage would have to be used to allow the current to flow through the diodes and this would give a reduced output from the alternator. The next test was to see whether there was a circuit from the alternator positive output to the diodes this test is done by putting the multimeter on diode test then put the positive probe on the rectifier's positive output and the negative probe on each of the diodes the reading should come up open circuit, if their was a reading this would indicate faulty diodes as they would be allowing current to flow in the wrong direction, if this is the case then the rectifier would need to be replaced. The next test was the negative diodes of the rectifier this test should give the same readings and if the there were readings that were not to specification this would give the same problems as the positive diodes and the rectifier would need to be replaced.

The next component to be tested was the voltage regulator this was tested using a transpo regulator tester, the specifications to test the voltage regulator can be found in the regulator test manual, by checking the model of the regulator, once the regulator has been wired up to the tester correctly the test can begin. This tester tells you how much output it allows the alternator to give, if the reading was above specification this would mean that the battery is being overcharged and this would cause the battery plates to swell and the battery would produce alot of hydrogen, however if the set point (regulating voltage) is not to high above the specification this would not affect modern day batteries as they are designed to take a higher charge. If the reading was below specification this would mean that the regulator is not allowing the alternator to produce its full charge ability and the battery would not be fully charged.

The last test was to measure the length of the brushes to make sure that they were long enough to make good contact with the slip rings, if the brushes were to short ths would cause a poor connection on the slip rings and this would cause a lot of resistance and even sparking this would result in a lower output from the alternator as current cannot flow freely and new brushes would need to be put in.

ALTERNATOR ON CAR TESTING

With on car testing the first thing that was performed was visual check to make sure that the drive belt was in good condition that there were no cracks and that the belt was tensioned to 5mm deflection, then the alternator wiring connections were checked to make sure that the connections were in place and making a good connection. Then the alternator mounts were tested to make sure that the alternator was held securely in place and that the was a good eathing point for the alternator circuit.

The next test was an OCV of the battery to make sure that its charge was above 12.4volts which is 50% charged if it is below this it could affect the results of the test and give incorrect readings. Then the regulating voltage was tested to make sure that the regulator was in good condition this test is done exactly the same as an OVC except the motor is running, the test is done by running the motor at 2000 RPM untill the charge has reached its peak then this would be the regulating voltage, if the reading was above 14.8 volts this would indicate a faulty regulator and the alternator would be over charging the battery and would give the battery a short life span.

Once the regulating voltage is at the maximum then a no load current test can be done, this is done by running the motor at 2000 RPM whilst no accessories are on this means that there is no radio or air conditioning to draw the current and create load on the circuit. To get a reading, use a ampmeter and place it around the alternators positve output cable then you will get a reading. If there is a higher than specification reading this could indicate that the battery is not fully charged and the alternator is still trying to charge the battery or there could be something wrong with the volatge regulator.

The next test was to measure the regulating voltage under load, this is done by hooking up the multimeter as if you are doing an OCV test except now the engine is running at about 2000 RPM and all the accessories are on like the headlights the airconditioning and the radio to create a load on the battery and to test what the output of the alternator if the reading is below specification then the alternator is not producing enough charge and the battery has to supplement this, this could mean that there is something wrong with the voltage regulator. Next the battery was tested to see how much current was drawn from it under load using the ampmeter on the positive lead this test is done with out the motor running. Next using the ampmeter measure the current flow through the alternator positive output the current flow from the alternator should be the same if not higher than the load that is being applied to the battery, this will indicate how much current is being supplied to the battery whilst the battery is under load. 

The last test is to test voltage drops on the positive and negative sides of the charging system this test is performed whilst the motor is running at around 2000 RPM, the positive side is tested by setting the multimeter DCV and placing the red probe on the positive battery post and the black probe on the alternator positive output. The voltage drop should be no more than 0.2 volts, the negative side is tested by placing the positive probe on the alternator body and the black probe on the negative battery post this result should be no higher than 0.2 volts, in a perfect circuit these readings should be zero. If the readings are higher than this, it will mean that voltage is being used up to pass through this part of the circuit and will result in a lower output from the alternator.