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Battery not charging?

Tiny
MR DON'T KNOW
  • MEMBER
  • 1976 CADILLAC DEVILLE
  • 8.2L
  • V8
  • 2WD
  • AUTOMATIC
  • 78,000 MILES
Bought a new alternator multimeter reads 15 volts and when I attached cable to battery it reads 12.36 volts and won't go much higher when throttle is given. I checked the wiring on the ignition switch and all is fine.
Thursday, June 6th, 2019 AT 12:34 PM

11 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good afternoon,

When did it read 15 volts?

Was the engine running when it read 12.36 volts?

Roy
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Thursday, June 6th, 2019 AT 12:48 PM
Tiny
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Read 15 volts when I started it. Checked the voltage from alternator then attached cable to battery and volts read 12.36 and wouldn't go much higher after I pressed on the throttle.
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Thursday, June 6th, 2019 AT 12:58 PM
Tiny
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15 Volts? Did you run the engine with a battery cable disconnected?

Roy
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Thursday, June 6th, 2019 AT 1:05 PM
Tiny
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I had the cable that connects to the alternator disconnected to see if was producing volts. Once it read 1.5 to 1.76 then it started to read 15 volts that's when I attached cable to battery.
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Thursday, June 6th, 2019 AT 1:08 PM
Tiny
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Okay, thanks.

No, it will not charge if the cable is off. The alternator needs battery voltage to charge the magnets for it to work.

You removing it and installing it while running most likely failed the alternator.

Never ever do that. Voltage spikes are never good for the system.

Roy
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Thursday, June 6th, 2019 AT 1:11 PM
Tiny
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Alternator still reads 15 volts but when I attached cable and start engine battery reads 12.36 volts. Can it be a ground problem?
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Thursday, June 6th, 2019 AT 1:14 PM
Tiny
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No, it is not charging if it is reading 12.36 volts. That is the battery voltage.

Roy
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Thursday, June 6th, 2019 AT 1:25 PM
Tiny
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Then why does the alternator read voltage before it is attached to battery but not charging it after it is attached?
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Thursday, June 6th, 2019 AT 1:28 PM
Tiny
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That is an internal issue. Again, you should never remove and attach the cable with the engine running. You failed the regulator and the diode trio.

Roy
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Thursday, June 6th, 2019 AT 1:32 PM
Tiny
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Okay, thanks. I replaced the alternator and it is all fixed.
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Thursday, June 6th, 2019 AT 1:35 PM
Tiny
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You are welcome.

Always glad to help.

Roy
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Thursday, June 6th, 2019 AT 1:38 PM

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