I have a mallory 685 hyfire VI ignition system in my car now. It starts and runs good so far but the tach doesnt work

I connected the tach wire to the tach terminal on the box. During the 'tach test' mode, i set the box to read 5000 rpms, and my tach moved to 5000 rpms. I tried 5500 and 3000 and its dead on. SO i'm thinkin its all good

When i went to fire the car for the first time today, the tach PEGGED at 7K rpms and doesnt read the motor. I shut the car down and its still pegged on that side...7000 rpms


WHY??

 

well, never mind...it now mysteriously works.

I started the motor again and its idling well enough for now. Tach was pegged still. I gave it a couple of quick revs and the tach magically came back down and its back to normal