Actually, I am using the diesel tach. I do plan on hooking the gas tach up just to see whether it works, but I've been told it will not.

I guess that connecting one side of the sender to chassis ground and the other to the tach input signal creates a potential between the signal line and ground (as the tach is also grounded).

It's actually not a nerve-wracking experience to drive the Trooper around (around the driveway, that is, until I get the fuel tank back in) as I didn't have to fabricate any mounts etc - I am confident in using all Isuzu pieces, which was one of the project goals. Driving my Acura after the rebuilt (by me) engine and transmission were first installed was much more worrisome.