912ULS tacho and mag drop puzzles

From own “experience“ I can say that you need to re-check the wiring on the back side of your key switch. The way you connected it does not seem to shortcut the individual circuits. It took me a while to actually start afresh cause I was convinced my wiring was faultless. Save some time and accept that there is a wiring error and give it another go. It’s not a big issue, re-read and re-do ;-)

What RPM-instrument do you have?

No matter what the front of the Tach says, it was probably made by someone else and Re-Branded.

Most Tachs are generic and will attach to the Points of a 1,2,3,4,6, or 8 cylinder engine and also to Alternators that can output from 1 to 20 or more pulses per revolution.

A normal 4-Cylinder, 4-Stroke engine Fires 2 plugs per revolution.

The Rotax 912 gets its Tach signal from a dedicated Tach Coil in the alternator that outputs ONE(1) Pulse per revolution.

This would be the same as a 2-Cylinder, 4-Stroke Engine.

The Tach needs to be set for 1 pulse per revolution which will be the 2-Cylinder, 4stroke,  setting on the Tach.

The Tach was probably default shipped for a Single-Cylinder Engine.

Look for a multi-position switch on the back of the gauge case.  

They are often very small and delicate requiring a Jeweler's screwdriver to adjust.

@Peter KL yes what you say makes sense and my thoughts at about 3AM had reached the same conclusion!

@Bill accept your point in general but this tach is scaled and colour marked with the right figures for a 912, it wouldn't suit much else. It's the one Rotax supply. 3 little round pins on the back. I don't know who make it but I don't think it's the Aviasport one, pics of that show 4 pins on the back. 

Out with the panel again!

Edit - it apparently has a calibration under a sticker on the back but I think it's just a trim, unlikely to cover a 2:1 range.

Update -

The tach is a Rotax one specifically for a 912, part no 966403. It's well out of warranty. I can't open it up without wrecking the case or drilling out the rivnuts that both hold it together and provide the fixings. It's correctly wired up to +, - and the trigger output. The trigger cct measures around 260 ohms.

The trigger coil presumably gives a double pulse as the magnet passes? I wonder if the instrument includes a diode to block one half and it might have gone short and is double counting? Clutching at straws! Anyway it looks like an instrument fault as the MGL works ok.

I've disconnected the kill wires from the ignition boxes and measured through them to ground via the keyswitch. All correct. Each is 0.3 ohm when it should be and infinite when it should be. All wires are physically where they belong on the keyswitch.

I'm baffled. Unfortunately I daren't run it in the garden again - neighbours - and can't move it to the airfield because of lockdown so further tests will have to wait.

All ideas appreciated in the meantime!

Mike,

Although I am not 100% sureI am beginning to recall that my „experience“ had to do with the shielding of the kill wires and where to connect it to the on the switch (GND <> GND). Up to a point I was willing to bet all in that my wiring and all connections were brilliantly perfected. Finally I compared my “solution“ with an installation that worked - I‘ll see if I can find a picture I took and post it here as a reference.

Regards

Peter