[eion]

Actually, I'm pretty sure that the KTS500 (the new one) will work on OBD1 systems. That aside, it does sound like it's time to either try swapping the OBC out, checking the connections on the tach/OBC (the clock sometimes suffers from solder problems, so it isn't beyond the bounds of probability that the tach could too), or using the wiring diagrams and a digital multimeter. I don't have access to the turbo-specific diagrams, but seeing as your boost gauge works already, I can probably hunt down the relevant OBC wiring diagrams for the non-turbo and that should work fine.