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I have this spool (image) and did some testing with a multimeter before I disassembled it. It's working range was [-3.3v,0v,3.3v]. When I tested it by just applying the battery after disassembly nothing happens.
Does this work in other ways then just applying the correct voltage?
The interesting thing with coils (inductors) is current, not voltage. It might be that the circuit connected to your coil uses some type of current-control. In the most simple case, this would be a resistor in series with the coil.
If this was the case, you might measure something like 3.3 V on the connected coil, and you would now measure something like the full battery voltage (7.2 V) on the wires leading to the coil when the coil is not connected.
Also, it may be that the internal resistance of the batteries plays a big role.
Can you try and measure the coil's DC resistance? Can you try and measure the current flowing into the coil? Can you look into the things connected between the battery and the coil?
I have no idea what the steering mechanism looks like internally, however... This simply looks like a large electromagnet. The force generated is dependent upon the geometry of the coil, number of turns and the current flowing through it. Given the geometry and turn count have not changed I suspect that the problem lies with your power source. My thought is the AAs simply can't source the current that your Ni-Cd batteries do. Give it a shot with the Ni-Cd pack and I bet you'll get some movement out of it.
