Try this...
- Remove the Hot input from the input to the TIU channel from which the engines get power.
- Power up the TIU via the Aux. Power port.
- Turn up track power to 18 volts.
- Connect the Hot wire to the TIU channel input.
This will put 18 volts on the track at the same time that the TIU issues the watchdog signal. See if things improve.