# Need Help with a DCC Loco



## MtRR75 (Nov 27, 2013)

I am a DC guy. I understand the basics of how DCC operates, but I have no interest in running DCC.

However, I picked up a used Mantua 2-8-2 which turned out to have been converted to DCC. (I didn't read the fine print carefully.) Obviously it does not run on my DC layout. I opened it up and found a Digitrax DH123 inside the tender. The decoder is shrink-wrapped in plastic, with only part of the connector showing. The connector has 9 wires going into it. Those 9 wires go into a massive tangle, from which about 4 wires emerge and go to the loco.

How can I modify this loco so that it runs on my DC layout? Is there a way to remove the decoder without cutting all of those wires? If I remove the decoder, will the loco run on DC?


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## JerryH (Nov 18, 2012)

Try it on DC. They can be operated on DC if no one disabled that CV. If analog operation (DC) has been disabled you will have to get someone with a DCC system to enable it. A Digitrax DHDP Dummy Plug can be used to replace the decoder and leave the wiring intact for DC usage as well. It comes as a five pack.


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## Cycleops (Dec 6, 2014)

The DH123 is one of the older Digitrax decoders. I've had a look at the instructions on their site and no mention is made of self detect DC running. It sounds like you've tried to run it as is on DC and no go. From your description it looks like it may be hard wired. The four wires would be two for the motor and two for the wheel pick ups. Alternatively the wires may terminate in a plug which plugs into a board in the engine. You need check that out.


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## MtRR75 (Nov 27, 2013)

Thanks for the replies.

I have ordered the plug to replace the DCC card.

I sorted out all the wires in the tender. Three are cut off (maybe the sound wires? There is no sound card in this loco). One is attached to the drawbar screw of the tender. Another is attached to the drawbar screw of the loco. The drawbar is non-conducting. The other four go to the loco. I have not taken it apart, yet, but I can see that two of them are soldered to the motor, so it does not appear that there is a circuit board on the engine.


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## JerryH (Nov 18, 2012)

From Digitrax.

8.0 Analog Operation of Digitrax Decoders
Analog operation occurs when Digitrax decoders are operated on DC or conventionally
controlled layouts. Automatic analog mode conversion is a feature
of most Digitrax decoders, which lets you run your decoder equipped loco on a
DC layout without having to re-program the decoder. There are a few things
you need to know about analog operation:
· Digitrax decoder equipped locomotives will not move until the voltage
is above 5 to 6 Volts DC. When operating decoder equipped locos on
DC power, increase the voltage quickly from zero to five volts for
best operation.
· Some decoder equipped locos may not operate smoothly on conventional
"pulse power." The decoders should be driven from a quality
smooth DC power supply when on a conventional layout. Some DC
63
throttles/power supplies with proprietary control systems, such as "tracking
control," exhibit pulse power characteristics and may give unpredictable
operation.
· Decoders that use the Digitrax 9 Pin Decoder Interface can be easily
removed from the loco and replaced with the DHDP Dummy Plug
(available separately). This is very useful when you want to run your
locomotives on conventional layouts without sacrificing low speed
performance. It also allows you to run your engine on layouts that
use pulse power. And, when you are ready to run DCC again, just
remove the dummy plug & re-install the decoder and you're ready to
go!


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## DonR (Oct 18, 2012)

A DC loco wiring is very simple, so you should be able to trace
that and thus know what wires of the decoder do what before
you cut. Then you'll cut all of them.

Here are the color codes from the decoder to make your
trace easier:

Red Power From right rail (loco on the track)
Black Power From left rail
Orange To motor left side
Grey To motor right side
Blue Common for headlight (positive polarity)
white To headlight. 
Yellow to rear light
green and violet not used likely.

First you want to trace the power pickups from the track. Some
use the loco wheels on one side, and the tender wheels for the
other rail. Your description makes me think that is the situation
with your loco.

Then with the shell off, you should be able to trace the motor
wires and thus see the decoder wires connected to them to cut.
Make note which motor wire is connected to the Red decoder
wire. This motor wire should be connect to the Right rail power
pickup wire. (This is important so loco will go forward when
the reverse switch is so set).

Likewise you should see the wires to the headlight. If it is 
incandescent it likely is 12 V. If LED leave the resistor
connected, but you may need a diode to limit it to forward
lighted only.

You should also be able to detect what wires in the
tender come from the wheels, if any.

After cutting the decoder wires and removing it
Connect the wires from the left rail of the track to the
wire from the left side of the motor. Connect the wire
from the right side of the track to the wire from the right
side of the motor. You may find a diode in one wire from
the headlight. That is to light it only when the polarity
is set for forward movement. You can test to see which
of the wires from the wheel pick up to use by placing the
loco on the track, power set to forward, loco trying to
move forward. Touch the diode wire to one of the two
motor wires. If it lights, connect to that.

After removing the decoder, you will likely need at least 
one wire from the tender
into the loco to provide the power from one rail. The power
from the other rail will come from the loco wheels.

If the tender has a 'back up' light you would need an
additional wire from the loco.

Let us know how it goes.

Don


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## MtRR75 (Nov 27, 2013)

Thanks for all of your suggestions. I ordered and installed the Digitrax dummy plug in place of the Digitrax decoder. Now it runs fine on my DC layout.


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