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How do you check for a bad diode in an alternator?
My headlights haven't been as bright lately, and a mechanic friend told me the problem could be a faulty diode. Can you tell me how to check for a bad diode in an alternator?
Natalie Todoroff · Updated on
Reviewed by Shannon Martin, Licensed Insurance Agent.
Learning how to check for a bad diode in an alternator is easy! You will need to purchase an inexpensive tool called a voltmeter, which you can find online at Amazon for around $13. But first, let’s quickly review what diodes are and how they work.
Your alternator is like a mini generator in your engine that provides charge to your car’s electrical components like your headlights, dashboard lights, and
battery
. Diodes convert the alternating current, the electrical current your alternator produces, into direct current, which is then used to power the headlights, dashboard lights, battery, etc. It’s helpful to think about diodes as translators: they translate what your alternator produces into something the rest of your vehicle’s electrical system can understand. Now, here’s how to check and see if they’re working:
- Grab your voltmeter and change it to a low setting on the AC (alternating current) voltage scale
- Turn on your engine
- Gently touch your voltmeter’s probes to the battery terminals
- The voltmeter should read 0 AC. Any other reading indicates a bad diode
To replace your alternator, you’ll probably pay between $350 and $400 for the service. Iif you’re looking to cut back your vehicle’s routine expenses, downloading
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