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.

[Delta Whisky]

The answer is in. It is a long story - the short version is: B&C worked overtime to help me solve the issue and after sending several different iterations of their unit - from their very first engineering evaluation unit to their pre-production beta unit to a second current production unit - they found the answer in a GRT trouble shooting guide. (I'd been all over their web site but hadn't come across it.) In it there is one short paragraph that says a small percentage of production units might have the problem I had. All it took was a 10K Ohm resistor in the tach lead - similar to or just like the answers several of you suggested for other EIS/EFIS and/or regulator manufacturers. So there you go, mixing and matching parts might require a little electronic trimming. All is well now. My thanks to you for your inputs and to B&C for all of their work in solving this issue.

[Spook712]

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


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[Delta Whisky]

The resistor is inserted in the tach line. I'm guessing the impedance of the RPM measuring circuit is pretty high so a high value, in-line resistor is needed to ensure spurious signal voltages are dropped below the measurement threshold.

[Spook712]

So in line in the + line?


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[Delta Whisky]

It sounds like your system uses both of the tach leads. (I'm assuming we are talking about a Rotax.?) If that is the case, I'd try it there. Good luck.

[Spook712]

Yes, the instrument has a RPM+ and RPM- input


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[Spook712]

Problem solved, a 220Ohm 2 Watts Resitor did the trick [emoji1317]


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[Delta Whisky]

This is good news. Thanks for the update - one more tool in the tool box.

[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

[Delta Whisky]

I'd go ahead and put the resistor in the tach line. It won't hurt anything and if your unit is susceptible as mine was (with the B&C unit) you'll be ready to go. Check out pages 14 and 15 of the GRT troubleshooting guide for more info. I'd also put the B&C unit in now and carry the original regulator as the spar. The B&C has a couple of features that sold itself to me - the most important being over voltage protection.

[jiott]

Agree, put the resistor in now when it is convenient. Mine went about 500 hours before I needed to install a resistor.

[Spook712]

I got it on ebay


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[Maverick]

I ordered what the GRT document listed (10k ohm resistor 1/4 watt) for the 912/914. When I install the B&C, I'll install this. I assume it goes between the engine tach lead and the GRT. Is this correct?

Since these look like they belong on a circuit board, is there a particular method for splicing them into the wire?

[Spook712]

I don’t know about the GRT System, I had to put mine in like thishttps://uploads.tapatalk-cdn.com/20210410/4939e9587d8c24a263aff19ef1f6b3df.jpg


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[Eric Page]

Since these look like they belong on a circuit board, is there a particular method for splicing them into the wire?
I would cut the resistor leads to about 1/2" in length, then bend them back on themselves 180 degrees to make a 1/4" hook. Do the same with the wires you want to connect. Slip three lengths of heat shrink tubing over the wires: two small pieces to cover the joints and one longer piece to cover the entire assembly plus 1/2" on each end. Hook the wires and resistor leads over each other and press them tightly together. Hold the assembly straight and solder the joints. Shrink the short pieces of heat shrink over the soldered joints, then the longer piece over the assembly. Support the assembly at each end and in the middle so the resistor leads can't bend due to harness weight or vibration.

[Delta Whisky]

Maverick - yes, put the resistor in the one lead going to the EIS. Follow Eric's instruction and all will be well. The GRT EIS uses only one of the tach leads off of a 912 so Spook's schematic won't be the one to use in this instance.

[Maverick]

My only concern is soldering. I can never seem to get the solder to grab onto the wires. I hate doing it. I don't know how electronics guys do it. Any tips for this? I've got my resistors and will do this once it gets too warm to fly which won't be long. . . unfortunately.

[Geek]

My only concern is soldering. I can never seem to get the solder to grab onto the wires.

Make sure you use flux (not the flux capacitor - soldering flux) and tin the wires before trying to connect the stuff up. Tinning the wire will let the solder "grab the wires" when you go to connect them.

G

[Eric Page]

As Gary said, flux is your friend. I use often a flux pen like this (https://www.ebay.com/itm/233326273595) when soldering wires (never use corrosive plumber's flux). You can't apply too much.

Be sure you're applying the soldering iron to the joint, heating it up, then melting the solder into the joint. Don't carry solder to the joint on the iron tip. You can apply a little solder to the iron tip to help it transfer heat to the joint, but you must add fresh solder once the joint is hot.

Be sure your iron tip is clean and tinned with solder. If solder balls up on the tip and doesn't coat it, it's not going to transfer heat well. It can be cleaned with a ball of brass wool (https://www.ebay.com/itm/324415429115) or a damp sponge (a small tuna can makes a good brass wool holder). Don't use anything more abrasive than brass wool or you'll remove the factory plating on the tip and it'll be ruined.

If you want to make it easy to get a clean and well-tinned tip, get some tip tinner (https://www.ebay.com/itm/224340281036); it chemically cleans the tip and tins it at the same time. Just roll the hot tip around in the tinner for a few seconds, and voila!