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Permalink Reply by John Austin on May 14, 2020 at 8:27am

My MGL XTreme EMS used a similar system for obtaining the tach signal on my Jab 3300, but you could also pick it up from the mag lead - which EFIS do you have - you might check and see if you can use the mag lead?

I found either tach source to be a bit "fiddly" in that it took very careful adjustment of the RDAC (the interface between the MGL EMS and the sensor leads) to obtain stable, full-range tach readings.  I had fairly satisfactory results using the mag lead, but that only provides the tach reading on one side of the dual ignition system, so I couldn't do a mag check of left and right mags with a single rpm display - I had to set up a second pick-up and tach display if I wanted to see the rpms on both ignition systems.  People would ask why I had two tachs and I'd say, "Didn't you notice this was a twin?"  ;>)

I finally ditched the mag pick-ups and went to a Hall effect gear tooth sensor.  It counts the magnets on my alternator as they sweep past and gives a rock-solid, reliable rpm indication.  If you've removed the alternator magnets in the Rotec conversion, a suitably positioned gear tooth sensor could still count the teeth on the starter ring gear and derive rpm or if your new alternator has a pulley, a couple of small magnets or bolts (the gear tooth sensor can count a ferrous bolt head - doesn't have to be magnetic) could be affixed (180 degrees opposite to keep the pulley balanced) to act as targets for the gear tooth sensor, but to get the correct rpm indication, you'd have to take in to account the pulley's rotation ratio to the engine's rotation if they're not 1:1. 

Some people use cheap Chinese Hall effect sensors from Ebay that cost $5 or less and work perfectly OK except they're not heat tolerant and sometimes have stability issues when they get heat-soaked.  The Honeywell GT1 I used is rated for 150-160 degrees C and very stable, but has been discontinued.  A few new-old-stock units are out there on the web, but are relatively expensive - most are well over $100 delivered. There are clones available on Ebay for about $30 that claim to meet OEM specs - I have no experience with them but that's what I'd try if I had the option to go to the Hall effect gear tooth sensor.

John

N750A

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Permalink Reply by Eugene Stone-Romero on May 14, 2020 at 9:53am

Hello John:

Sincerest of thanks for your detailed and highly informative message. I will follow up on the leads that you provided.

Best regards,

Gene

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