Rev counter issue

[mbcamper]

I've purchased a rev counter and volt meter gauge from these people:

http://rspec.co.uk/b-gauges-b/clear-len ... nter-gauge" onclick="window.open(this.href);return false;

The volt meter is working fine, but the rev counter I got from them is behaving oddly.



So at idle it reads as expected and when you are driving along without applying much pressure to the accelerator again it's fine. However as soon as you apply pressure to the accelerator, the rev counter starts twitching upwards by up to an additional 1500rpm. As soon as you release the accelerator is stabilises again. As it is, it's basically useless.

I've wired it as follows:
- -ve to the body
- +ve to the ignition switch
- signal to the green wire which comes from the coil and picked up from just before the instrument panel.

Has anyone seen this kind of problem when connecting a cheap rev counter?

[DoubleOSeven]

I'll follow with interest as I also have the cheapo Rev counter to fit tomorrow. VDO want £150 for their one and that puts you in the replace whole binnacle arena and I want to stay fairly stock. Anyway, I'm going to get the sender feed direct from the coil rather than picking it up from behind dash. Only other suggestion would be try putting the earth wire onto the earth crown above the fuse box to ensure it's a good connection.


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[itchyfeet]

I'd try running a fresh wire direct from coil neg to prove the rev counter is faulty.
you could just run a wire temporarily through the van to do the test.

[DoubleOSeven]

Similar cheapo (£21.43) dial seems ok. Direct feed from coil.

Backlight a bit bright in comparison to oil temp and being a cheapo one, it's a sealed unit so cannot remove bulb to cover it with plastic sleeve. So they are working on the dimmer but differing brightness. I'll upgrade the oil temp bulb to match the brightness.
https://vimeo.com/167778085

[itchyfeet]

I opened my cheapo air fuel guage by running a small screwdriver around the rear of the crimped bezel and opeing it up enough to pop it off, I then cut the naff colour changing led circuit and swapped smoked for clear window.

[multisi]

Original vw rev counters are available through vw classic parts , mostly diesel types and one petrol. About £110.

[mbcamper]

Thanks for ideas so far. I think I'll try running a temporary wire from the green wire at the coil and see if that helps.

The other thing I was curious about is the TDC sensor I've discovered in the wiring diagram in my Haynes manual. Apparently this is for connecting to VW test equipment, but could this be used as a more reliable rev counter sender? Has anyone even found the plug for this?

[mbcamper]

So as suggested i ran a temporary cable from the coil green wire directly to the rev counter. I also connected my digital multimeter alongside as a comparison. It actually made no difference and the rev counter was jumping an extra 1500rpm under load whilst my digital meter was stable.

So 007, your gauge looks very similar to mine. Did that also come from Rspec?

[DoubleOSeven]

[img]//uploads.tapatalk-cdn.com/2016052 ... 1f5652.jpg[/img]

I purchased off Amazon from a random company called 'special parts'.


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[DoubleOSeven]

No idea how this works I'm afraid.

(Look at the state of that engine!)


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[mbcamper]

Thanks for the picture of the TDC sensor. This explains why I was not able to find it on my van; it because its not there! It's had a replacement engine at some point and I guess they stripped it out.

I had a reply from Rspec about my rev counter issues and they said that signal readings from the coil can be unreliable and suggest I use another source like a crank sensor or injector. I've also spotted in the small print on the rev counter that they don't accept returns if the user has sensor issues from the coil! Unfortunately I've got a carburettor DG van so I'm stuck with my coil.

It's a little frustrating as I read an thread recently on this forum with a few people successfully using very cheap rev counters; but not me. It does however almost work, there is clearly some improvement which can be made to the signal.

I know this can be fixed as my multi meter is getting a good reading so I'm thinking that my multimeter has got better filter circuitry. My electronics knowledge is quite limited, but I'm sure a simple low pass filter could be made with a resister and capacitor which would fix this for a few pence.

I'll see what I can find out, but if there are any electronics people out there maybe you can tell me what I need to do.

[tobydog]

What happens when you rev the engine out of gear?

[mbcamper]

I'd say the fluctuation is less when free revving, but it's still there a little. Under driving conditions, It is most apparent when you have your foot on the accelerator. As soon as you ease back off the accelerator the reading stabilises.

I don't own a oscilloscope, but I'm sure that would tell the story.

My thinking is a to add a low pass filter. As I say my electronics is very sketchy but:
- If you get 2 sparks per revolution of the engine.
- Say the max rpm is 6000 (I'd never push it that hard though)
- That's 100 revolutions per second
- So you get 200 pulses per second from the coil.

I'd need a circuit which cuts off above 200 Herz maybe?

Someone correct me if I'm talking rubbish!

[tobydog]

Don't think that you need a filter.

I would try it with a lead from the coil as previously suggested, but also with a direct 12volt feed. If that doesn't work then perhaps the rev counter is faulty?

[kevtherev]

Filters?
I connected my TIM tachometer to the coil, everyone does, this bloke is talking nonsense.
Perhaps that gauge is not for connecting to coils, but I have never heard of one.