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How To:- Diagnose Alternator Problems

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In response to many posts about alternator problems, I've written this guide to help with finding out where your problem lies.

You will need a multimeter or voltmeter able to read 0-20v

First thing:-

On a MK1 check these fuses.

  • AM2 (7.5A)
  • Charge (5A)
  • Engine (10A)

All in the engine bay fusebox

On a MK2 check these fuses

  • ALT SENCING (7.5A in front compartment fusebox)
  • AM2 (7.5A in front luggage compartment fusebox)
  • ECU–IG  (7.5A in engine bay fusebox)

Now carry out the following checks with a good, fully charged battery fitted:-

  • 1. Measure the battery voltage with the engine and ignition off, should be over 12v (more like 13v) take a note of the voltage for reference
  • 2. Start the engine and check the voltage across the battery again, should have risen to over 13.5v. 
  • 3. With the meter still on the battery increase the rpm to 2000, voltage should be at least 14v.

If it fails any of these checks, then you either have a faulty alternator or wiring. (assuming you paid attention and have a good, fully charged battery)

Next carry out these checks:-

  • 4. Take the plug out of the alternator, switch the ignition on, (engine not running)
  • 5. Use the meter to check from each of the 3 pins of the PLUG to a good earth, you should get around the battery voltage (that you noted in step 1) on each pin
  • 6. Use the meter to check from the large bolted terminal on the alternator to a good earth, you should get around the battery voltage (that you noted in step 1)

If you don't get battery voltage at any of these 4 points, you have a fault on the wiring.

If you do get the battery voltage at each of those 4 places then the alternator is faulty.

If you have a wiring fault it will be on the wiring from the points where you don't get battery voltage, use the meter to track back the wiring and find the fault.


Jimi Lawson

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