Why won't the compressor engage on my out door a/c condenser unit, when 122 volts are present at the compressor and there is a full charge on the a/c system.

SOURCE: Copleland scroll compressor failure

Make sure ALL moisture and noncondensables are evacuated while the system is down.I would have the line flushed to remove all remnants of burnout.Actually with the age of the unit and amount that needs repaired I would change the unit out to a 410a system and start over.

SOURCE: COMPRESSOR NOT RUNNING, HOT

The high side/low side pressure equalization you describe is normal for a compressor sitting idle.

If you have an ohm meter you can check the compressor windings for opens and shorts. If your compressor is 3 phase as you state (this would be very unusual for a residential unit) you should have continuity across all 3 phases and no continuity to ground. Take the compressor out of the circuit before you check.

If the compressor does have crankcase heat, it should have been left on for at least 24 hours before starting the unit. Another cause for failure could be overcharging which could have resulted in refrigerant slugging back to the compressor.

Does the unit have any low or high pressure control safeties? If so, check them too - some have resets. You said the condenser fan ran so it's not likely the breaker blew, but check anyway and also check to make sure you have available power on all 3 phases.

SOURCE: p8 eroor code

P8 error is abnormal pipe temperature indoor. If you are running it on heating P8 error will appear because the disc pressure is too high. It could be overcharged.

SOURCE: compressor not turning on

is the compressor actually turning over ?? if not check the contactor, or capacitor is actually ok...

if it does turn over, are the stop valves open ??

SOURCE: Condenser fan turns slowly and then compressor shuts down

clean bushing and allign them very well.