Light was added for 85 ROW 16V, as well as for the new 32V cars.

Seems there is a good chance its interference, so I'd assume it.

Disassemble and go through it, piece by piece.

Takes a weekend and a few tools. Prerequisite of ownership, well, usership. Simplest thing to work on, car-wise, on his long laundry list of special high end cars.

 

No this is not true. My belt light came on under the same conditions. It turned out to be an electrical issue with the timing belt circuit. A little crimping fixed it

 

The 85 Euro S is an interference engine. Go figure.

 

Yes - highest factory-rated compression of all the 928's on the 85-86 Euro 4.7L at 10.4:1

The later '84 euros are also the high compression interference motor too - there was a mid-year piston change. a search of rennlist will bring up the engine numbers Porsche documented for the change for the auto and manual engine number series'

From 80 to early '84 the K-jet "Euro" 4.7L 928S are non-interference as long as there isn't significant carbon buildup.

 

The OP doesn't seem to really know what the timing belt is or what problems breaking one can cause based on the brevity of his post.