Rotorway Forums

I have a long standing intermittent but consistently occurring issue with my MGL RPM-2 engine tachometer display.  Using the dual arc setup once I reach operating RPM, the engine RPM intermittently drops off and then recovers.

I run a Fuel Tech computer on a Joe Huff turbo.  Engine RPM on the Fuel Tech display is steady.  Rotor RPM display is steady. 

I have done all the standard troubleshooting including purchasing a new unit, adjusting the sensitivity, swapping the display sides, changing the increments, etc.  On some flights it only occurs once, and on other flights it is consistent.  I have yet to have a flight where it did not occur.

I am to the point now where I think I am going to have to chase the signal wire all the way back to the origination which will require removing about every cowl and wire bundle.  That will have to wait until next annual.  

In the meantime, do any of you MGL experts have any ideas I might try?

Not an MGL expert and there's a lot of things that could be wrong but it sounds like you get a clean enough signal for the fuel tech display? Is the ground wire from the MGL going direct to the engine block and not being shared with other loads? Double check the dipswitch setting for high frequency filtering. Make sure the tach signal wire is routed safely away from spark plug, fuel injector and alternator wires to reduce possible interference. The ~1k ohm pull-up resistor that you used between pins 6 and 14 wired in parallel, try replacing it.

Other than the obvious (inspect for chaffing/corrosion, poor connections, etc..), I'm not sure what else to suggest. Mine's worked great from the start. Is it also possible you have a short occurring intermittently somewhere unrelated that is being picked up by the tach? If nothing suggested works, then I'd say it's time to take the cowlings off and hang test with new wire & fresh connections.