OK first off it's been so long since I worked on a "9" I can't remember what they look like!
OK, OK maybe not, but it's been a while! And I never worked on a NJT "9" that had a DDEC.
So far I ain't bat'n to good am I?
Well I do have a little insight or experience in this area.
Our first Setra '89 model S215 W/8V92 DDEC in it had similar issues as what your having prior to us purchasing it.
We listened to the head "tech" at the company that had it and were skeptical of a coolant sensor causing the shut downs. But he said "can't be that we already tried that!"
Well I just happened to have one laying on the desk that I needed to send back to Setra as they had sent the wrong one to me for a customers bus. (the customers bus had a 60 Series in it!)
So we took a gamble and took the sending unit (little black box like Jr describes, not the actual sensor!) with us when we went to look at it on a Sat. and first thing I noticed was that the sensor it had was the same as the one I'd put on the 60 Series. So 1st thing I did was asked if we could "test drive it." They said sure, but you ain't gonna get far before it shuts down and you'll have to wait until it restarts to bring it back.
I told them I'd take that chance. So as soon as we rolled out we found a large church drive to pull off at and unhooked the sender and hooked up the one I'd brought and left it hanging. We drove it for 30 miles or so with no issues. Went back and stopped and took our sender off and hooked the original back up. Just as I pulled into their lot it shut down.
They came out and met us as we got off the bus and told us "see that's what it does to us, if ya buy it you'll have to have it towed home.
Well long story short after the deal was made I switched it and drove it home 138 miles with no issues there. (others yes, but not there!)
OK now after we had it a little while I noticed the coolant low it the sight glass on pre-trip so I added some. While loading a passenger or someone told me yer bus is leaking anti-freeze!
So I went and checked thinking I had a "leak" but it was just coming out the overflow.
So next morning it was down again and I added some again. A little while later it was running out the overflow.
So after 3 or 4 times of this I decided to go ahead and start it and see what happened before adding any. After checking the oil, coolant, belts etc. I started it and finished doing my pre-trip just before taking off I took one last look at the coolant level and there it was right where it was supposed to be.
So I always waited to add after that and seldom had too. And anytime dad or another driver would drive it the first thing they would do is add coolant, and then complain it was "leaking" later!
(yeah they never listen to me!
And we've have a newer Setra ('95) with a 60 Series that does the exact same thing!
So if it's not actually showing bubbles it may not really be a problem.
My guess is shut down is a faulty sensor or sender!