Engine code 21 help?

[blazer9903]

I'm working on an '88 NA 4age right now, its running great aside from the worn synchro in 2nd. For all intensive purposes, the engine is stock. (has a monza cat-back) Though its running very nice, I'm getting a check engine light - I have reset it and it gives an all clear before startup, but after startup, it immediately codes 12 and 21. I know 12 is the o2 sensor, which I'll replace this weekend, but 21 "no RPM signal within 4 seconds of startup" - thats the one I can't seem to find the cause of. The distributor and coil were replaced before I got my hands on the car, and all the wiring looks good. I'm getting 12v off the distributor harness and variable voltage increasing with the engine speed at the terminals on the coil. The tach is working fine. It doesn't seem to effect the performance at all, but the damn light is just annoying as hell. I'd finally gotten to the point of getting ready to just live with it, but my intention is to restore this car to "new" condition and I see that there are a lot of people on here with more knowledge than me. Any help is appreciated!!