Tach sensor with P-Mags

[RV9APlane]

I am using dual P-Mags with my Skyview system. I had Stein Air design my panel and as part of the documentation they designed in a 30k resistor on the RPM wire coming off the left P-Mag going to the EMS. The people at Stein stand by this however the people at E-Mag say that a resistor would not be needed since the voltage coming off the P-Mag is already reduced, not hundreds of volts as in a magneto. They suggested I ask you all as the tie-breaker. What do you think?

Thanks!
Bruce

 

[mmarien]

Which P-Mag control lead is Stein Air using for tack signal?

The recommended method of connecting the P-Mags for tack signal (RPM) is to use #6 control lead instead of #4. It's made to connect to an EFIS - no resistors needed. See the wiring diagram in the P-Mag manual. I connected them directly to the standard pin 32/33 on the EMS. You need to set the pulse to 2.0 on the EMS SETUP > ENGINE INFORMATION page.

 

[RV9APlane]

I am using lead #6 connected to Pin 33 on the EMS. I have received another email this morning from the guy at Stein Air who is sending me the 30k resistor. He seems to think it's necessary for some reason. What are the implications if I use it versus not using it?

 

[dynonsupport]

The relevant page is 7-34 in the install guide:

Digital Ignition and Other Pickoffs
The SV-EMS-220/221’s standard RPM inputs can read frequency-based RPM signals, provided the peak voltage goes at least 5.1 volts above ground, and crosses back down below 2.0V relative to ground. If the peak voltage exceeds 50 volts, use the included 30 k[ch937] resistors as described in the P-lead pickoff (Lycoming and Continental) Section above. Like the other methods above, you must know the number of pulses per revolution for your RPM transducer.

And from the P-Mag manual, page 8:

Pin 6 is a courtesy (optional) connection to provide a digital tack signal – two pulses per revolution (reconfigurable to 1ppr using EICAD).
The default tach voltage is 12 volts, although some ignitions were produced with a 5 volt signal. See notes on the Wiring Diagram at the end of this manual for switching tach voltage signal, if needed.

While the Stein Air guys are generally very knowledgeable about our products, in this case they happen to be wrong. We don't need or want the 33K resistor, and with it in place you are likely to get no or erratic RPM readings.

Also we have low voltage inputs for RPM, so if you actually did have a 5V output, you would hook direct to those.

 

[RV9APlane]

Ok, thank you. I have emailed Stein Air with a link to this thread. I appreciate the advice and help.