Alternator voltage 120 vs 150 amp?
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Thread: Alternator voltage 120 vs 150 amp?

  1. #1
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    Alternator voltage 120 vs 150 amp?

    from the Electrical troubleshooting sticky, does it matter if you have a 120amp Vs 150 amp?
    from the sticky :
    - Voltage should be 14.2 to 14.4 volts. Anything less provides only a partial charge. If it is 14.1 or less volts, it needs to be fixed.
    do both alternators use the same volts for testing?
    btw, i have the 120 amp
    thanks
    98

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  3. #2
    Premium Member kmagnuss's Avatar
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    Re: Alternator voltage 120 vs 150 amp?

    I could be totally off here... but I'd go with the 150 if you need to replace one.

    But yeah... amps don't effect volts. A good way to think about it is like a pipe. Volts represents the size of the pipe... amps represents the amount of liquid being pushed through the pipe.

    It's been awhile since I switched from electrical engineering to business... so please correct me if I'm wrong.
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  4. #3
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    Re: Alternator voltage 120 vs 150 amp?

    Quote Originally Posted by kmagnuss
    But yeah... amps don't effect volts. A good way to think about it is like a pipe. Volts represents the size of the pipe... amps represents the amount of liquid being pushed through the pipe.

    It's been awhile since I switched from electrical engineering to business... so please correct me if I'm wrong.

    If viewing electrics as liquid, how about pipe size being resistance to flow, volts being pressure and amps being flow rate?

    Nothing otherwise constructive to the OP but I thought it could be my turn to be available for correction..........

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  6. #4
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    Re: Alternator voltage 120 vs 150 amp?

    Should be similar readings but depends on the draw from other electronics. I can't say you see 14v with a 120. From the 150A alternators on Td5 Discovery and Defenders, you usually see a full 14v or so when testing. I've never seen 14v from my 100A alternator on my early p38, more like 13.8v. It has struggled a few times this winter, so I suspect that's why the 120A alternators were used as well, and then the 150A ones on the later Thor engines.




  7. #5
    Premium Member kmagnuss's Avatar
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    Re: Alternator voltage 120 vs 150 amp?

    For the record, I've NEVER seen 14.4 v consistenly on my 150a rebuilt alternator. I normally hang around 13.9-14.1.
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  8. #6
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    Re: Alternator voltage 120 vs 150 amp?

    Quote Originally Posted by kmagnuss
    For the record, I've NEVER seen 14.4 v consistenly on my 150a rebuilt alternator. I normally hang around 13.9-14.1.
    Quote Originally Posted by Rich998a
    Should be similar readings but depends on the draw from other electronics. I can't say you see 14v with a 120. From the 150A alternators on Td5 Discovery and Defenders, you usually see a full 14v or so when testing. I've never seen 14v from my 100A alternator on my early p38, more like 13.8v. It has struggled a few times this winter, so I suspect that's why the 120A alternators were used as well, and then the 150A ones on the later Thor engines.
    so, can i assume that the voltage numbers in the sticky are not realistic for a 120a alternator? can someone confirm that for a 120a?
    is 13.5-13.7v bad or requires fixing for a 120a alternator?
    98

  9. #7
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    Re: Alternator voltage 120 vs 150 amp?

    If you get more volts at the battery witht eh engine runnig than you do stopped then I for one would not go and buy a new alternator.
    P38A DHSE shiped UK to SA, very rare in SA!

  10. #8
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    Re: Alternator voltage 120 vs 150 amp?

    Quote Originally Posted by waveydavey
    If you get more volts at the battery witht eh engine runnig than you do stopped then I for one would not go and buy a new alternator.
    Could you explain that statement.You should get a lower voltage across the battery with the engine not running as opposed to running.
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  11. #9
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    Re: Alternator voltage 120 vs 150 amp?

    An alternator should always give out about the same voltage, being the voltage at which a lead-acid battery (like those in a car) charges best. This is around 14V. The amperage the alternator can deliver is merely an indication of under what load it can still supply that charging voltage.

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