| [ Total Views: 1210 | Total Replies: 18 | Thread Id: 103139 ] |
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 shokwave2
A.k.a.: Matt Hamilton
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| posted on August 18th, 2013 at 06:25 PM |
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Tacho jumpy at idle unless multimeter connected?!?
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?
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 helbus
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| posted on August 18th, 2013 at 06:36 PM |
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Is it points or electronic ignition?
Is it generator or alternator?
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shokwave2
A.k.a.: Matt Hamilton
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| posted on August 18th, 2013 at 06:38 PM |
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12V, points, alternator. Tacho connected to coil, not alternator.
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barls
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| posted on August 18th, 2013 at 07:26 PM |
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did you fit the diode they ask you to for points.
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shokwave2
A.k.a.: Matt Hamilton
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| posted on August 18th, 2013 at 07:30 PM |
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No not yet, i just read about in the last half hour. Will have to buy one tomorrow. Cheers.
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barls
A.k.a.: Mr indestructible
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| posted on August 18th, 2013 at 07:48 PM |
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mine came with it. it was a male and female fitting each end and about an inch in length
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shokwave2
A.k.a.: Matt Hamilton
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| posted on August 18th, 2013 at 07:58 PM |
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Don't think mine came with a diode. I'll check the workbench where i threw the packaging.
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barls
A.k.a.: Mr indestructible
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| posted on August 18th, 2013 at 08:36 PM |
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have a look in the spares bag that came with it. ie the globe covers.
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shokwave2
A.k.a.: Matt Hamilton
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| posted on August 18th, 2013 at 08:54 PM |
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Nothing in there but the globe. I'll drop into Jaycar and pick up a diode or two.
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oldskoolguy
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| posted on March 5th, 2014 at 07:15 PM |
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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
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modnrod
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| posted on March 6th, 2014 at 10:14 AM |
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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
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psimitar
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| posted on March 6th, 2014 at 10:13 PM |
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| 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
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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  |
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modnrod
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| posted on March 7th, 2014 at 06:39 AM |
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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.
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Lucky Phil
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| posted on March 7th, 2014 at 07:58 PM |
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Wow! Pscimitar! You geek you!
Outta my league,
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psimitar
Son of Jim - Creator of Good
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| posted on March 8th, 2014 at 12:11 AM |
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| Quote: | Originally
posted by Lucky Phil
Wow! Pscimitar! You geek you!
Outta my league,
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Oh, i feel all warm n fuzzy inside now
madness is in the eye of the beholder |
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psimitar
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| posted on March 8th, 2014 at 12:21 AM |
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| 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.
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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 |
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modnrod
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| posted on March 8th, 2014 at 05:10 AM |
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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........
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AA003
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| posted on March 8th, 2014 at 09:47 AM |
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| Quote: | Originally
posted by psimitar
So as there are 4 sparks were revolution then we are looking at 24,000 tacho input pulses @6000 rpm.
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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!
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psimitar
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| posted on March 8th, 2014 at 06:12 PM |
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| Quote: | Originally
posted by AA003
| Quote: | Originally
posted by psimitar
So as there are 4 sparks were revolution then we are looking at 24,000 tacho input pulses @6000 rpm.
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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.
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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 |
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