HI guys thanks for all the replies. I figured it out. I had the one alligator clip screwed to the accessory terminal thereby fouling everthing up in the DC mode. I was using a DC engine not equipped for DCC. However, something was shorting out. Later that night the DC system worked great. I must have moved something that corrected the short. Worst part is that during everything I burnt out something in my NCE SB5 booster and had to send it to NCE (after conferring with them). Hard lesson learned.
wjstix Just a long shot, but it may be that under DCC the engine got the 'message' to move in the split second before the DCC system's circuit breaker kicked in due to the short. When you hooked up the alligator clips with DC, it may be the decoder was OK with the engine going backwards, but when you changed it to go forward, it still had the DCC command from before in it, so took off going forward.
Just a long shot, but it may be that under DCC the engine got the 'message' to move in the split second before the DCC system's circuit breaker kicked in due to the short. When you hooked up the alligator clips with DC, it may be the decoder was OK with the engine going backwards, but when you changed it to go forward, it still had the DCC command from before in it, so took off going forward.
Good guess, but that's not how it works. When a decoder first receives power of any kind, it goes through a checksum process. The first thing it looks for is DC voltage (or no voltage which is assumed to be DC). If it doesn't see signs of DC voltage, it looks for a DCC voltage / signal.
This is where run-aways occur if the option to look for DC voltage is on. The decoder gets the wrong pulse when looking for DC voltage and thinks it found it, when in fact the voltage on the rails is DCC, so the engine takes off at track voltage thinking it's running on DC.
By disabling the DC aspect in CV29, the decoder won't even bother looking for a DC voltage during the checksum process.
Mark.
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What you are describing really isn't possible. There is either something you are missing or not telling us. Unless you have the DC throttle cranked up, there is no possible way it could take off at full speed just by changing the direction switch.
If the throttle (rheostat / potentiometer) were shot, causing full voltage to pass, it would do the same thing in both directions.
If the DC with the alligator clips (which I'm assuming you clipped directly to the tracks) didn't cause a short, then it would appear the short must be with the wiring between the track and your DCC system. In adding to the layout you might have reversed the wiring on a new section or something. Worst case scenario, unhook all the DCC wiring and start reconnecting it one section at a time and test it each time to make sure it works.
You could use a meter and verify the voltage to the rails under both circumstances...DCC and DC. I suspect the decoder you have in the locomotive is much to receptive to DC current, meaning it's in 'dual mode'. This means the decoder is meant to sense which type of power you are supplying the rails, but some decoders don't behave nicely when they get full DCC voltage at one go...which is what happens as you power up the rails with DCC. You have to disable the dual mode, which is done in CV29. Google 'CV29 calculator', and find which values meet your needs. The calculator will give you a final figure, and you programme CV29 with that value. See if that helps.
As for the short, I don't know what to say except you may have a defective circuit in the DCC system and not in the wiring. Otherwise, the DC shouldn't work either.
I have NCE DCC and I'm now getting a short as the SB5 booster is blinking. I disconnected the DCC and put a DC power pack with alligator clips to the track. Ran a DC engine one way using the throttle, however when I reversed direction the engine took off at full speed and did not respond to the throttle. I reversed the clips on the track and it did the same but in the opposite directions. Previously I ran the DCC both ways with no problem. This is a new layout. Most recently I fixed some track and turnouts and hot glued some tunnel portals after which the problem occurred.
Can anyone help. Not very good with electric.