93 512 Tach reading high

The tachometer on this car seems to be reading high, by the time the engine hits rev limit, the tach is reading just over 9,000 rpm. At idle, it checks out as being accurate. It seems as it goes through its sweep, it starts reading higher and higher than what the actual revs are. I had an instrument cluster technician have a look at it and it checks out as being in good working condition. The strange thing we noticed however, was that the voltage signal going to the tach would start out high, and decrease as the revs went up. The technician said he had never in 20 years seen a gauge that had a dropping voltage signal like this.

So essentially I need to know where the tach signal comes from on this car and what the voltage specifications are supposed to be. I no longer have the Ferrari manual nor do I have any wiring diagrams. If anyone knows where I can find any, it would be much appreciated.

Seeing as the car runs individual coil packs, I assume the rev signal comes from one, or both of those. However I don't know if the signal gets altered by another box before it gets to the tach. Thanks in advance!