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Alternator Testing Guide

Is My Alternator Faulty? How to Test an Alternator

Our ‘how to test an alternator’ guide provides three 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. Alternator problems result in some of the following common effects:

•    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.

Alternator Wiring Diagram

Step 2: Inspect Alternator Wiring Connections

If the fan belt is not to blame, perform a thorough inspection of the alternator wiring. Loose or damaged wires will hamper the electrical circuit and reduce the performance of your alternator. Most alternators have two wires: a feed from the ignition which supplies the battery warning lamp (D+ L) - and a thicker cable which supplies charge to the battery (B+).

Test: Check the connections between the battery and the alternator. Also undertake an inspection along the entire alternator wiring, paying attention to any cracked casing and exposed wires, which may need to be replaced. Providing everything is OK, move on to step 3.

Step 3: Test the Alternator Warning Bulb or Its 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 running.

Quick checks: If your battery warning light is not working at all, it might just be the bulb – although this doesn’t eliminate a faulty alternator. If your battery light continues to glow when the engine is running, this signifies a problem with the alternator.

Test: When the ignition is on – the battery warning lamp should glow. If your bulb does not glow you will need to locate and disconnect the lamp terminal (connection D+ L). Using a standard voltmeter, 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.

  • If there is no voltage on the lamp wire, this indicates a problem with the bulb lamp or bulb wiring.
  • If there is 12V on the lamp wire and the bulb is good, the alternator must be faulty and will need replacement.

Need a Replacement Alternator

If you found anything suspicious and would like further expert guidance feel free to contact us on +44 (0)1752 339871 or email sales@startermotor-alternator.co.uk. Alternatively visit the alternator section of our store to find a replacement alternator today.