• Could be a ground issue. The instrument to engine ground was may be fouled. Cleaned it and recheck operation.

• Check that the wiring is not adjacent to ignition wires.

• Check that CYL in FAC LIM of the EDM is set correctly to 4 or 6 or 8 for Bendix dual D2000/3000 magnetos or 12 for a Unison Laser ignition.

• Check location of sensor. Make sure sensor is mounted as illustrated in installation manual.

• Check with a degausser (available at Radio Shack): 4 cyl should read 3600 rpm and 6 cyl should read 2400 rpm; 8 cyl 1800; 9 cyl 1600; 4 cyl dual mag 1800; 6 cyl dual mag 2400.

• Make sure the correct sensor is being used. For BENDIX series -20, -21, -200 Non-pressurized -#420815-2 — Pressurize is # 420806.    For Bendix 1200 Non-pressurized -# 420815-2 —– Pressurize is # 420807. For the Bendix dual mag – D2000 or D3000 – No # -just specify “Bendix  Dual”     ——-         For  SLICK 4000 or 6000 -Non-pressurized – #420815-1 — Pressurize is # 420809.  For the Slick 600 Pressurize is # 420806.

•With oscilloscope or Frequency Counter-( If they do not have these, ask if their digital voltmeter has the ‘Hz’ option on the dial. They can use that if it does. Meter’s red probe to the green wire on RPM sensor and the meter’s black probe to black ground wire on RPM harness. Do not use engine ground.) —- They should see these values per RPM / HZ:  500 RPM=12.50HZ; 1200 RPM=30.00HZ; 1550 RPM=38.75HZ; 1900 RPM=47.50HZ; 2250 RPM=56.25HZ; 2600 RPM=65.00HZ; 3500 RPM=87.50HZ