912ULS tacho and mag drop puzzles

Thanks Bill that's interesting new info to think about. I have no prior knowledge on the ccts used, I was assuming pulse counting.

Yes the connections are correct. The MGL E1 connected to the same wires reads fine. (Paralleling them is OK, I had exactly the same on a previous a/c with no problems.) I will try reversing the two wires just in case.

Unfortunately I can't do any more tests until our lockdown finishes and I can move it to the airfield. Running it in the front garden was a one off!

Only high-frequency instruments use the Pulse counting method.

If you are measuring in the cycles per SECOND range and can wait a second for the update that would be acceptable. e.g. Audio Frequencies and above.

Keeping in mind that 6000rpm is only 100 Revolutions and Pulses per second...

If you are measuring revolutions per minute and only get 1 pulse per rev, you will have to count a full minute's worth of pulse between updates.  Hardly practical.  e.g. 5243 rpm ± 1rpm

If you only count for 1/10 of a minute (6 seconds) the resolution drops by 1/10th.  e.g. 5240 ± 10 rpm.  Slow,  but maybe usable for Rotor Speed that is Sluggish by Nature.

1/100 of a minute update (0.6 Seconds) would be usable if you could accept the low resolution.  e.g. 5200 ± 100rpm.

- - -

At slower pulse rates, you need to measure pulse repetition periods and calculate the frequency. e.g. 0.499 mSec = 2003 rpm. Updated 2000 times a second.

And by averaging 10 pulses the resolution goes up and the update time remains usable.  e.g. 10 pulses = 4.991 mSec = 2003.6 rpm updated 200 times a second.  Still way faster than you could ever read the numbers.

Most instruments purposely limit the update to once or twice a second otherwise the eyes can't process the number.

Try to borrow a frequency generator to check and adjust your tach that 100 pulses equals 6000 revs 

Next stage in the puzzle!

Prompted by the suggestion of a frequency generator, I wired up the tach to a 12V battery and a 12V transformer. Thought if it worked, it should be equivalent to 3000rpm (50Hz mains here) and so should indicate around 6000rpm.

Nope, it showed 3200.

In the meantime I've snapped up a guaranteed used tach I was offered. Wired that up and it showed exactly 3000.

So back to wondering what on earth could be wrong in the aircraft wiring - not forgetting that the MGL monitor reads correctly.

Mike Said:

... Thought if it worked, it should be equivalent to 3000rpm (50Hz mains here) and so should indicate around 6000rpm.

The 3000 RPM indication is correct.  The 3200rpm indication on the original Tach is Not!

It appears that the Original Tachometer is overly sensitive to noise.

An Unloaded transformer is going to transfer some noise from the Mains to the Tach.

You might try adding a Resistor across the tach signal wires to lower the Impedance/Resistance of the circuit.

The Noise will be the first to succumb to the additional load.

- - -

An Excerpt taken from the UMA Rotax 912 Tachometer Installation Manual:

If the tachometer’s needle (Reads High) when the engine is running, install a 200 to 680 Ohm ½ Watt resistor between the (Signal) wire of the tachometer and ground, starting from higher resistance.