How To:- Diagnose Alternator Problems
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
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.
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