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Tacho jumpy at idle unless multimeter connected?!?

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shokwave2 Officially Full-On Dubber

289 posts · joined Mar 2010 ·

#966827
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?

shokwave2 Officially Full-On Dubber

289 posts · joined Mar 2010 ·

#966831
12V, points, alternator. Tacho connected to coil, not alternator.

barls Administrator Causer of Chaos and Mayhem

9,473 posts · joined Jun 2004 ·

#966842
did you fit the diode they ask you to for points.
“We all pay for life with death, so everything in between should be free.” Bill Hicks
the chaos project - click me

shokwave2 Officially Full-On Dubber

289 posts · joined Mar 2010 ·

#966843
No not yet, i just read about in the last half hour. Will have to buy one tomorrow. Cheers.

barls Administrator Causer of Chaos and Mayhem

9,473 posts · joined Jun 2004 ·

#966848
mine came with it. it was a male and female fitting each end and about an inch in length
“We all pay for life with death, so everything in between should be free.” Bill Hicks
the chaos project - click me

shokwave2 Officially Full-On Dubber

289 posts · joined Mar 2010 ·

#966852
Don't think mine came with a diode. I'll check the workbench where i threw the packaging.

barls Administrator Causer of Chaos and Mayhem

9,473 posts · joined Jun 2004 ·

#966864
have a look in the spares bag that came with it. ie the globe covers.
“We all pay for life with death, so everything in between should be free.” Bill Hicks
the chaos project - click me

shokwave2 Officially Full-On Dubber

289 posts · joined Mar 2010 ·

#966866
Nothing in there but the globe. I'll drop into Jaycar and pick up a diode or two.

oldskoolguy Officially Full-On Dubber

329 posts · joined Sep 2011 ·

#985153
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.
"The best trophies are stone chips"
1969 1500 Beetle "Otis"
2008 Golf GTI

modnrod Fahrvergnugen

978 posts · joined Mar 2011 ·

#985214
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

psimitar Son of Jim - Creator of Good

2,506 posts · joined Jan 2009 ·

#985275
modnrod wrote: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
Um, it has been 10yrs since I did my electronics degree but doesn't f=1/T and T is number of seconds for a single cycle to occur?
6000rpm = 166rev/sec and therefore 1 rev takes 6milliseconds.
Plonking that into the calc gives an R value of 9.1Kohms.

Like i said it's been a while so don't mind if I've remembered things incorrectly.
madness is in the eye of the beholder 👍

modnrod Fahrvergnugen

978 posts · joined Mar 2011 ·

#985282
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.

👍

Lucky Phil Wolfsburg Wizard

480 posts · joined Aug 2010 ·

#985355
Wow! Pscimitar! You geek you!
Outta my league,

psimitar Son of Jim - Creator of Good

2,506 posts · joined Jan 2009 ·

#985375
Lucky Phil wrote:Wow! Pscimitar! You geek you!
Outta my league,
Oh, i feel all warm n fuzzy inside now 😂
madness is in the eye of the beholder 👍

psimitar Son of Jim - Creator of Good

2,506 posts · joined Jan 2009 ·

#985376
modnrod wrote: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.

👍
Um, ok now you're making me actually use my brain for a change.

as the VW crank is of the 180degree design then one full rev on say no.1 cyl will take 2 revs but due to the cam n dizzy being gear to half that then 1 dizzy rev, which is what we are counting. So as there are 4 sparks were revolution then we are looking at 24,000 tacho input pulses @6000 rpm. Now the tacho pulse input has a divider built in to allow use with 4/6/8 cylinder engines but to filter the tacho positive feed then 24k pulses per minute is the figure we need to use.

am i right?
madness is in the eye of the beholder 👍

modnrod Fahrvergnugen

978 posts · joined Mar 2011 ·

#985378
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........
😵

AA003 Custom Title Time!

1,189 posts · joined Oct 2011 ·

#985391
psimitar wrote:So as there are 4 sparks were revolution then we are looking at 24,000 tacho input pulses @6000 rpm.
No there are 2 sparks per engine revolution so it is 12000 tacho input pulses @6000 rpm.

The distributor and camshaft spin at 3000 rpm when the engine is at 6000 rpm.
I read it on samba, so it must be correct.

Sometimes Volkswagen dealers sell spare parts. Amazing isn't it!😂

psimitar Son of Jim - Creator of Good

2,506 posts · joined Jan 2009 ·

#985409
AA003 wrote:
psimitar wrote:So as there are 4 sparks were revolution then we are looking at 24,000 tacho input pulses @6000 rpm.
No there are 2 sparks per engine revolution so it is 12000 tacho input pulses @6000 rpm.

The distributor and camshaft spin at 3000 rpm when the engine is at 6000 rpm.
Cool. Can't always be right especially when i wrote that past my bedtime 🙂

Still, at least between us we got to the bottom of it and now know what filter parts to use 😀
madness is in the eye of the beholder 👍