After some testing tonight, I've got the tach working. There are 2 wires coming from the tach sender on the injection pump, one black and one black/red. I've tried connecting both of them to the tach signal input with no success, and was left wondering what they both were for.

After digging through another non-USA Isuzu service manual, I found a wiring diagram showing the tach sender. In it, there are 2 wires listed (black/red and black/p, which could mean purple?).

The bk/p wire was run to a ground while the bk/r wire ran to the EGR/QOS (glow) controller, so I figured that had to be the signal line. I reconnected the bk/r line to the tach as signal and grounded the black wire and just like magic, I had a working tach.

Not sure why one would have to be grounded, and why the sender isn't just grounded to the pump body. Maybe the pump isn't grounded? Anyway, it works perfectly. I am using a C223 (1986 diesel Trooper) tach, which has a different redline and (according to Jerry Lemond) is designed to work with the signal from the diesel sender - a gas tach is not supposed to work, I have not confirmed this though.