EMD SD7 Troubleshooting

[Don Stafford]

A friend gave me a Life-Like EMD SD7 to which a decoder had been unsuccessfully added.  My sole DCC conversion experience is limited to TCS K1D4-NC drop-in in a Kato GG1, so I could use some help at getting started with troubleshooting.

The installed decoder in the SD7 is programmed to address 03 - I can turn the lights on and off - and the lights, (yes, it has also had a rear light added), indicate forward and reverse - almost correctly.  Almost, because the thing wont run, and because on "reverse", the light on the long nose forward (F) end comes on and on "forward", the light on the short nose cab in flashes on and off at a steady, constant rate.

I'd appreciate any assistance anyone could offer to get me started.

[seadowns]

Hi!

Find out what decoder brand you have before you via CV8.
First of all I would reset that decoder to factory values and then start it at standard adr. 3.
If it doesn't run check the installation and the motor.
light direction can be changed via CV33=1 and 34=2 which is forward and rear light. You can switch the numbers to reverse the lighting.

Regards
Mathi

[Don Stafford]

Thanks Mathi,

Think I have the decoder identified as a TCS M1.  Resetting CV8 to factory defaults didn't change anything.  Lights still respond to forward and reverse and off and on with function key "0", but engine will not respond on either address 3 or 0.  Wiring seems visually to be correct as far as I can see.

Can I apply 12 v. to the wires connecting the two motor brushes to the points where they are soldered to the decoder without risking damaging it in order to test the motor?

[skipgear]

NOOO!!!!

Applying voltage to the motor with the decoder still connected will let the magic smoke out.

Disconnect the motor before testing it.

[seadowns]

Well,

the M1 manual says you have to write the number 2 into CV 8 to reset the decoder to factoey standards.

I would very carefully put power direct to the motor contacts while the decoder is attached. I did this very often with no consequences but I used not more than 3-4 Volts DC and this low voltage obviously does not harm the decoder. I did it just some split seconds only to watch if the motor starts turning. If you use more power.........................I don't know, probably you will toast it, as skipgear stated.

Regards
Mathi

[richg]

I belong to a bunch of train forums and some have put 12 vdc directly to the motor leads with the decoder connected. Smoked the decoder in an instant. Lower voltage, you are still taking a chance.
I have used a voltmeter to check and see if the decoder is sending any power to the motor. That always works.

I have some meter test leads made with EX Hook clips.

http://catalog.e-z-hook.com/viewitems/test-leads/e-z-micro-hook-test-leads-2?

If you want to work at what I call the component stage, get the proper test apparatus. Makes troubleshooting much easier. Takes out the guess work.

A multimeter can be had for about $5.00 each from Harbor Freight. I have three of the meters for  few years.

Rich

[Don Stafford]

Referring to Tony, Rogers and Rich's responses:
1. Can I safely apply test voltage to the motor if I first unsolder the top brush wire, which is accessible, then apply voltage to the brush and the point on the decoder where the wire from the bottom motor contact comes up and attaches to the decoder?
2. Can anyone direct me to something that either of you 3 guys or anyone else has written on how to test decoders, both in- and out-of-circuit?

I really want to learn how to do this, and though I've heard and read about how important it is to go back and test between each step of decoder hardwire installation, but I'm having an awfully hard time finding actual "how to" information.  Have any of you or other forum members written such documentation?  It would be a gold mine!

[Don Stafford]

...also,
3. Any idea what's going on with the flashing light in one direction and solid on in the opposite?

[skipgear]

Don
As long as nothing is shorting in the frame, yes, removing one decoder wire from the motor will break the circuit between the motor and decoder and will allow you to test the motor.

To test them outside of a loco takes a test rig. I have an old motor and a couple LEDs wired to board with screw terminals representing the pins on an NMRA 8 pin plug. I needed it to test and program Digitrax sound decoders because they won't program without seeing a load from the motor and lighting for some reason.

The flashing light could very well be the way it is programmed. Try the CV8=2 reset mentioned above. Someone may have had the headlight programmed as a beacon in reverse.

[seadowns]

Hi Rich!

This is the very difference! You said they've 12V DC directly to the motor and fried the decoder.
ok, I would not recommend this to an unexperienced tinkerer, but I said not more than 3-4 volts. In many cases is 1-2V enough to check if the motor starts turning, most of them don't need more power.
I would NEVER use full power and I did it less than a split second just to see its working
I did this with DZ, Uhlenbrock, Lenz, Kuehn and D&H-decoders without any troubles but if you want to stay on the safer side unplug the decoder before doing so.
If a motor does not turn with app. 3V DC there must be something wrong with it and you have check it anyway.

Regards
Mathi