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?
helbus ·
Is it points or electronic ignition?
Is it generator or alternator?
shokwave2 ·
12V, points, alternator. Tacho connected to coil, not alternator.
barls ·
did you fit the diode they ask you to for points.
shokwave2 ·
No not yet, i just read about in the last half hour. Will have to buy one tomorrow. Cheers.
barls ·
mine came with it. it was a male and female fitting each end and about an inch in length
shokwave2 ·
Don't think mine came with a diode. I'll check the workbench where i threw the packaging.
barls ·
have a look in the spares bag that came with it. ie the globe covers.
shokwave2 ·
Nothing in there but the globe. I'll drop into Jaycar and pick up a diode or two.
oldskoolguy ·
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.
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....
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....
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.
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.
👍
Lucky Phil ·
Wow! Pscimitar! You geek you!
Outta my league,
psimitar ·
Lucky Phil wrote:Wow! Pscimitar! You geek you!
Outta my league,
Oh, i feel all warm n fuzzy inside now 😂
psimitar ·
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?
modnrod ·
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 ·
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.
psimitar ·
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 😀