Alternator Failing?!
Thread Starter
Intermediate
Joined: Apr 2024
Posts: 29
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Alternator Failing?!
I installed headers and since then I’ve had a charging system fault pop up while driving but never at startup. Before I recharged my battery the other day. The voltage while driving would be low 11’s and low 12’s. Since the recharge and cleaning my grounds. I used a multimeter to check the power at start. With the engine off
Battery: 12.3 Volts
With the engine on:
Battery: 14.5
Alternator 14.5 (roughly) maybe 14.7?
BIG HOWEVER,
after 20-30 minutes of driving my voltage on the multimeter with the car running is
Battery: 12.2-12.4
Alternator: 12.4-12.7
Dash Voltage reading: 11.4-11.6
Im not quite sure what the issue is. Is this an alternator problem for sure? It seems like when the alternator gets hot it starts failing. What brands of alternator should I buy? I’ve heard of people buying 3-4 from autozone because the kept failing
Battery: 12.3 Volts
With the engine on:
Battery: 14.5
Alternator 14.5 (roughly) maybe 14.7?
BIG HOWEVER,
after 20-30 minutes of driving my voltage on the multimeter with the car running is
Battery: 12.2-12.4
Alternator: 12.4-12.7
Dash Voltage reading: 11.4-11.6
Im not quite sure what the issue is. Is this an alternator problem for sure? It seems like when the alternator gets hot it starts failing. What brands of alternator should I buy? I’ve heard of people buying 3-4 from autozone because the kept failing
Thread Starter
Intermediate
Joined: Apr 2024
Posts: 29
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Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,621
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From: Cape Coral, Florida
If you are reading say 12.5 volts at the alternator (alternator B+ to alternator case) and 12.5 at the battery you don’t have a voltage drop issue…may be a failing alternator…get your current one overhauled if OE and not replaced.
Tech Contributor
Joined: Jan 2007
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From: Dyer, IN
Are you aware that the red wires from the alternator connect to the positive battery cable at the starter?
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
The positive battery cable and alternator B+ cable meet at the starter solenoid if you didn’t know that…if there is a bad connection or high resistance the battery will not be charged…the alternator may be delivering 14.5 volts but if you are seeing 12.5 volts at the battery you have a 2 volt drop…you should have less than 0.5 drop on the positive and negative sides of the charging system.
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Thread Starter
Intermediate
Joined: Apr 2024
Posts: 29
Likes: 1
The positive battery cable and alternator B+ cable meet at the starter solenoid if you didn’t know that…if there is a bad connection or high resistance the battery will not be charged…the alternator may be delivering 14.5 volts but if you are seeing 12.5 volts at the battery you have a 2 volt drop…you should have less than 0.5 drop on the positive and negative sides of the charging system.
Im just now learning this actually but that makes a lot of sense! The only reason Im skeptical about it being a starter connection is that the charging system works until maybe 25-30 minutes into driving. Does that sound right?
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,621
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From: Cape Coral, Florida
If you can have your DMM hooked up to the alternator B+ and the negative lead to the case while you drive you can monitor the alternator voltage…if it drops the alternator may be failing….if the alternator is producing 14.5 volts but after 30 minutes the battery is only charging at 12.5 then you have that bad connection or high resistance….are you checking the alternator charging voltage at the stud and not the nut ??…I like using my thermal imager as a quick check.
Last edited by C5 Diag; Aug 17, 2025 at 04:36 PM.