Most e-units swap the connections to the motor armature to reverse the locomotive. The armature and field are wired in series, as is normal for a universal motor. The field winding is usually grounded to the locomotive frame, as is the frame of the e-unit. A switch built into the e-unit connects the e-unit coil between the center-rail pickup and the frame, so that the e-unit can be left in one of its three positions if reversing is not desired.
Most e-units sequence through forward, neutral, reverse, neutral, forward, etc.
E-units are notorious for buzzing.
Here is a link to much more information:
http://pictures.olsenstoy.com/searchcd2y.htm
Bob Nelson
Eberhard, are you under the impression that Lionel direction control is by a high-voltage pulse, like Maerklin uses? The Lionel reversing unit ("e-unit") operates by interrupting the track voltage, not by increasing it. So it should operate at a lower voltage than the motor. It will step every time you put voltage to the track.
Except for a few locomotives which had no room for vertical mounting of the e-unit, Lionel e-units have no springs but depend on gravity to return the armature.
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