The mechanic is right that the voltage should be higher than 13.0 at the
battery with the engine running. The real question first is what does it
read at the alternator? With the engine warmed up, rev it for a few
seconds and then let it idle while you measure. While idling at 1000 the
alternator should put out no less than 13.5V. That is the lowest
acceptable at the alternator and min 13.8-13.9 is more typical. If the
reading is too low at the alternator then the alternator has a problem
and replacing just the voltage regulator module in it may fix it.

 If voltage is ok at the alternator then see what it reads at the
battery, making sure everything is turned off except the engine still
running. There should be only a slightly lower voltage at the battery
than at the alternator.

Mark

Woody Halsey wrote:
> Dear List,
>
> Can anyone tell me what voltage readings I should be getting from my
> battery:
>
> 1)     with engine off
> 2)     with engine idling
> 3)     with engine at high RPM (if I can figure out how to test that)
>
> My mechanic states that since the readings do not go above 13V when the
> engine is running, the alternator must be malfunctioning.  Is that true?
>
> P.S.  My paint/rust/insulation/interior detailing projects are finally
> completed. Of course I will never take it out of the garage again for
> fear of getting wet, but she sure looks nice. My surgeon wife likens her
> to an elderly lady with a face job: beautiful on the outside, but just
> waiting for a key organ to cease functioning.
>
> Thanks,
>
> Woody
>
>
>
>