Alternator: Testing and Inspection
Alternator:
Background and Application Information:
A very valuable alternator test that has been around for years is called the "ripple voltage" test. This test was usually performed using an engine analyzer's ignition scope. The DSO performs this test very well in just a matter of seconds. It tests the performance of the alternator's diodes. When a vehicle keeps eating batteries and everything else checks out, or when chasing down an elusive voltage spike in the system, this can be a very valuable test. It is a fairly simple test, but for it to work properly, the procedure has to be followed closely.
Connect and Setup the Digital Storage Oscilloscope (DSO):
1. Connect the "COM" probe to the negative battery terminal, or preferably the alternator case.
2. Connect the "CH1" probe to the positive battery terminal, or preferably the B+ output terminal on the back of the alternator.
NOTE: Connecting the DSO probes at the alternator itself rather than the battery terminals yields a less dampened, more clarified signal.
3. Set the voltage scaling to 20 mV/div. Alter as needed later.
4. Set the time base to 1 ms/div. Alter as needed later.
5. Select "GND" coupling and position the trace on the division line in the center of the DSO display.
6. After ground position is set, select "AC" input coupling.
7. Select "NORMAL" acquire mode.
8. Set the "TRIGGER" mode to "AUTO."
Exercise The Charging System:
With the Key On, Engine Off, turn on the high beam headlights, put the A/C or heater blower motor on high speed, turn on the windshield wipers, and rear defrost (if equipped) for about 3 minutes. This removes the "surface charge" from the battery and makes the alternator's regulator allow the alternator to "full field," or charge at maximum output. This keeps the voltage regulator from kicking in and out, complicating the waveform test. Now start the engine and let it idle.
Good Alternator Diode Pattern (Typical):
Expect This Waveform Result
A waveform that looks like a string of back-to-back rolling hills should appear on the DSO display if the alternator's diodes are good. The rolling hills should be roughly equal in height and should have no breaks, chops, or dropouts. This waveform example shows good alternator diodes. Transient spikes may be seen occasionally along the waveform's rolling hills. This is normal on many systems. Bad diodes will appear as very noticeable dropouts in the waveform, dropouts that have two to three times the amplitude of a normal rolling hill.
IMPORTANT NOTE:
As the alternator does its job of charging the battery, the rounded rolling hills will become sharp edged, usually on their left side. This is from the regulator lowering the alternator's load by varying the field circuit. If the battery keeps charging up too fast to get a good read on the diode waveform, put a light load on the battery with a carbon pile battery load tester. This will keep the regulator from kicking out, making diagnosis easier.
For More Information
Description of Automotive Signals
Diagnosis Using A Labscope
Labscope Quick Reference Appendix