Odd tach question?
 

I have been playing with the tach on my 83 300TD. It has never worked since I got the car. Once while messing around I happen to hit the AC and suddenly the tach was jumping all over the place... this only lasted a day or so. I was under the impression from that that I was dealing with a bad ground situation being the dash was replaced by PO, there is no telling what might have gone missed etc.

Today I replaced the pickup, using my known good one from the new engine I have sitting here, I also popped on the known good tack amp I have. I have also replaced the tach itself with one I got from Vstech. Still nothing.

Was working on something else and noticed that Fuse #3 was out. Put it in and suddenly the tach works? What's fuse #3 have to do with the tach? Or, is this just a coincidence...

Tachs not smooth, seems to hang up a little, might be the old tack, going to toss the original one on to test, but it seems to be reading the RPMS correctly.

Tach amp and tach needs power to operate. Maybe fuse 3 supplies power to it?

Jumpy tach can be pickup gap too wide (try adjusting it to a narrow gap), or a flaky tach amp.

Not jumpy, more like the needles on too tight and causing friction.

Ok, from further reading....

#3 feeds the amplified signal to the tacho

That would explain that...