Fluctuating Tac problem

The tach wire comes off the rear area where the trigger coils are located  It usually has a black sheath that houses two wires and comes up to plug into your panel wires either in the engine compartment or maybe on the back side of the firewall inside. Look in the Heavy Maint. manual section 74-00-00 and it shows where it starts at the rear of the engine and has all the info you need. You have 5 trigger coils there. If you have a tractor prop then that coil is on the left side and above the other two trigger coils that are for electrical needs. The wires going through the firewall may have rubbed through from abrasion so look there too. You can ohm these two wires and or do the old quick method by starting the engine and wiggling the wires from inside the panel and outside by the engine.

p.s.

Be safe and careful around a spinning prop. It will hurt.

I had this problem after several years of flawless tach operation. It turned out I need to add a resistor inline to the tach signal wire. Many folks with various tach readout instruments/glass panels find this needs to be done, but some never need to do it. It is fairly easily installed and is a complete fix (assuming you don't have any wiring/chafing problems that should be fixed first). I believe the Rotax installation manual, and also some instrumentation or EFIS installation manuals give instructions for installing a correct resistor (my Dynon panel manual addressed this issue).

Hi James,

Good point. I forgot about that. 

This is why good interaction between users can really be a help. There are just so many configurations and instruments any more. 

"It turned out I need to add a resistor inline to the tach "

Long long way from an electronics expert however in my installation the resistor  "bridges"   the positive (incoming signal) and earth at the terminals/connecters on the back of the Rotax tacho. Went from a jumpy tach to a nice consistent reading.

Glad James was on the ball and remembered this. Way to go James. :)