I've looked around here for an answer to see if I could get one without posting, but I haven't found anything that sounds like my issue.

I've got a 79 12A, and while driving it semi-long distance, after most of the trip, the tach stopped reading rpm and started reading voltage. All of the spark plugs are firing, what could be causing this?

Thanks in advance!

 

Bump, cause I'm still having this problem.

 

Well, no one responded to this, but I thought I'd post it anyway, we figured out that the problem was a dead voltage regulator, within the alternator. Got a reman alternator and now she's purring and my tach works.

 

Glad you figured it out. Good job.