I played a bit with both engines tonight. I'm running with the ZW set at 18v going through Fixed 1. I laid an engine on its side and connected the red alligator lead to the pickup and the black alligator lead to screw on the frame. I also tried connecting to the other truck block. (No electrical difference.) Both engines respond pretty much the same way: The rear wheels start turning as soon as I scroll up the speed. The front wheels do not move until the speed setting is somewhere between 6 and 10 mph. The problematic unit's front truck starts turning ~6mph, while the "good" unit starts closer to 10mph. Go figure. If I put a finger on a turning rear wheel, the front wheels pickup speed in proportion to how much slower the rear is turning. The more drag on the rear wheels, the faster the front ones go. The rear truck has a fair amount of torque available to the wheels. The front truck has virtually no torque. No stuttering or jerkiness was noted with any of the 4 trucks.
I put the track back into the picture. The problematic SD continues to jerk down the track in forward. I had it up to 12mph with no change. The other SD jerks a very little bit on starting, then smooths out, even at 3mph.
BTW, both units are RK, bought in 2005 with 5v boards.
Chris
LVHR
