Just installed a VDO tachometer today and hooked up a multimeter to check the accuracy. It was spot on reading 850 at idle and when reved. But when i removed the multimeter the tacho showed higher and jumping around at idle, anywhere from 1100-1500. What would be causing this? All i did was attach the multimeter to - side of the coil and to a ground, and the tach works fine. Is this a grounding issue somewhere, with either the coil or tacho?

Is it points or electronic ignition?

Is it generator or alternator?

12V, points, alternator. Tacho connected to coil, not alternator.

did you fit the diode they ask you to for points.

No not yet, i just read about in the last half hour. Will have to buy one tomorrow. Cheers.

mine came with it. it was a male and female fitting each end and about an inch in length

Don't think mine came with a diode. I'll check the workbench where i threw the packaging.

have a look in the spares bag that came with it. ie the globe covers.

Nothing in there but the globe. I'll drop into Jaycar and pick up a diode or two.

Just adding my 2 cents worth to this. The tacho in my Beetle was jumpy, fitted 1/2 watt 10k resistor close to tacho, still jumped occasionally. Fitted diode N14004, still jumped. Checked to see that wire was clear of leads etc. , still jumpy.
Fitted resistor at coil end, tacho still jumpy and radio interference now!
Finally decided to fit 1/2 watt 100k resistor at tacho end and it seemed to fix it, and no radio interference.
I think it might be that different quality tachos react different to the noisy standard ignition system, my tacho is a $30 Ebuy special, which for the most part I'm happy with now.
Rob.

I've got a cheapy SAAS tacho on my bike (*sigh* don't ask, it was in the shed when I needed a tach one day............) that also jumps around everywhere.
Bloody irritating isn't it?

A simple low-pass RC filter will usually stop it in it's tracks (HAHAHA! Small geek electronic humour there).
Like this one....



Nice little calculator pinched here, type in the Frequency in Hertz (4cyl-6000rpm = 3000Hz F).........
http://sim.okawa-denshi.jp/en/CRtool.php 

Quote:

Originally posted by modnrod I've got a cheapy SAAS tacho on my bike (*sigh* don't ask, it was in the shed when I needed a tach one day............) that also jumps around everywhere. Bloody irritating isn't it? A simple low-pass RC filter will usually stop it in it's tracks (HAHAHA! Small geek electronic humour there). Like this one.... Nice little calculator pinched here, type in the Frequency in Hertz (4cyl-6000rpm = 3000Hz F)......... http://sim.okawa-denshi.jp/en/CRtool.php

Uhm......yup! Sorta....when I first looked at it I thought, "Hang on.......", but then I got distracted.

6000rpm =12000 coil fires per minute on a 4 cyl with a dizzy =12000/60 = 200 pulses per second (Hz).
200Hz = 8.2 K Resistor.

Phew. Oh yeah, and Jaycar will have all these components, they still stock discrete parts.

Sorry guys. My ningcompoople rating is directly proportional to the amount of sleep I get..........
Thanks Psimitar.

Wow! Pscimitar! You geek you!
Outta my league,

Quote:

Originally posted by modnrod Uhm......yup! Sorta....when I first looked at it I thought, "Hang on.......", but then I got distracted. 6000rpm =12000 coil fires per minute on a 4 cyl with a dizzy =12000/60 = 200 pulses per second (Hz). 200Hz = 8.2 K Resistor. Phew. Oh yeah, and Jaycar will have all these components, they still stock discrete parts. Sorry guys. My ningcompoople rating is directly proportional to the amount of sleep I get.......... Thanks Psimitar.

HAHA! It's 2:35am, I just sat down on watch with my first coffee, and quickly flicked here coz I have a spare moment........Jeez my head hurts!

The pulses you want to filter, the ones that drive the tacho, come from the coil. A 4 stroke motor fires every second rev only, so in one rev on a 4 cyl the coil will fire twice (two posts on the dizzy coz it spins at half speed), a 6 cyl three times,, an 8 cyl four times, etc. So for a 4 cylinder 6000rpm = 12000pulses/minute / 60 (minutes to seconds) = 200Hz.
A 202 Red motor six spinning out to 7600rpm (HAHAHAHA! Hehehe.....yeah right!) = 7600 x 3 = 22800 / 60 = 380Hz.

Is it? I think?
I might go back to bed now........