EDM 800 with erratic RPM readout in flight

rhranzau

I’ve a JPI EDM 800 installed on my 77 Lance. It has worked well over the years, but for a while now has had erratic RPM read outs which make it unreliable for Leaning the engine accurately. The unit was sent in to JPI and was refurbished, checked 4.0 on deck. I confirmed the spark plug wires are not within 3” of the mag pickup. Erratic rpm is still there, manifold pressure is accurate. Has anyone had this problem? And know a solution?? Would shielding of the mag pick up harness with shielding tape work/help? Perplexed.

Scott Sherer

Did JPI have any recommendations? Have you measured your RPM with a different instrument?

rhranzau

They said to check the connections which i did and also installed a new rpm pick-up on the mag itself. no improvement. So talked w the folks at GA Avionics and a few good questions have me going to look for a loose/wobbly wire in the engine compartment. ? So the erratic rpm only happens while in the air, and not during engine run-up? so I'll check this weekend and follow up on this with you. The RPM on the old gauge is not erratic, the rpm on the JPI is, in flight.

Scott Sherer

OK, thanks for the update.

rhranzau

Finally figured it out. The cable from the mag to/thru the firewall was not secured. So rerouted and zip tied /secured the cable and problem went away. Tell tale sign was no problem on the ground but when flying and airflow into cowling caused the jump in readout which led to sporadic tach readings.

Scott Sherer

I'm glad you found it. Next time you take your plane in to the shop, have him replace the zip tie with an aircraft approved clamp. The zip tie will be okay till you bring it in but it won't last long in the engine compartment, perhaps only a few months with the heat, cold and vibration.

rhranzau

Thanks and will do! AC spruce to the rescue.