Weird tach indication problem

[Delta Whisky]

Here's the situation - with a GRT EIS - take off is normal until the "out of range" warning light illuminates. A quick look at the EIS identifies the issue: 9000 rpm. That's not possible with the nose up and a constant speed prop so the heart rate goes down to normal. A little investigation reveals that the rpm indication goes crazy at a true rpm of just over 4100. The tach indications are fast changing, unpredicable and wide ranging - always between about 5000 and 9000. Bring the throttle back under 4100 and everything is normal. Above 4100 and it goes crazy again. Same symptoms on three after-inspection test flights. 58 hours TT. All connections on and between the tach sensor and the instrument are soldered. A visual of the solder connections does not reveal any appearances of a cold solder joint.

As always, your ideas, experiences and suggestions are appreciated.

Thanks, Darrel

[jiott]

I had a similar problem. I installed a resistor in series with one of the tach leads and the problem was solved. I believe the Rotax installation manual says that this might be necessary and gives a value for the resistor.

[Delta Whisky]

Thanks Jim - I see a 1K Ohm in series for a "point type" ignition in the GRT installation manual but can't find anything in the Rotax installation manual. I have found several postings in a variety of forums from folks with the same problem (worked fine, now, suddenly doesn't) but nothing definitive for a solution. The answers that seem to work are related to different grounding schemes or resistor placement as you suggested. I'd rather try a bunch of these before even thinking about changing out the pick up coil so I'm off to try your suggestion first. Thanks for the response.

[jiott]

Delta W, I was mistaken about the installation instruction for the tach being in the Rotax manual. It is in the Dynon Skyview, which I have, installation instructions for the engine monitor sensors. That is where they say if you have erratic rpm readings especially at hi rpms you may need to install a 60k resistor in the tach wiring. Can't say if this is appropriate for your GRT system, but it sure fixed my problem. What is weird is that I had no problem whatsoever without the resistor for 3-4 years, then the erratic readings showed up for no apparent reason.

[Shadowrider]

G3X installation manual just calls out that a resistor is required. Stein air builds a pcb board with the resistor installed on for ease of installation. They sell them and are like 20$

[Delta Whisky]

Thanks for your input gents. I'm pretty sure I found the source of the problem but want to run another test or two. Will post the answer/solution when they are known for sure. A special thanks to Jim for his post: "What is weird is that I had no problem whatsoever without the resistor for 3-4 years, then the erratic readings showed up for no apparent reason." That was pretty much what I was claiming and it got me to thinking - what changes, if any, had I made, intentionally or unintentionally, and that got me going.

[VictorV]

My guess is that at higher RPMS the digital pulses from the RPM sensor are close enough together and the voltage is high enough that
they "blur" the pulses so they appear to be closer together than they really are resulting in the higher RPM reading. A resistor will
bring that voltage down a bit so the pulses are a bit cleaner. In essence the pulses are wider at higher RPMS and the width is
dependent on the voltage. It would be an interesting experiment to connect an oscilloscope to the signal from the sensor
and see what it looks like with and without the resistor.

V.

[Delta Whisky]

Thanks for your input Victor but it appears that the source of my problem is the newly installed B&C regulator. (Turn it off and the problem disappears.) They (B&C) are working really well with me to determine the exact source and any and all remedies. I just received a second unit to see if the issue is unit dependent and I hope to have time this weekend to install it and perform more tests. It would be great to have either a scope or signal analyzer but my contacts in the electronic world are back in Texas. I have shielded the tach signal line, routed it behind a fuselage tube, and added more length to the shield on the power input lines. That activity raised the threshold at which the problem starts (now about 4330 rpm) but didn't solve it completely. More news at 11:00.

[Maverick]

I bought my engine from salvage and the regulator is the original as far as I know. No entry in the logs about replacing it. However, I bought a B&C to carry in the plane based on recommendations made in another post. I have a GRT EIS and have not had any problems during this first 40hours and I don't have a resistor in the tach line. Given the proper operation now would adding a resistor now be counter productive or would it not be an issue with the current regulator. My concern is that I go somewhere, loose the regulator, install the new one and then find I have the issue with the B&C and need a resistor for the B&C.
Fred

[Spook712]

How do you put in the resistor?
Just from + to - on the RPM leads?


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