How to test an alternator?
BATTERY AND ALTERNATOR (GENERATOR) TEST
If the battery discharges when the vehicle is not driven, there may be a constant drain or current draw causing it to discharge when the ignition is off. Depending on the draw and the condition of the battery, a full discharge can happen overnight or it may take a few weeks. Although a small static drain on the battery is normal (for example to operate the clock or radio memory), a large drain such as a relay sticking on or a faulty switch will cause the battery to quickly discharge. Make a static current draw test as the first step when experiencing battery discharge.
If the current draw on the battery is not excessive and the battery still discharges, the condition of the battery should be tested. Battery testing determines the state of the battery charge. The most common methods are open-circuit and load voltage testing. Batteries with filler caps can also be tested by checking the specific gravity of the electrolyte.
Inexpensive specific gravity testers are available at most auto supply stores.
Some vehicles are equipped with batteries with a central gas venting system with anti-flash protection. The antiflash protection consists of a small round fiberglass mat with a diameter of approximately 15 mm and a thickness of 2 mm. Its purpose is to vent gasses that form in the battery.
The gasses that form while charging can vent centrally through an opening on the upper side of the battery cover. The anti-flash protection is also installed to prevent ignition of the flammable gasses in the battery.
Open-circuit voltage test
An open-circuit voltage test checks battery voltage by connecting an accurate digital voltmeter to the battery posts after disconnecting the battery ground cable. Before making an open-circuit voltage test on a battery, first load the battery with 15 amps for one minute.
See Table a for open-circuit voltage levels and their corresponding percentages of charge.
Causes of Charging System Failure:
A charging system malfunction is identified by the battery
1. OVERCHARGED BATTERIES caused by one or a combination of the following:
A. Defective Battery.
B. Defective regulator.
C. Poor sensing lead contact to regulator or rectifier assembly.
2. UNDERCHARGED BATTERIES caused by one or a combination of the following:
A. Loose belts; corroded, broken, loose or dirty terminals; broken wiring; undersize wiring;
B. Alternator field circuit malfunction caused by one or a combination of the following:
a. Poor contacts between regulator and brushes.
b. No residual magnetism in the rotor. (Refer to TSB-1034 to restore residual magnetism.)
c. Defective regulator.
d. Damaged or worn brushes.
e. Damaged or worn slip rings.
f. Poor connection between slip ring assembly and field coil leads.
g. Rotor coil shorted, open, or grounded.
C. Alternator generating section malfunction caused by one of the following:
a. Stator phase(s) shorted, open, or grounded.
b. Rectifier assembly grounded.
c. Rectifier(s) shorted or open.
ALTERNATOR TEST WITH MULTIMETER
Start the engine
Select DC Volts mode on your multimeter
Clamp the black lead to the negative battery terminal. Clamp the red lead to the positive battery terminal.
the voltage should be between 13.5v and 14.5v