[Fulltiming]

Marius, as Bill pointed out, the center position is non-merged. The batteries should be merged in either the up position or the down position. If you have a really low voltage on one of your banks of batteries, and that is the bank that the switch is using to power the relay, they would not merge. Newell put in the three position switch to eliminate the potential of not having enough voltage to engage the relay. If the voltage of both banks is enough to engage the relay, it doesn't matter if you put it in the up or the down position BUT if one bank of batteries is really down and you ONLY had a two position (Off-On) merge switch, you might not be able to energize the relay to merge the two banks of batteries.

You should find that under most circumstances the up position merges the batteries just as the down position does. Are you saying that the up position does NOT merge the batteries? If so you either have a bad switch, a bad connection, a short in the wiring (or miss wired), or one of your battery banks is seriously depleted.