DCS and TMCC/Legacy work independently of each other. So if you have a Lionel command base, and you are running through a Fixed Voltage TIU channel, it will respond to the Lionel remote, regardless of what the DCS TIU is doing. The only exception to that rule is if a fuse is blown in the TIU. Then nothing works.
Presuming you have everything DCS configured properly, one or more of your signal generators could be bad. Or, depending on exactly how the TIU was constructed there could be a TVS or more that have shorted, driving the signal output down. There have been several highly technical threads about this issue and possible fixes on this forum. If you want to delve into some heavy duty electronics theory and practical application thereof, I invite you to read this thread
At this point, I will defer to Gunrunnerjohn or GGG. Either of them are highly competent and capable of repairing a Rev L TIU. BTW, Rev L TIUs are definitely worth repairing!
Chris
LVHR
