Question about 2005 Datsun 720

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Battery charges when idling but not at speed. 81 datsun

New battery, new alternator. Battery charges when idling but loses charge at speed.

Could it be the regulator?

81 datsun truck (720)

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  • korpdeath77 Mar 04, 2009

    thanks muchly chief,


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This sounds like the altenator belt is loose. When the altenator is under a load the magnetic field causes the altenator to require more fource to rotate. At higher engine rpms the belt can be slipping on the altenator pulley. There is one other possibility I can think of and that would be that the voltage regulator is faulty and and causing an overvoltage at higher engine speeds and therfore opening up the field circuit to prevent the altenator from over charging the battery this is a safety function of the regulator. I would check the belt first.

Posted on Mar 04, 2009

  • larry bud Mar 05, 2009

    I would suspect the regulator. if you have access to a volt meter connect it to the battery and rev the engine up and see what the voltage reads. a typical altenator should put out about 14.2 volts. If the voltage exceeds above this then drops off the voltage regulator is detecting an overvoltage and will open the charging circuit to prevent the battery from over charging giving it the potential to explode due to the build up of gasses. The trip should occur around 15 volts or so if this is the problem. Just keep in mind the faster the altenator turns the more voltage it is capable of produceing and this is were the voltage regulator comes into play to maintain a constant charging rate.


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