RPM indication jumping

GalinHdz

GalinHdz

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Joined
Mar 3, 2008
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KSGJ/TJBQ
During cruise flight my engine RPM indication will, once in a while and randomly, jump from cruise to over 3000rpms and fall right back to cruise even thought the engine has not surged or otherwise changed. All other engine indications remain normal.

I have a Lycoming O360 running dual P-Mags. The tach output is connected to EMS pins 32/33 without the 5v diode. Has anybody else experienced this issue?

8-)
 
Hi
I too have this problem. it is more pronounced during hot temperatures. Once in while, the RPM jumps out of limit and settles back in a fraction of second. RPM pick up arrangement is also the same. Looks like a design problem
 
You can try adding some series resistance to see if that helps. 30K is the default recommendation. But if you're seeing high readings you can try adding another 30k to see if that curtails the voltage spikes without losing any pulses (if you add too much resistance you'll end up not seeing enough of the pulses you're after and you'll then you'll see erroneous dips instead).
 
I installed the 5V Zener Diode that was provided by P-Mags and the problem went away. I guess I should have RTFB before doing the installation.

8-)
 
These spurious rpm variations with conventional mags can be eliminated. I use the MakerPlane PTACH in my Harmon Rocket because it was impossible to find the sweet spot with series resistors.

There is more info elswhere in the Forum, but the summary is that it eliminates the signal variations caused by rpm, mag condition, plug age and cylinder condition. This is especially a problem with 6 cylinders, but some 4 cylinder engines have issues as well.

Cheers,
Vern
 
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