Hello Artronix, I can try and give you a hand with your PLC problems. I am a bit confused with your terminology, but before we get into your problem, here is some general PLC knowledge to get you started.
A PLC is a Programmable Logic Controller. It was developed in 1969 for General Motors. The PLC mimics relays and is configurable/reprogrammable. Before the PLC, when their was a change to a production line, electricians would have to rewire a rats nest of relays and switches.
One single PLC can replace 1,000's of switches and relays, making installations and modifications an easy endeavor. Today the PLC is used as a controller in industrial automation process.
PLC's use whats called "Ladder Logic" as its programming language. Its a simple, graphical interface that was intended to give electricians the ability to modify a program without requiring the need to learn Assembly or Machine code.
To understand ladder logic programming you can think of the Ladder "rails" as power rails, a "rung" will connect power rails together, with "logic" riding the rails.
"Logic" is simple and there are just a few simple symbols you need to remember; Inputs | |, Outputs ( ), and Functions [ ].
This is what a PLC ladder looks like:
Keeping it simple, inputs are either, N.O. (Normally Open), or N.C. (Normally Closed). Just a note, there are a few other types of inputs but NO & NC are simple and used most often. Any symbol that has two horizontal lines is an input | |. If the lines are open the input is N.O., if there is a slash the inputs are N.C.
An output is designated by ( ). an output can be something that is connected to the PLC or it can refer to an Internal relay or input within the PLC.
A function [ ], not pictured, is a box. The function of the box will be something like a counter, compare, math, etc.
If an input is energized, the current will flow to an output, turning the output on, or "changing the state".
So, a PLC does not use Logic Gates. There are equivalent operations to and, or, nand, xor, etc. but it's important you understand a PLC is not a field-programmable gate array (FPGA).
But back to your question,
I am going to assume that by 'alternator' you are not talking about a power generating device. I'm almost positive your intention is to:
Turn on a pump to fill a tank;
Turn off the pump when tank is full;
Turn on another pump to empty the tank;
Stop the pump when empty and repeat;
You have a few parts you would need to set up. A low level sensor, a high level sensor, and two motors/pumps.
Your two sensors are inputs
your two motors are outputs
The trick with this problem is to use interlocks.
Also you need to 'Latch' your pump on and reset the latch when a level sensor is toggled.
I hope this helps, I can write more latter but its late and I must sleep. Good luck and take a look at http://www.thelearningpit.com/lp/logixpro.html