Simple 4 Step Alternator Testing Guide

Is My Alternator Faulty? How to Test an Alternator in 4 Simple Steps...

Our ‘How to Test an Alternator in 4 Simple Steps’ guide provides four basics checks that will assist your diagnosis of alternator problems. If your vehicle passes the following alternator tests, your alternator might not be to blame. Some of the following common effects of failure are:

• My engine wont start / car wont start
• Battery is flat
• Battery warning light stays on / not working properly

Our Alternator testing procedure couldn't be easier. Following our three step alternator tests in a methodical order ensures fault detection as early as possible and covers car, marine, commercial, plant & agricultural applications.

Step 1: Check the Fan Belt Tension

Approximately 10% of alternators are replaced incorrectly due to inadequate diagnosis – when in fact a replacement alternator drive belt (fan belt) may have fixed the problem. If the alternator belt is slipping, it will not drive the alternator – and hence charge the battery properly.
Test: Find your recommended fan belt tension in the vehicle service manual. If the belt is too loose, adjust accordingly and see whether this provides a fix to your problem.

Step 2: The Alternator Volt Meter Test

Test: the alternator using a standard multimeter. To perform this test you need to measure the voltage across your battery. This is achieved by placing the positive (red) probe on the positive battery terminal - and the negative (black) probe on the battery negative terminal.

Set the device to the DC scale and with the engine running, measure the voltage. With the engine idling, we would expect to see at least 14 volts, which suggests that the alternator is charging the battery sufficiently. Any reading below 14v requires further investigation, so move on to the next test.

Step 3: Test the Alternator B+ Cable

The B+ thick cable connection allows the alternator to pass charge to your battery. This is therefore a crucial component of the electrical setup, because without continual charge the battery will go flat!

Test: Using your multimeter on the DC setting, place one probe on the B+ terminal on your alternator - and the other on the battery negative terminal (It doesn't matter which way round). The reading should be similar to that measured in step 2 at over 14 volts. If there is a significant variation between the two results, this indicates a break in the wiring and further investigation is required. Otherwise proceed to step 4... 

Step 4: Test the Alternator Warning Bulb & Wiring

The battery warning light alerts you to a possible alternator problem. Under normal conditions the battery light glows when the ignition is turned on - and disappears when the engine is started.
Quick checks: If your battery warning light is not working at all, it might just be the bulb. If your battery light continues to glow when the engine is running, this signifies a problem with the alternator.
Test: With the ignition on – the battery warning lamp should glow. If your bulb doesn't glow you will need to locate and disconnect the lamp terminal (connection D+ L in the above diagram). Using a standard voltmeter (multimeter), test the voltage on the lamp wire (with the ignition on). You should discover a reading of 12 volts, which disappears when the ignition is turned off.

Note: If there is 12V on the lamp wire, and you have completed this 4 step testing guide, then the bulb is fine and the alternator must be faulty.

Need a Replacement Alternator

If you found anything suspicious and would like further expert guidance feel free to contact us on 0845 564 2681. Alternatively visit the alternator section of our store to find a replacement alternator today.