Ah, I understand. The decoder needs that 8V in order to "wake up" the electronics, after this it can then determine if it's running on DC or DCC and react accordingly. It's quite normal behaviour. LGB advise not to consist MTS and DC locos on the same DC power due to this difference in running characteristics.
The loco's DCC decoder is going to be either part of the main circuit board or may possibly be a separate decoder plugged into the main board. If it's a plug-in then it can be removed and some switches changed to make it analogue DC only. If the decoder is integrated onto the main board, you've got no choice but to remove the board and rewire - seems a bit of a shame to do that as it's an expensive bit of kit.
Here's a link to the service diagram of 23194.
http://www.champex-linden.de/lgb_pr...CEE8B6FB61485256EE000700BB2/$FILE/23194-1.pdf