Anonymous asked in Cars & TransportationMaintenance & Repairs · 1 month ago

What is the most likely cause of an alternator that charges fine when starting the car but within a minute becomes too weak?

Battery tests good. I replaced the brushes then had Autozone test the alternator outside and it tested good. I reinstalled. Same problem. What else could be the cause?


I cleaned battery terminals and alternate tested bad by Autozone when inside the car before I replaced the brushes and brought it in for bench testing.  (Where it tested good.)

7 Answers

  • M.
    Lv 7
    1 month ago

    How are you determining that the alternator charges fine?  (Statement 1)

    How are you determining that the alternator charges too weakly?  (Statement 2)

    You measured something?

    What did you measure and what were the results?

    -Engine overhaul mechanic and general automotive mechanic since 1972

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  • 1 month ago

    When you tested your battery did you do a voltage check , or was it load tested . If it's reading out at 12V . Which is what you want it to be .

    A load test puts a load on the battery and mimics the power  draw like a starter motor would . That's the only real way to determine if your battery Is  putting actual current out .Volts dont  start cars Amperage starts cars .If your going off the voltage ,and assuming it's good based on that . It may have sulfated cells which cause the battery to lose  the ability to flow those amps to start your car .Volts can be 12 ,and  the battery can still be no good . 1more thing if your battery is bad . A bad battery can also cause the alternator  to have problems too . That's one killer of alternators , Is when people use it or unknowingly use the alternator to charge that battery It's a myth that that's what it's for . it will maintain vikts Inna healthy battery But s bad battery will strain it usually killing the volt regulator and or diodes inside the alternator . You can take a volt meter while the car is running and check the battery voltage It should be 13.7 up to 14.5 volts If it's under 13.7 it's going bad replace it . If it's over 14 .5 the voltage regulator is bad Replace it . If it's between 13 .7 and 14.5 it's perfect leave it alone After that then the battery load test . Now you know what to look for a d what can happen if the battery is bad Cause a bad battery will damage the alt . Then your out a grip  of cash . Try to avoid that much as possible . Good luck 

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  • Dan
    Lv 5
    1 month ago

    It's probably a Honda. They have a load detection module that shuts off the alternator after the engine starts.

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  • 1 month ago

    Put a volt meter on the battery terminals and start the motor. You need a minimum of 13.7 volts to charge the battery. If your alternator is not charging check your fusible links. Fusible links are the larger fuses in the engine compartment fuse box. Most alternators go bad because one of the 3 diodes fails and you only get around 12.3 volts out of it. 

    If the fusible links are all good you either have a bad alternator or a wiring problem.

    Source(s): Mitsubishi Master Tech
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  • 1 month ago

    Car needs a new battery. 

    How old is the current one ?

    Over two years or three years ?

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  • 1 month ago

    you need a test to determine exactly how much voltage the battery will take battery tested ok ..the alternator worked fine ..with tester linked up it was still charging battery with all lights on tickover ...however the problem was the battery would not take as much charge as it should means you would need to drive the car for two days to fully charge battery ..normal two hours max ..bench test for alternators are not good 

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  • Anonymous
    1 month ago

    A bench test is not an accurate test for alternators.  The alternator must be tested on the car under load using a carbon pile tester.  Your alternator is likely toast if voltage drops under load.  A bad alternator might have bad windings, a bad voltage regulator or the bearings are bad.

    First test if the battery light is ON when key is ON, engine OFF.  If battery light is not ON,you have to fix this issue first.   Check output at B+ terminal of alternator  with engine running (negative lead at battery negative post) and voltage across battery.  If voltage checks good at B+terminal and bad across the battery, you have a wiring problem or a bad ground.  Voltage is bad at B+ terminal,  alternator is bad.  If you have good voltage at battery with engine running,  load test the alternator using a carbon pile tester.

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