I have '72 240 with a Pertronix Igniter II magnetic points replacement with their .6-ohm coil. I later added the Pertronix Digital HP multi spark ignition box. It made my L24 with triple DCOE 40's run great. But the tach signal was very flakey. As in none for 20 minutes then a bouncy needle and eventually it worked until I shut it off, then the same routine. They offered many suggestions, resistors etc. but it never improved. I sent it back to them for testing and they said the tach signal was fine. My 280 tach works great just connected to the coil (-) terminal, so I know the tach is good. I recently put on an MSD 6AL, and it ran great but also no tach signal. There is a fix by putting on their Tach Adaptor for analog tachs. My 280 tach now works perfectly all the time. I wish Pertonix would offer a fix for their Digital HP box. I have another car with a digital tach so I'll use ii on that.