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Repair guides and support for the fourth generation of Ford Mustang, including the Mustang GT and Mustang Cobra.

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Car starts, but the battery loses it's charge

I have a 1997 Ford mustang and got new battery it starts fine my battery gage dont show battery is fully charged and I got a new alternator and I checked the fuses but my battery and alternator wont keep charge is anyone having this problem

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Use a Voltmeter (function found in a digital multimeter) to check that the voltage across the battery terminals is between 13.8 - 14.5 VDC when the engine is running.

Be safety aware when you test as the engine is running!

If it is less than 13.8VDC then you will not be charging the battery.

I think that the voltage regulator is inbuilt into the alternator in your car, so since you got a new alternator you may have to check if its' output is OK and if so then check why you're not getting the correct voltage at the battery terminals.

If you can't do this yourself go to an auto electrician and get them to check.


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Lead acid car batteries don’t like being discharged and tend to “expire” in ~5 years. Check when your battery was made and if it’s 5 years old or older, try replacing it and see if that helps first. In some cases they're still reasonably strong even after that so in some cases a battery test may reveal your old one is still serviceable. Usually the battery is near the end of it's life after 5 years but you sometimes get lucky.

If you do not get a better result with a replacement battery, you may need to try another alternator (especially if you used a rebuilt unit in the past) that is OEM or new but not rebuilt. This is the second common reason for no starting.

If you have issues with it even with a new battery and a quality alternator, you likely have a parasitic draw somewhere that will take some time to find, or something like the anti theft system or ECU may be failing. With the anti theft system it may be bypassable, but I am not going to discuss that here or provide links on how - you're on your own. If it's an ECU, they usually need pairing to the BCM and anti theft system and can be over $1k in some cases. At that point you probably want to go to a mechanic unless you own a scan tool which can do the pairing already ($$$).

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Also be sure that your terminals are connected right and your body grounds are perfectly clean…a simple loose terminal is a definite reason why there is no charge and much cheaper than an alternator…a few turns of a wrench is always the cheapest way out!!

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Bear in mind I wrote my answer when I knew less about cars so I missed this possibility. When I seen European depreciation I picked up a lot lol.


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