Mystery Tacho

Yes 12 volt systems. 750 is about right for a Minor, it could run lower but the revs drop a little when the headlights are on. I've been looking to see if I could find a suitable variable resistor but no luck - I'll try the piggy back resistors and see what happens.
 
If you want to check a tacho, you can make an accurate strobe lamp by wiring a 240V neon in series with a diode and connecting it to the mains (be careful!). In the UK it flashes exactly 50 times a second. By the light of this lamp, a tippex mark on the crank pulley will appear stationary at 1500, 3000, and 6000! RPM, accurate to better than 0.5%. I have one. It works.;)
 
Great idea, what do you think about increasing the resistor rate to slow down the tacho, for nowI would be happy to get is as good as the one I compared it too.
 
OK did some more investigation this morning. First of all I put two of the 330 ohm resistors in parallel which should have given a bit more resistance but had little effect on the operation of the tacho, then put two in series with again little difference to using one resistor. So I tried the two in parallel in series with one then two resistors which didn't do a thing for the accuracy:( What the hell I thought and just connected the gauge straight to the 12v signal and low and behold it works, a bit sluggish but pretty much the same readings as the tacho I'm comparing it against. Not sure what the resistor is meant to do but it doesn't look like it's necessary for a 12v application, so to sum up the signal wire from the dizzy side of the coil (yellow)is connected to the positive terminal post on the gauge and the negative post(black) is straight to earth, the wires coming out through the grommet on the back of the gauge aren't used - about as simple as you can get:)
 
A tacho's good for at least 20bhp but just wait until the new stereo is in sssssssssssmokin :cool:
 
Someone had the same idea :confused::confused:
bigbang2016banner.jpg
Big Bang Fri 22nd to Sun 24th April 2016

This springtime VW festival features VW Drag Racing, invited race classes and VAG only RWYB with Live Action Arena, Live music, Show & Shine, traders and a chilled out family atmosphere with kids entertainment! Full details at www.Big-Bang.co.uk
 
colinthefox said:
If you want to check a tacho, you can make an accurate strobe lamp by wiring a 240V neon in series with a diode and connecting it to the mains (be careful!). In the UK it flashes exactly 50 times a second. By the light of this lamp, a tippex mark on the crank pulley will appear stationary at 1500, 3000, and 6000! RPM, accurate to better than 0.5%. I have one. It works.;)
Click to expand...

This was posted on the MMOC forum, another way to check calibration found by accident.

http://www.mmoc.org.uk/viewtopic.php?f=4&t=63612&p=580830#p580830
 
Yes but the writer didn't appear to explain the 60 in the equation, which made me think for a minute;)
 
Last edited:
Panky said:
It turns out that what we thought were variable resistors are in fact multiple contacts on the wires not used (maybe to set up for cylinder number variations) so not adjustment for zero and span as I'd thought. I cant see any other easy way of adjusting the reaction but could a higher rated resistor in the signal wire do the trick? any ideas
Click to expand...
The things referred definitely ARE variable, as I can move mine!
 

Attachments

  • 2016-04-23 14.50.32.jpg
    2016-04-23 14.50.32.jpg
    1.2 MB · Views: 7
You must log in or register to reply here.
Back
Top