Jumping Tach Signal

[MD64]

MS1/Extra rev 029y4, v3.0 board, running fuel only on my Jeep 4.0l engine. Tach signal is coming from a hall sensor in my distributor. Signal is pulled up to an ~8v supply through a 1k resistor (OEM supply is supposed to be above 7.1v). Hall sensor ground is going to XG1 through SPR4. Hall sensor signal and ground wire are twisted and shielded from the sensor to the Megasquirt. C12 is a .1uF capacitor. The Megasquirt is grounded directly at the car's -ve battery terminal. Car picks up RPM signal and runs, but RPM signal jumps a few hundred RPM every second or so. Jumps in RPM increase at higher RPM. Attached are a datalog showing this, and the tune I was running. Thanks for the help!

[Matt Cramer]

Do you have it built for the "low to high" or "high to low" Hall sensor input? I'd try the opposite build and see if that does anything. Some sensors only have one stable edge.

[MD64]

Tried the opposite build and it worked. I now have the engine running spark too. Thanks for the help!