I went to town earlier after I checked the voltage on C121 and when I came back, All the lights came on dash and it was able to crank and start, it was so weird, So I was able to hookup my Xtool scanner and a code of P1000 came up and that is an issue with pcm or a disconnected battery and I know I don't have a disconnected battery, so could the pcm be going out or bad?

 

When we get a P1000 that means we lost power to the vehicle. It could be a loose or bad connection.

It could be corrosion or loose or bad connection in either the positive OR ground side of the electrical system.

Just measuring voltage at any point in the electrical system is not accurate because if only one strand of wire is making the connection, you will get a 12 volt reading and yet the load gets insufficient power to function. The circuit needs to be checked while under load ( Operating or attempting to operate)
Corrosion can affect circuit operation because it can be affected by something as simple as humidity.

 

ok i will recheck all negative and positive wires and connections and check a load test on it as well

 

ok I checked all connections and cables, positive and negative no corrosion and all are tight , running a load test now with head and dome lights, AC and radio on and running I am getting around 13.65 v output after about an hour of running

 

I believe it's a bad alternator now down to 13.40 V output after about an hour and half

 

What we really need to know is how many amps the alternator is capable of and if the voltage regulator is regulating that CURRENT (AMPS) output.

Some vehicles are good at 13.4 and some others can go as hi as 15 volts and be considered normal. Every vehicle is different and best to go by manufacturers specs..

 

After I replaced alternator and belt it cranked and started for a couple of times then after about 3 or 4 days will not crank or start, no lIghts on dash except oil light, when I turn on switch, could ignition switch be bad?

 

If you connect your voltmeter to the clean battery terminals, what reading do you get ?
Next turn the key on and tell us what reading you get now ? Leave the key on, turn the headlamps on and watch the voltmeter reading. It should go down, how much and how fast can tell us a lot about the condition of the battery.
If the battery is questionable, it will have trouble holding a charge and may not accept a charge. The reading you get at the battery with a voltmeter only tells us the state of charge , but not if the alt is capable of putting out a full charge OR if the battery is capable of accepting the charge.

What is required is an ammeter either connected in series with the cable coming off the alternator OR a clamp on or amp meter you can lay next to the battery positive cable , engine running . With out the equipment to do an accurate diagnosis, you are limited as to just how much useful info you can obtain.
There are all kinds of measuring tools that make the job simple and easy, but most folks just don't have them.

To answer your question, I do not believe the ign sw is the problem.
We need to be sure the battery is good, receiving a good charge from the alt and that battery power is available to all the components and systems.
With out that starting point satisfied, we cannot offer any addl helpful info.