When the truck is in park, and I step on the gas the engine revs up, however when in comes down the battery voltage indicator goes from 14 volts down to 10. The lights dim and the heater blower gets weak, the trucks wants to shut off momentarily. The voltage then comes back up to 14. I just change the battery and the alternator, what could be cause this problem?

Usually indicates low chassis voltage. Check ground connection to chassis.

amps or volts
11 to 12 amps indicates nothing but if it is volts then it indicates that the alternator is not charging the battery
Battery voltage is 12 -13.5 volts and when charging is 14.5-14.8 volts dc
the rest of your statement indicates that the alternator or voltage regulator is faulty and you should have the charging system checked by an accredited auto electrician

replace the voltage regulator
amp gauges read amps or current flow
volte gauges read volts so if the volt gauge reads 6 with the engine running indicates that the regulator is not coming into service until the battery voltage drops and that triggers the regulator
an accredited auto electrician will be able to test the complete charging system and advise on the fault

I would get a second opinion on your alternator. Does your battery light come on while your driving? That can be an indicator of an alternator going bad. Also, there's an easy way to check the alternator yourself if you have access to a multi-meter. Turn the multi-meter on to DC volts and start the car. Put the positive lead on the positive battery terminal and the negative lead on the negative battery cable. While the car is idling, the meter should read 12.5 - 13.5 volts. Have someone rev the engine. With the increased RPM's the meter should read over 14 volts. If the voltage goes down when you rev the engine, your alternator is bad.

Good Luck!
~Brandon

This could be the alternator. Bring your vehicle to a shop and have them test both the battery and the alternator. I have seen brand new batteries come off the shelf and fail so it could be either

With a multimeter check the voltage at battery terminals. Should be more than 12.6Volts when engine off, and at least 14 volts with engine on. If the battery voltage is less than 12.6 volts then you need a new battery. If the voltage is less than 14 Volts with running engine, then the alternator is not working properly. A stop at AutoZone might help. They scan codes, and check alternators for free.

Well even a new battery can be drained down is you have something on all the time, in about an hour.

So start with checking for a drain.
Remove the battery cables in order to clean and grease them.
Then only put one on.
With everything off if you jump the gap of the cable that is off to the battery, with a test light, is should not light up. If it does, then you have a constant drain that will kill a batter over night.
You have to check for things like interior light, and disconnect everything, one at a time, to pin it down.

If you don't have a drain, then the next thing is to check continuity.
When you turn the ignition on, first the dash indicators come on.
The battery should read 12.5 volts.
Then you try to crank it.
The voltage should drop to 10 while cranking, and come back up after.
If it does not drop to 10, the starter is blocked and not drawing.
It could be the neutral safety switch, so try in neutral instead of park.
It could also be the ignition switch.
Use a test light to see of the small wire plugged into the starter solenoid (the small cylinder on top of the starter), get power when keyed to starter?
If it does not, then ignition switch or neutral safety switch.
If yes, then starter solenoid is probably bad.
But try turning the engine over at the alternator by hand or wrench, and try again. Could be binding between starter and drive plate.

Once you get it started, then rev the engine up and verify alternator goes over 14 volts, to recharge battery.
Otherwise battery will go dead in a day.

These lights will come on if your alternator isn't charging fully. With a multi meter test your voltage at the battery with the engine running. The voltage should be above 14 volts. If it is below 14 you will need to replace the alternator.

Check that the alternator and charging circuit is actually working. You can do this with a voltmeter across the battery terminals. Check the value with the ignition off - you should see 12.1 and 12.5 volts. Then start the engine and leave it on idle - you should see between 13.8 to 14.2 volts. Watch the voltmeter as the gas is pressed down. With the engine revving around 2000 rpm the voltmeter value should increase up to something like 14 to 14.2 V but not increase any further with an increase of engine revs. If you see voltages of 14.5V or above then the voltage regulator needs replacing.

Note that if you constantly see voltages of 12V or less with the engine switched off then the battery should be replaced as it will not hold charge and gradually drain down as you are experiencing to a point where it will not turn the car engine.

If you don’t see any increase in battery voltage, or the battery voltage falls, when the engine is switched on then either the alternator or the voltage regulator need to be replaced.

Check that the alternator and charging circuit is actually working. You can do this with a voltmeter across the battery terminals. Check the value with the ignition off - you should see 12.1 and 12.5 volts. Then start the engine and leave it on idle - you should see between 13.8 to 14.2 volts. Watch the voltmeter as the gas is pressed down. With the engine revving around 2000 rpm the voltmeter value should increase up to something like 14 to 14.2 V but not increase any further with an increase of engine revs. If you see voltages of 14.5V or above then the voltage regulator needs replacing.

Note that if you constantly see voltages of 12V or less with the engine switched off then the battery should be replaced as it will not hold charge and gradually drain down as you are experiencing to a point where it will not turn the car engine.

If you don’t see any increase in battery voltage, or the battery voltage falls, when the engine is switched on then either the alternator or the voltage regulator need to be replaced.