[Vwdiesel] Tacho Find

[Roger Brown]

Val Christian wrote:
> 
> How about using knowns?  Like transmission ratio and tire size?
> If you don't trust your speedometer, then take a GPS with you
> and do a couple of straight runs.  You should be able to repeat
> within 1% or worst case 2% accuracy.
> 
> Val
> 
> >
> > A few days ago I was down my local Car breakers and I
> > spotted a diesel tacho in a lorry. Couldn't leave it so I
> > had it :o)
> > It looks like a motor cycle rev counter in size about 3"
> > across.  The scale 'max's out at 3500 rpm. It must have been
> > attached to the alternator 'W' connector but as the alt. was
> > gone I couldn't see the pulley ratio ( although this
> > wouldn't tell me the No. of poles...) The three wires
> > (excluding the light) were black/yellow streak, red /orange
> > streak and brown. Definitely not British Standard colours!
> > It turns out it works on my alt with the black one being
> > batt+ red being signal and brown earth(-ve)
> > I thought to myself how can I check the calibration without
> > another  diesel tacho?
> > Well I reckon that if I was to use a gas strobe on a gas car
> > parked alongside my 'Q' and used it to strobe the diesel fly
> > wheel at different speeds of the gasser (noted with its
> > gasser tacho) and then adjust the speed of the diesel to
> > match that'll do it (clever eh :o)
> >
> > Alternatively I'll just throw this back into the sea and
> > convert one of my oem gasser tachos...

You could always try something like this:
	http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=41727
or this:
	http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=41726

The calibration issue was what led me to design the opto-sensor driven signal generator I use on my tachometer:
	http://www.4crawler.com/Diesel/CheapTricks/Tachometer/index.shtml#Analog

It was easier to build  the "calibration" in the signal generator than to mess with calibrating the system after the fact.  

--
   Roger