Just put in my new {reman} O'Reilly alternator took 2 month old battery out of other truck fired it up and read voltage across battery of 14.2 volts drove around for 30 min and began to notice my voltage start to drop went back to house turned on headlights and turned up fan and read voltage of 13.5 volts across battery turned off lights and fan and was reading 13.6 volts ..have i ajust put in a def alternator or have i got another problem ..this was happening with other alternator only lower readings...help me im falling...

 

what rpms are you checking it ?

 

idle speed...

 

You just drove 30 min with a new battery and alternator.
The battery should be fully charged and the alternator right on top of it so the voltage may not go down or up.
What were you expecting to see?
The voltage should only rise when the battery calls for charge.
When it's up, the load from the lights and fan can be supplied by the alternator under electronic regulation that may not always show as increased voltage.
At idle the alternator may not be spinning fast enough to raise the voltage in response to the load.

 

what would be causing the gauge to drop down to the low end of normal scale after driving for 30 mins i would think that it would remain constant but it drops almost to lowest point of normal scale. and when night driving my lights are slightly dimmed just by turning my fan on

 

From personal experience, O'Reilly has become my least favorite place for such parts. I've gone through as many as 3 alternators in a row before I got one that would even charge. Did you have them bench test it before you left the store? Go back to them or Wally World and have them connect their electrical system tester.... there is no charge. If you get any faults and have to pull the alternator, when you exchange it, have them bench test it before you leave. Next time try Car Quest.

 

The output voltage of the alternator is proportional to the alternator regulator temperature. So when you start the vehicle up in the summer, you will have approx 14.2 V and this will drop while the alternator heats up underhood. 13.5 Volts out of a hot alternator is normal. These keeps the alternator from overcharging the battery. Is the battery warning light on in your dash? If this light is on, then you have a problem with the alternator not outputing.

 

No light is on ..gauge is reading different than it used to ...i'll just keep an eye on things ...i thank everyone for their input ..this is really a great site for info