Fluctuating tacho reading

[10ixwwybk2]

I have twin Skyviews with a Lycoming O320 engine.
My rpm is rapidly varying such that it is very difficult to read the value. This is at higher rpm settings, 2200 and above. From new it was ok but it has been like this for the last 20 hours or so.
I went to the airfield today with a range of resistors.
I first identified that the problem was with one mag only. The right mag with a 60k resistor was fine. I stepped up the resistor to 160k on the left one but it was still fluctuating.
Has anyone had any experience of this, should I get higher value resistors or looking for something else.
Robin

 

[dlloyd]

I used to have an occasional jumpy tach reading. Never happened long enough to narrow down one mag or another. Right mag was high time and the oil seal failed. Overhauled, never had the problem again, no other changes.

 

[Raymo]

As you may already know, the EMS will use the reading from the first mag it senses, BTW. If you turn off the one that fluctuates, it should go to the steady one and remain there.

How many hours on the mags?
Is the shielding terminated at the magneto end or switch end?

 

[dynonsupport]

Actually, SkyView displays the higher RPM of the two inputs in real time. It does not switch like the D10 series does.

 

[10ixwwybk2]

Thanks for the comments.
The mags are about 200 hours old.
The shielding is terminated at the magneto end.
I could disconnect the 'faulty' one but then I wouldn't be able to do a mag check.
As I have said I have increased the resistance to 160k, has anyone done this much or more.
Should I just keep increasing it until it works or stops reading.

 

[jakej]

Robin
I know of a mag in an IO-540 that had similar problems with one mag but have not seen that in the O-320 (had 2 of those for 20 years) - From my experience you should not have to have wildly different resistors between the 2 mags - I'd pull the LH one & get it checked out.
In recent times the have been problems with the points & condensors & also had a failure of one @ 125hrs.

Or you could try one of these http://store.makerplane.org/magneto-p-lead-tachometer-signal-stabilizer-p-tach/

HTH

 

[10ixwwybk2]

Yesterday I disconnected the faulty feed and the other one gave me good steady reading which I had not seen for a long time.
Is there any good reason not to leave it at that.
Jakej is suggesting that this could be indicative of a mag fault even though it is running well with no significant mag drop.
I would be prepared to fit a transducer to the mag if necessary.

 

[Raymo]

Check all the plugs and wires to make sure they meet resistance requirements. A plug that has too high resistance can cause the mag coil to fail prematurely. Not sure what it would cause relative to the tach, though.

 

[jakej]

robint - yes that is what I'm suggesting is a possibility here, it has happened to another poster here. Short version is his mag was bench tested & found to be ok however fitting another mag fixed the problem. Fitting the P-Lead Tach unit may be the most cost effective way of stabilising the tach.

 

[vlittle]

It's very important to ensure that your plugs are all in good condition and have identical gaps. Any problems with the plugs, wires or mags will cause erratic readings.

The P-TACH device helps to stabilize tach signals, but won't fix misfire problems in the ignition system. I tried misgapping plugs as an experiment, and I got erratic readings at take-off power. Restoring the gaps fixed the problem.

Vern

 

[austinglasair]

Robint,

I know it's been a couple years, but I am wondering if arrived at a final solution to your tach issue.

-Austin