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MTH 20-3315-1  4-6-4 Royal Hudson  -  3 Rail - Proto 2.

I have no marker lights.  I have the shell off and traced the wires back to the small board and all looks to be fine.  No damage to wires and connectors were tight.  I removed and re-seated the connectors to the board.  I've done both factory and feature resets.  I have headlight, number board and tender reverse lights.

No joy.

Any help much appreciated.


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The Mux usually takes ground from the boiler shell.  That shell needs to be connected to the chassis to complete the ground.  Take a jumper and connect chassis ground to the boiler shell.  This way when you put power to the engines rollers and ground, the boiler shell sitting away from the chassis will see ground.  The yellow plug should be the markers.

If you want to put current to the markers to test them, use a 9V battery with a 1K resistor in line on the red wire. 

It is very common when the LEDs are put into the marker lamp that a wire will come off one lead of an LED. 


The photo above is a harness I have been using for years to test the LED markers in a locomotive.  You can see the 1K resistor in the shrink wrap next to the lower left of the battery.


This photo shows the test result of the package connected.   On the posters locomotive, I would want to confirm if the LEDs are working or not working.  The LEDs are the end of the food chain in this situation.  If he gets no action from the LEDs, I would unplug what I had in photo number 1 and plug those LEDs into the Mux.  If they light, the issue is in the LED marker harness.  He can troubleshoot for a broken wire (probable cause) or replace the LED harness.



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Last edited by Marty Fitzhenry

Thanks Marty

All I have on hand is a 100 ohm resistor as pictured.

I fully agree that the LEDs need testing.  Is there another way to test them without the resistor for now until I can proceed with building what you have?  I have DC on the work bench.  

Would the testing device you pictured be good for all LED testing or only for MTH?  Layman here


Thanks for the help





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Last edited by Soo Line

The markers are series wired for PS-2 so if one fails, they both go out.  Your could move the markers to the rear as a test.

As far as mux, did you try a feature reset?  Turn smoke on and off, turn cab light on and off. 

Mux board is not grounded to the shell.  As long as harness is connected to the 10 pin on engine chassis it is wired into the circuit.  G

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