>This is a good point except, as I've said before I have built a few
>directional lighting sets using Mark Rollins' diagrams. They use the same
>diode matrix as in the P2K units I've seen, except I have a light for both
>foward and reverse directions. After installing these in Athearn units I
>experienced the same effect of the rear unit starting before the leading
>There is no loss of 1.2V as you descrided above so what could be causing
OK. Mechanical points first.
I've had all manner of locomotives, steam and diesel that simply ran better
in one direction than the other. It /can/ be a matter of the worm shaft
binding as the slack runs out in the worm/spur interface, and if so often
thrust bearings can help. I had one which drove me nuts until I found that
the motor itself was the culprit- it ran faster in one direction. ???? I
did have one where it didn't even run at all in one direction, but that was
simple. It had picked up a stray spike which was clinging to the magnet,
ratcheting on the armature. One way, no problme. Other way, instant lock.
If thrust washers are needed, NWSL has them in many sizes and incude both
SAE and Metric sizes. They seem to be bearing bronze and are precision
As long as we're on the topic of diodes, though, let's consider the most
basic approach. Take a standard bulb, place one end on the most convinient
ground, place the other lead on one end of a diode, and the other end of the
diode to the motor's "hot" brush. The bulb will light when the motor is
running in one direction, not the other. If for your purposes, the bulb
lights in the wrong direction, just reverse the diode. But the bulb's
brightness will track the motor's speed- it won't be a constant brightness.
So, next we take into consideration one of intrinsic characteristics of a
semi-conductor junction: It is not a dead short, but rather it is a very
high resistance in one direction of electron flow, and a low but finite
resistance to opposite electron flow. Therefore, when a current is passed
through the diode in what is called the "forward" direction, there is a
small voltage developed across the junction. The exact voltage depends on
the nature of the semiconductor, but for most common silicon diodes it runs
about .6 volts. This voltage remains constant across a fairly wide ranges
of applied voltage to the diode as a whole.
So, if you place that same diode between the engine frame, and the other end
to the brush that used to be connected to ground, you can now place a small
bulb across the diode. The motor current will pass through the diode, and
also through the bulb, lighting it. As soon as you try to reverse the
motor, the voltage drop across the diode goes to the full applied track
voltage, and your little, low voltage bulb will pop. NOt terribly useful,
and the small bulbs are normally rated at 1.2- 1.5 volts while you're only
providing them with .6 volts. They'll be pretty dim.
NOw is where the wiring gets a bit complex. First, take a pair of diodes.
Wire them with the little band on one end attached to the diode with the
band away from the first. When you pass current through this pair, in the
forward direction the voltage drop is a nice, useful 1.2 volts. But we
still have the one-way only problem. So, take another pair of diodes wired
the same way as the first, but then connect them in reverse to the first
pair. At one end, you have leads going to /two/ diodes, one to the band,
the other the end away from the band. Each of the first diodes connects to
the other in each pair as above, then the leads at the other ends come
together again and connect to the opposite side of the power distribution
By playing with the makeup of these diodes, leaving one off or adding more
you can get headlights that brighten in one direction and go off in the
other, or just go dim in the other. But for a simple, constant brightness
in both directions setup, you can use just a simple plastic encapsulated
diode "bridge" from Rat Shack or whomever. Run one of the leads intended to
connect to AC to the ground, the other AC lead to the motor or pickup
bracket. Connect the - and + wires together, and connect your light
directly to the two AC leads. Then find a good place to put the plastic
This basic system has limitations- it's quick, and cheap, but it's also
limited. First, if you want to use it to light cabooses, passenger cars
etc, you need to place something for a "ballast", a current limiter in
series with the diode/bulb setup. If you just use the diodes, and apply the
full 12-18 volts, the current through the diodes will heat them and they'll
self destruct. Luckily, the most common failure mode of the diodes is to
short, so the bulbs won't also be fried.
The most common ballast is a small light bulb, rated for the full voltage
and with a high enough current rating that it will pass enough current for
the target bulbs to light as well. In many cases, a common auto dash light
bulb will work.
Of course, now you have a big bulb, generating heat and light you need to
get rid of, but no one said it was all easy.
Finally, if you are using one of the many new, high efficiency motors common
in the hobby, the motor won't let you use this series circuit. All the
current required to light the bulb has to pass though the motor, and the
motor might not draw enough current, even at stall, to light them. So for
these, you need the ballast, or some sort of "active" electronic package.
Let's get back to the original topic. LL engines are a maze of hardware
with wires going every way. But see if you can find the wires that go to
the truck pickups, and to the motor brushes.
Temporarily disconnect all of them. Use clip leads or wet noodles,
whatever, to connect one motor brush directly to one truck pickup. The do
the same for the other motor lead and a truck pickup on the other side of
Test it. If it still runs faster in one direction than the other, it's a
mechanical problem. Note: For this test to mean anything, you should really
make sure the electronics board is /not/ in the circuit. One end can still
be connected, but the bulbs should not light, etc.
I've got four LL E8 units to trouble shoot. None of them run the same
speed, and including the forwards and reverse speeds, nothing runs together.
They're pretty far down on my priority list, though.
Fred Dabney, watching the action from BNSF MP 1112, El Paso sub.