Tach Drops to Zero During Left Mag Check

I'm hoping someone can point me in the right direction.

During run up, the right mag has a normal drop of 75-100 rpm. The left mag causes the tach to drop to zero, although it sounds like a normal drop of 75-100 rpm. Would this be caused by the left mag p-lead?

It is a Lycoming 0320 E2D, run up rpm of 1800.

Sounds like you have an electronic tach and the tach is connected to one mag and not the other. You'll need to connect it to both of you want to read RPM from both mags.

If you have Slick or Bendix mags and a mag vent-hole sensor installed, one will do because the rotating magnet is still being sensed with the mag grounded off. If you use the p-leads (as with the Dynon EMS-D10 I had) you need both.

I have this on my plane as well. There is only one mag check wire going into the EFIS, so your wiring had to allow for using 1 wire and 2 P-leads.

Another option is to use a hall effect magnetic RPM sensor that can be screwed into one magneto.