Tachometer mismatch

[Glen Davis]

Tachometer mismatch:

The K model my friend owns developed the following problem recently. It has a JPI 830 as well as a conventional analog tachometer. During a run up the conventional analog tachometer showed a drop of approximately 75 RPM on both mags. The sound coming from the engine corresponded with 75 RPMs dropping. The tachometer on the JPI 830 indicated over 1000 RPM drop on each mag. The sound did not correspond with a 1000 RPM drop so the JPI was reading incorrectly. A call was placed to JPI and the technician said the problem is EMI, electromagnetic interference. When asked for a further explanation or a way to fix it the JPI representative said “just ask your mechanic”.  Has anyone else had this happen to them with an 830 or other digital mag?  A follow up question is it appears there is only one lead from the JPI 830 to one of the mags. If I am correct about that, how does the JPI accurately read a mag drop on run up on both mags if only one mag is connected to the instrument?

[A64Pilot]

For the EMI thing he could either shield the tach input (only ground one end of the shield) or get an iron ferrite and loop the tach wire through it a few times. Shielding is more mainstream aviation

So far as how could it work with only one mag connected, I don’t know other than to suppose it’s so sensitive that it can still measure the pulses even when the wire is grounded.

Best place to get that answer I would suspect should be the manufacturers help desk, because I’d call them back and say my mechanic isn’t an avionics tech, so what does JPI suggest as a corrective action?

[N201MKTurbo]

It is kind of strange that it would excessively drop. EMI would be expected to induce false pulses that would make it indicate a higher RPM. 
 

If we look at what is happening when we do a mag check, we are grounding P-leads. I assume the JPI is getting its tach signal from the P-leads. how could grounding one P-lead affect the signals on the other? I can’t imagine how that would work, but it may be worth checking the wiring to make sure it is correct.

[EricJ]

6 hours ago, Glen Davis said:

Tachometer mismatch:

The K model my friend owns developed the following problem recently. It has a JPI 830 as well as a conventional analog tachometer. During a run up the conventional analog tachometer showed a drop of approximately 75 RPM on both mags. The sound coming from the engine corresponded with 75 RPMs dropping. The tachometer on the JPI 830 indicated over 1000 RPM drop on each mag. The sound did not correspond with a 1000 RPM drop so the JPI was reading incorrectly. A call was placed to JPI and the technician said the problem is EMI, electromagnetic interference. When asked for a further explanation or a way to fix it the JPI representative said “just ask your mechanic”.  Has anyone else had this happen to them with an 830 or other digital mag?  A follow up question is it appears there is only one lead from the JPI 830 to one of the mags. If I am correct about that, how does the JPI accurately read a mag drop on run up on both mags if only one mag is connected to the instrument?

Usually the sensor is triggered by the magnet rotation in the magneto, which happens whether the magneto is firing or not.

EMI may be adding enough noise to the signal that the JPI can't distinguish the pulses from the noise and so counts fewer pulses.  It does seem odd that it only does it during a mag check.    Is the signal from the sensor to the JPI shielded?    There may also be a loose ground somewhere since that can do the same thing.

[kortopates]

Eric is correct, the EDM doesn’t use p-leads for signal but a Hall-effect sensor on the mag counting rotations. It makes no difference to the sensor if the mag is firing or not.
Never heard of EMI noise problem with them, nor this issue with them but i would check all connections closely at the mag. i’d expect you have a poor connection there.
Be careful it’s a rather delicate sensor installation.


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