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By MikeW
Does anyone here know anything about the inner workings of the tacho that Rotax supply for the 912 please? Part no 966403, specifically for the 912.
I have one that overreads by a factor of exactly 2 and I'm wondering if there's any chance of fixing it. The trigger pulse from the coil is double as the magnet passes it, a + spike then a - spike, I'm wondering if maybe a failed diode might be causing it count both spikes?
Not easy to open up but nothing to lose!
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By mikehallam
If you're right, surely you could put a diode anywhere easy to reach in the feed wire without exposing the innards of the tacho.

Although on my 912 the worst problem is fluctuating readings when around max allowed rpm. Usual 'cure' is then to fiddle to improve the contact of the wire ends to the the four little connection stubs and the instrument behaves itself !
By MikeW
Yes, I was thinking of trying that, but of course it might be a bypass rather than a block.

A fluctuating reading at high revs is a common complaint. Usually cured by putting a 220 ohm resistor across the two wires from the trigger, have you tried that?
By MikeW
4 wires? Normally 3 pins -
ground plus one trigger wire
The other trigger wire

Ah do you have the Aviasport one? I believe that has a 4th pin, I think it's to switch a Hobbs, not sure.
By MikeW
You'll find loads on eBay at 10 for £1 or thereabouts. Go on splash out!
(You might get one free from Rod L - I don't know if he still keeps all sort of electronic bits.)
By MikeW
MikeW wrote:Ah do you have the Aviasport one? I believe that has a 4th pin, I think it's to switch a Hobbs, not sure.

Sorry that's cobblers. See for the truth.
It gives you the wiring dia too - and a hint to try swapping over the two trigger wires if the reading is erratic.
It and the Rotax one bear a remarkably close resemblance to each other!
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By mikehallam
The Aviasport does have three pins.

Latest news is that mine was proven this week to read increasingly lower rpm than true as rev's rose, when compared with a plug lead sensor digital counter.
Upshot is a new meter is on back order and running as she was at some 3 to 4oo rpm true above indicated 4,800 has neatly answered by fears re my excessive fuel usage. The consumption at ~5,200 rpm agrees with the Rotax book figure.
By MikeW
Your initial problem was fluctuating readings at high rpm. What happened about that? Did you put a load resistor on and if so did that fix it?
To my surprise putting a 220 ohm R on fixed my problem of the reading suddenly doubling at ca 2000rpm. However I've subsequently realised that compared with my MGL E3, which I think is digital from end to end and has no calibration provision and should be accurate, the Rotax badged Aviasport analogue meter under-reads about 400rpm at 5700. The E3 agrees closely with the preset rpm positions of the Airmaster CS prop controller (the take off rpm is factory set at 5700 and the climb at 5400).
That's not conclusive but suggests the under-read may be an artefact of the resistor.
I bought a used identical tacho and it also under-reads at high revs but not quite so much.
When I can get back to any activity on it, I may play with different value resistors, see if say a 500R will stop the jump but not affect calibration.
Alternatively there is a calibration adjuster on the meter down the hole at the centre of the back, may be worth a twiddle.
I'll be interested to hear how your new tacho behaves.