Weird tach indication problem

[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.

[jiott]

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

[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.