Thanks for the input. From what I got out of the threads, I suspect that what I pulled is a unit that was for a car with ABS. I think I have two options here.

First, maybe I can adapt the ABS unit so it gives me correct readings for my non-ABS differential. From the looks of it, I can maybe replace a couple of divider circuits with some resistors. I might look into this option since it seems simpler than the second. Needs more research though.

The second option is to purchase a new 2115 IC as it seems to be the source of the problem with my original unit. Basically, my original unit seems to work fine except for I seem to be getting no signal out of the taxi meter output (pin 3). I can buy one of these (from China) for around $15-$20.

Interesting enough, I came up with the same solution as Art about using an AC adapter as a speedometer input signal. Makes bench testing much easier.