Is there a sensor on the tach related to transmission shifting timing?

1991 190e 2.6

Bought as a project car, it's turned into something more than that... lol

Owned it 7 months. Tach and temp sensor display have never worked.

Snooping around found blown fuse that controls both, replaced it, fuse immediately blew again. Forgot it, moved on to other issues. Weeks later, tried again, and somehow ended up having the tach work for a mile or two, so I knew the tach and temp display were functional at least.

Today, while adding oil, my temper flared and I started messing with this again.

Blew another fuse, looked more closely and found this fuse also controls the water nozzle heaters... traced the wiring, found expose connector parts touching hood. Taped them. New fuse. Works great.

So, there were other issues which leads me to post this.

My tranny has always had to be forced to downshift on uphill grades (deep pressure on acclerator or manually downshifting), and, it would often die during downshifting while coming to a stop. A stall.

After the tach and temp display began working, the transmission is shifting like a dream in all conditions, and I can't make it die...

And, just so I can say I asked, why in the world would an engineer create a reliance for rpm signals on a secondary device (tach) that itself has several single points for associated failure (nozzle heater, external temperature sensor, on same fuse?)

Or am I way off base, and all this is wrong?