I thought a little project I just finished yesterday might be of interest to Jabiru engine owners.  I use a MGL XTreme EMS with the RDAC XF interface to monitor my Jab 3300.  Typically, you pick up a tach signal from one of the mags or the alternator.  These sources usually work OK but I found that adjusting the dip switches and pots on the RDAC was pretty "fiddly" and difficult to achieve stable rpm readouts across the entire rpm band from idle to WOT.

Eventually, I started using one of the cheap (typically less than $5 !) Chinese Hall Effect Sensors found on Ebay.  These are mounted so as be triggered by the 3 flywheel magnets as they sweep past the sensor.  These sensors typically detect each pole of the three magnets, so that's 6 poles per revolution.  If the MGL EMS is set to 6 pulses/rpm, you get an accurate tachometer reading.  I found this method to be much more stable than previous methods.

However, I noticed than when it gets really hot in the summer, if I fly and get the under-cowl temps up and then shut down and then fly again before the cowl temps have cooled down, the tach reading would become erratic until I had climbed-out and the improved airflow had cooled the sensor. Usually, the sensor would then stabilize for the remainder of the flight.

This is a pretty irritating thing since your tach is pretty important when taking off!  Most, if not all, of the cheap Ebay sensors don't specify a max heat rating. I did some research and found that quality, "name brand" sensors usually specify max heat limits of either 85C (185F) or 150C (302F).  Typically, the 150C sensors are much more expensive than the 85C sensors.

I found a Honeywell GT-1 gear tooth sensor at Sager Electronics for only $24.75!  This typically sells for 2-4x more elsewhere.  My delivered price - shipping + tax - was about $40 - still very reasonable.  The GT-1 is rated to 150C and even 160C for "brief" periods.

To install it, I soldered a 1 KOhm pull up resistor between the +12V lead and the signal lead which goes to the RDAC.  I drilled and tapped a 1/4-28 hole in the left upper arm of the alternator spider.  The GT-1 was bolted directly to the spider arm.  The sensor is 1.25" long from it's mounting flange, resulting in an approximately 3mm air gap between the flywheel magnets and the sensor.  Looks like this:

The result?  I set the XTreme EMS to 3 pulses/rpm (the GT-1 counts the entire magnet as 1 pulse and doesn't count 2 poles/2 pulses) and the tach worked perfectly the very first try!  It was accurate and extremely stable across the entire rpm range of the Jab. I didn't even have to adjust the RDAC from its previous settings!

Now all I have to do is wait for some hot weather and see if the sensor heat-soaking issue is resolved - I'm pretty confident the GT-1 will not be affected due to it's heat rating.

By the way, some may note that the GT-1 is a "gear tooth" Hall Effect sensor.  These contain a small internal magnet so that ferrous gear teeth will trigger the sensor by disturbing the internal magnet's flux field.  Of course, I really didn't need a "gear tooth" sensor in my application since the targets are the flywheel magnets, but gear tooth sensors work just fine with external magnetic targets and since this was a bargain price for the superior heat rating, I chose the GT-1.

John
N750A