How to Check a cars alternator

Learn how to easily check to see if you alternator is bad or if its your battery. This test is conducted on the electrical system of a car/truck. All or mostly all of the cars and trucks on the road are 12 volt DC (direct current) electrical systems.

Difficulty: Easy

Instructions

Things You'll Need:

• You will need a digital voltage meter.

1. First, you need to know how to use and read a digital voltage meter. The meter should have a DC setting some may have 12v and 24v settings and some may have numbers like 20, 200, or 2000, if you have these numbers you want to be on 20, if you have a 12v setting then use it.

2. Now, lets check the battery first to make sure that it is still good. With the engine and key turned off, take the probe from the positive (usually red) lead and place it on the positive (+) terminal of the battery. Place the negative (usually black) probe from the negative lead on the negative (-) terminal of the battery or ground it to the engine's block. Your reading on your meter should be in between 11 volts DC and 12.5 volts DC. If your reading is below 11 volts then charge your battery, if your reading is still to low after charge, then your battery needs to be replaced.

3. Now, to test your alternator you do the same test but with the engine running (make sure cooling fan is clear and free to spin). While your engine is running your meter reading should be in between 13.5v and 14.5v. You can also take the positive battery cable off while the car is running, if the car/truck shuts off the alternator is bad, if it keeps running the alternator is good.

Tips & Warnings

• Keep in mind your car/truck may have ignition fuses and relays as well as the ECU. So if your battery test and alternator test are both good, then your problem is somewhere else.
• Never cross positive and negative or grounded wires, leads, or batteries.