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Voltage Drops at Higher Speeds:

If I drive my truck under 35 MPH, it's in the 13.8/14V range. If I go over 40 MPH it's in the 11.8/12v mark and go down further.. to 10v.

Drive in first with the engine RPMs at the same level as at 40+ MPH. Drops? Yes, then it is engine speed related. (Almost has to be anyway, but...)

Possible problems (in my order of checking):

  1. Belt(s) loose and slipping.

  2. Pulley slipping

  3. Bad brushes.

  4. Bad regulator.

  5. Other internal faults in the alternator.

One might ask how could the regulator be bad if it charges OK at idle... It may fail when asked to control voltage when speed is increased--this is not common but can happen.


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