Does anyone know how the PCM555 computes the RPM? Crank Sensor? How do you know yours is going bad?

Thanks!

Correct. Crank position sensor. There can be a fault code logged if the pcm is scanned. In my case the engine was running and shut down. Attempted restarts resulted in backfires thru the intake. Tach was showing no rpm when engine was cranking. Replaced CPS. Fixed.

The 496Mag/HO crankshaft position sensor can be going bad before throwing an error code; known from personal experience. I had a pair of 496HOs in my AT, and one was consistently a few hundred RPM behind the other. I tried EVERYTHING. Once I replaced CPS, all better.

Did you have to do any special re-learn process? Replaced mine with a 300 buck merc part....nothing. Diacom shows RPM counts while cranking, but the tach does not. Dropped the old one back in and it starts. (my problem is a stall consistently at 10-12 minutes)