How do I set up a 12 station Rain Bird irrigation computer? It was done partially by a supplier that is now not contactable...

only 2 possibilities.
the solenoid mounted on top valve is defective
or there is a short in the wiring.

Check station module lock. It should be set.to the locked position.

I don't feel you can repair system by yourself. Excuse me if you can; so do it.
You should take it to an authorized Rain Bird repair service. Look for it on some listing they might have given you when you bought your stuff or contact them from here:

http://www.rainbird.com/corporate/contact.htm

Which brand / model sprinkler control do you have?

Check iside cover for rainbird 1-800 number and call them. Make sure you have the model number. They maybe able to email the directions or send you the manual. Additionally check youtube sometimes the go step by step on your model.

Look at the valve wiring to see what color each valve has 1 of the 2 wires connected to it. This is your common connect the same one to common at timer. Start zone 1 and touch each of the remaining wires to the pin to start a zone. Then you can connect them in any order you want in the pins.

Okay, then you answered your own question. I like it! However, did you test the wire to see if it will start a different valve and vise versa?

You should be able to put two valves on one station. If it is too much for the timer either a fuse or a "virtual fuse" on the circuit will fail. Just turn the unit OFF, separate the two valves and turn unit back ON to reset "virtual fuse".
I am an irrigation contractor and we have many timers with doubled zones.

Gary

Try contacting Rain Bird directly here, to request a copy of the manual via mail:

Rain Bird Online Store
6991 East Southpoint
Tuscon, AZ 85756

Online Store Phone Orders - (877) 727-8772
Available 7am - 3:30pm PST

Tech Support - (800) Rain Bird
Fax (800) 843-4162