Tachometer convertor?

[ninjapower]

After days of trying to wire up a VFR400 tach to my CBR600FM I have given up. I have been through every avenue and no luck. It's not the VFR tacho as I have two of them. I can't get it to read either by the LT of the coil or the pulse wire from the CDI. When you rev it to probably 10,000 RPM it only reads about 3,000 RPM.

Here is my question; Does anyone know of a Tacho convertor or where to look for one?

This can be done I'm sure. As an example if you had a in line 4 cylinder and put a V8 engine in you can get somewhere a device which coverts the pulse so that the Tacho will read correctly.

Any suggestions welcome... I'm stuck.

[finpos]

Bit of a long shot -

Take your spare tach to bits. inside, you'll find half a dozen electrical components. What you have there is a frequency to voltage converter. One side of it is hooked to your pulse input, the other side goes to the tach needle (it's really a voltmeter).

Now, look if one of the components is clearly marked out, maybe by some printing on the circuit board. You're in luck, it's a resistor which calibrates the frequency to voltage converter. You can read the value of that resistor off the coloured bands on it (consult the internet), then go to maplins and get a couple which are twice and half of it's value. A bit of judicious soldering/experimentation and you will fix it.

f.

[ninjapower]

Sounds like a fun... right nothing to loose so screw driver out! Cheers.
Something like this then?
https://www.reuk.co.uk/OtherImages/resistor-colour-code.gif

[finpos]

Precisely that The calibration resistor is usually a the one with the highest tolerance (i.e. lowest % number)

f.

[ninjapower]

Ok I got it in bits and inside was a adjuster. So I plugged the tacho in, started the bike up and turned the adjuster anti clockwise and to my suprise the revs picked up.
Revving the bike on the limiter I managed to get 11,000 RPM which probably should be around 14,000 RPM, still it's usable... well was as the unit was sealed for life. It's not anymore... broke the glass and destroyed the caseing. Still at least I have learned something today lol.

https://excoboard.com/forums/22303/user/200305/290665.jpg

I guess its for calibration.

[finpos]

Oh, smashing, it's a variable resistor. I've never seen a tach with one of those in before. Normally there's just a fixed value resistor.

You can probably change it's calibrated range to make it entirely accurate, but you'd need to be able to trace out the circuit on the board to do that and also make some measurements with a multimeter.

I'm guessing it was sealed with a crimped ring around the glass - you'll never get it off, the trick is to cut around the back of shell!

f.

[Ichy]

[Vincent]

Theres a guy that makes programable CDI units for two strokes his name is Borut and the company is called Zeeltronics. He made me a black box to run an RGv tacho on an RD350. The RD has a single coil for two cylinders while The RGv has 2 coils so the revs were reading half what they should be. - £20. As for removing clock bezels and replacing them, I've tried it but could never get the bezel tightened down properly afterwards. I did see a proper tool for replacing bezels on the internet once but lost the link and could never find it again
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[Ichy]

I agree Vincent, replacing the bezels isn't easy. I place the clock glass down on a piece of thick leather and form it around with a drift I made. I do have a fair amount of experience as a sheet metal worker so I probably find it easier than most.

Here is one option
https://www.stargis.net/webslinger/Docs/vstar1100.pdf

Or this which may be what Finpos was talking about.
https://www.davebarton.com/pdf/52mmfaceinstall.pdf
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[ninjapower]

On the second tacho purchased from eBay, I drilled a hole at the bottom to access the adjuster. Much better idea.
However with the adjuster turned to the max it reads 10,000 RPM on the limiter but should be reading 14,000 RPM.

Another question for the wizes of electronics... The tacho is currently wired to the negative side of the coil, is it possible to wire the tacho to the other negative side of the coil as well. probably talking daft but would this double the revolutions or would it do nothing at all and blow my coils? Don't want to try something I'm not sure about. Cheers chaps.

[finpos]

Probably a bad idea to try to connect them together, you'd probably end up feeding the same pulse/timing signal to both coils, probably not what you're after. You might be able to do something with diodes to stop that happening, but I wouldn't like to try.

With regard the calibration, I suspect you'll need to change one of the other (fixed value) resistors on the board to get it back into range.

f.

[Ichy]

I can't think of an easy way around this without changing the variable resistor for one of a different range.
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