I just installed a new autometer volt meter in my CJ.
When I turn on the switch the meter is reading 18 volts.
When you mean you turn the ignition switch 'ON'...
Is that with the engine running, or NOT running?
If your volt meter jumps to 18 volts when the engine IS NOT running, but the switch is in the 'RUN' position,
Then you have a bad gauge.
If the volts are around 12 when the engine is 'Off' but the switch is in the 'RUN' position,
THEN it jumps up to around 18 volts when you start the engine,
That means the alternator is shot and the voltage regulator isn't doing it's job.
Im also having trouble with either the brain box or the coil burning up I dont know witch one it is.
Which is it?
Either the coil is what needs replaced, the two aren't interchangeable,
And when you say 'Brain Box',
Do you mean the ignition module or the timing computer?
Usually the only way to kill a coil is to leave the ignition switch in the 'Run' position too long without engine running,
Or to NOT have sufficient 'Ground' to the engine heads.
Lack of 'Ground' to the module will make it act strange and eventually burn the module up.
I tested the battery and its reading 12 volts. Any ideas
Start with taking a battery voltage reading after the vehicle sits a couple of days.
Make sure you note down EXACTLY what the voltage was,
Then start the engine, let it run 10 minutes or so,
Then take the voltage at the battery again.
If your battery voltage starts below 12.2 volts, you have a problem with the battery.
If the voltage gets above 14.7 volts, you have a problem with the alternator/voltage regulator.
With the engine turned 'Off',
Take a RESISTANCE reading between the alternator case and the battery negative terminal.
If you show any resistance more than an Ohm or two,
You need to install a dedicated ground wire between battery negative terminal and alternator case.
Start the engine, and do the same resistance test between alternator and negative battery terminal.
If your resistance goes up, then you need a larger, more direct connection between alternator case and battery terminal...