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.