OK, '90 Cityliner I presume we are talking DDEC-II and Allison ATEC-1, maybe a 748?
On the Neoplan's, you will get a DNS light if the rear start switches are set to the "rear" positions.
You can also get a DNS light due to any number of ATEC or tranny problems. Check to make sure you have a working "Check Transmission" light on the dash -- it should come on momentarily, then go out a few seconds after you turn the key on. If you don't get a CT light, find the light and check the bulb.
With a working bulb, any codes in the tranny forcing a DNS condition should also give you a CT light. In which case, you can ground the diagnostic pin, and read the codes from the sequence of light flashes.
If you have a working CT light, but it does not stay on when you start the engine, then the problem is likely elsewhere -- but try to read out the codes, just in case. You may have to check all the signals on the Maximum Feature Throttle Interface, a little black box that connects the DDEC and the ATEC together.
Let me know how the light diagnostic goes, and I'll see if I can give you any more help. Your Cityliner electrical system is very close to my Spaceliner.