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Old 07-22-2022, 01:49 AM
plippo plippo is offline
Join Date: Jul 2022
Location: Järvenpää, Finland
Posts: 1
Default VM1000 RPM input voltage?

We're trying to sort out a random RPM display drop (no real RPM drop, engine is rock solid) in a VM1000 unit that came with the plane (DR107) built in the 90's. The plane has LS Plasma CDI which provides the RPM signal to the VM1000 via ~5.5V zener diode dropping the pulse voltage from 10V to about 4.5V.

We've already checked the grounding and signal paths using PC oscilloscope and now start to think that the zener may be a part of the issue instead of doing something useful. However I have not been able to find any information about the electrical specs of the VM1000 inputs, so just cutting the zener away does not feel too smart before knowing more.

I've already contacted Klaus from Lightspeed and JPI but have not received any information (probably busy prepping for Oshkosh), so I thought to ask here. I'm sure someone here has encountered this bit of info before...
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Old 07-22-2022, 06:36 AM
Southern Pete Southern Pete is offline
Join Date: Jul 2015
Location: England
Posts: 194

I ended up installing one of these beside the VM.
Reggie is the man who knows but I don't have his contact details.
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Old 07-22-2022, 06:54 AM
lr172 lr172 is offline
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 7,474

The standard in the automotive aftermarket for tach signals is a 12V square wave, with a configurable # or pulses per revolution, typically 2,3, or 4. That said, I would expect that many EMS systems would be able to manage pulses with a lower voltage, but that would be based upon their design. I can't imagine any EMS that couldn't tolerate a 12V signal, given that most systems provide that, so pulling the Diode out may be worth trying. Have no idea what a VM100 is.

N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 07-22-2022 at 07:00 AM.
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