How do you test an alternater?

Answer 1

Testing an Alternator

I do it in two ways. First, I do my own voltage output test using an ordinary Volt-Ohmeter.It can be either analog [dial-type], or a digital meter.

To set up the test, the meter test leads must be plugged into the proper jacks on the meter, the function selector switch [usually a rotary type multi-positon switch] set to the proper test mode [DC Volts], and the proper "range" [usually the 50 Volt scale]. The vehicle engine should be turned OFF, and then place the test lead probes [the "pointed ends] in contact with the battery terminals.

The RED [Positive] test probe should be held on the battery POSITIVE terminal [there will be a + mark, or the word POS embossed in the battery case top, next to the terminal. Also, the positive terminal is slightly larger in diameter than the negative terminal].

The negative meter probe can be touched to either the negative battery terminal or any bare metal part of the engine block or vehicle body.

Observe and record the voltage indicated. Normally for a fully charged battery this voltage should be about 12 to 12 1/2 volts.

Next, while making SURE that the meter and test leads are well clear of any moving parts [radiator fan, belts, turning parts of the alternator, smog air pump, Power Steering pump, etc.] start the engine and allow it to idle.

Run the test again, observing and recording the voltage. It could range anywhere from what was observed in the first [engine off] test to some higher voltage.

A properly functioning alternator should cause the voltage to read in the range of 13 to 16 or 17 volts. If the idling voltage is low [like 12 or 13 volts], have an assistant/helper gently accelerate the engine above idle speed. If this allows the voltage to increase to 13 to 17 volts [usually 16 is the highest most voltage regulators allow] then the alternator is working.

IF there is no significant increase in voltage between the engine OFF and engine running/accelerated modes, then there is something wrong with your charging system.

This does not necessarily mean that the alternator is no good, as there are several things which can interfere with proper charging.

If my own "home test" does not show the alternator to be putting out voltage, then I take my vehicle to one of the major, chain-type auto parts stores, as they usually have a handheld charging system analyzer and thus can check the entire [not just the alternator] charging system in their parking lot. In our area [southeast Texas] they do not charge for this service. Good luck.