Overcharging
car was charging 15.4 volts so that problem fixed tempted to replace Alternator.!
then once i punched it gauge pegged 18 volts
You have a classic situation where the alternator's rotor is getting unregulated power.
First things to check are that the alternator and regulator are wired correctly.
If the second brush terminal, or the green wire connectoed to it touch ground anywhere, then power to rotor is unregulated.
What is this mean?
It means the rotor will be creating a maximum strength electromagnetic field full time.
If everything is wire right and good, then the next is to check for voltage drop between the alternator output and the regulator's control connection. The closest places to measure that will be ballast resistor, then the alternator field, or the bulkhead. Back probe the terminals while the engine is running.
If the alternator is running at 14.2 and the regulator is seeing 13.5 or less, then resistance between them that's contributing to the problem.
Instead of measuring voltage above ground, another way is to measure the drop direct. Put one probe on the output stud and the other probe on the terminal in question. Then you know the drop is in the wiring not the ground connection.
At 15.4 Volts, current is flowing to the battery - but its OVERCHARGING.
At minimum, it will cook off the acid in the battery.
Second, if the battery was low on charge to begin with, its going to draw an enormous load through the wiring. This is a good way to damage all the terminal connections and the fusible link. I don't like to see 20-30 amps for more than a half minute or so, and 10-15 amps on the ammeter for more than a few minutes doesn't thrill me.
Charging Battery with Alternator - Warning