Resurrecting this old thread in hopes that someone can help me diagnose tach issues I am having with my RV-8 running dual P-Mags on an IO-360-M1B.
The tachometer reads 0000 until you run the engine up to a high idle -- I estimate about 1200 RPM (by ear) -- then it bounces around between 500 and 900 and continues to show that until you run the engine up to about 1700. There it begins to become more accurate and stable (though not perfectly stable). From 1800 to about 2200 it is almost perfectly stable, though it will occasionally bounce down to zero while I pull the prop back in a low RPM cruise. I have an Electronics International R-1 tach.
The airplane acted like this since I bought it (with the current setup). I discovered that both P-Mags had tach wires going back to the instrument and I disconnected one from the right mag after reading in the installation manual that you should only use one mag for the tach signal, but the problem remains.
I chose to disconnect the lead at the tachometer because the tach wires go through the firewall and then disappear into a giant maze of wire bundles and I cannot easily see the back of the tachometer. Does anyone know how I can tell which setup was selected by the person who installed the P-Mags? I.e. how do I know what to look for if the resistor method was used. Or, how do I troubleshoot within the instrument if the "count the pulses" method was used.
Otherwise, does anyone know what might be causing this behavior with my tach?
Much appreciated,
Dan