How To Diagnose A Failing Alternator
When your car starts showing signs of electrical issues, it can be very frustrating to figure out the cause. The possibilities include the battery, the alternator, or something else in the electrical system. Many times electrical issues are caused by a failing alternator. Alternators can fail slowly. They will still charge the battery, but not as well. If left unchecked, your vehicle’s alternator will fail completely and may leave you stranded. We came up with this guide to help you identify when your alternator is failing and how it can affect your vehicle.
Electrical Issues - Is It The Alternator, Or The Battery, Or Something Else?
There are plenty of signs that indicate an issue with your electrical system. Some common problems include:
- Slow to start, cranks over for an extended period
- Dashboard lights flicker
- Power windows slow to operate
- Unable to start
- Radio is in-operable
- Battery light on dashboard
The symptoms listed above can be confusing because the vehicle may seem to running fine otherwise. Sometimes these issues can be linked to a failing or failed battery. Battery failure is not uncommon, considering they typically last about 3-5 years. Alternators typically last longer than batteries, but they can fail between 5-10 years of operation. When an alternator fails, something inside the alternator has worn out. The possibilities include:
- Bad diodes
- Failed internal wiring
- Worn alternator bearings
- Failing voltage regulator
Begin By Inspecting For Common Problems
If your battery is in good condition, you should troubleshoot the alternator. The first thing to do is inspect the components related to the alternator:
- Drive belt - is it worn, cracking, or loose?
- Wiring harness - look for a loose connection or corrosion
- Alternator control circuit fuse - is it blown?
- The lead between the alternator and the battery - the connections should be secure and free of corrosion
If the parts you inspected look good, move on to diagnosing the alternator itself.
How To Diagnose A Failing Alternator:
One of the most sure ways to diagnose an alternator is to get a voltmeter and measure the charging voltage.
- Let the car sit for a few hours, so you can measure the resting voltage of the battery.
- Make sure all accessories that draw power are off.
- With the voltage meter, check the voltage of the battery (with the engine off,)
- If the voltage is between 12.6V and 12.8V, then the battery is fully charged. (The alternator may still be failing.)
- If it's under 12.6V, the battery is not fully charged. If the voltage at 11.8V or below, the battery is considerably discharged, and may be bad. It's possible that the alternator is still good and just the battery is bad.
- The next step is to start the engine and immediately measure the voltage at the battery terminals.
- When the engine first starts, the voltage should read between 14.1V and 14.7V. After a few minutes, the voltage will drop.
- If the voltage drops to under 13.7 V, then the alternator isn't keeping the battery fully charged.
- If the voltage remains between 13.7 V and 14.7 V, then the alternator is still charging the battery properly. This means the battery may be bad, or that there is another problem.
- If the voltage right after the engine starts never makes it above 14V but instead sits around 13.7V-13.9V, the alternator is getting worn out, and will probably fail soon.
If you don't have a voltmeter, another way of measuring voltage is to get a USB charger with a voltmeter that plugs into the 12v outlet in your dash. These are now commonly available for $15 or so.
What Alternator Should I Get?
If your alternator has failed and you’re considering what to replace it with, check out this blog post regarding OEM alternators versus aftermarket units. OEM alternators may cost a bit more upfront, but over the long run they are a better value. We stock many different OEM Ford alternators, and you'll find that our pricing is some of the best around.