Newly fitted rev counter fluctuates wildly

[ripley]

Hi,

Fitted a rev counter (this one: https://www.brickwerks.co.uk/vdo-gauge- ... 00rpm.html) to our van (petrol, 2.1 MV) over the weekend and something's not right.
The needle fluctuates wildly and is reading far too high. For example, at idle a rpm of around 3000 is indicated but the needle is bouncing around from 2500 up to 3500.
The signal wire is taken from the green wire going to the 14 pin multiplug that connects to the instrument panel. I believe this to be correct but for testing purposes I also ran a wire direct from terminal 1 of the coil, to the tacho and get the same readings.
I've checked and double checked the setting of the encoding dip switch on the rev counter and that's set right.
Not sure where to go from here.
Any thoughts?
Many Thanks.

[itchyfeet]

Have you checked your 12V and earth are good?
Did you disconnect the thin green at coil (connects to dash) when you took the direct feed? just possible something in the dash is causing a problem.
You can speak to Brickwerks if you bought it from them.

[ripley]

12V and earth check out fine, but I didn't disconnect the thin green wire when I did my test run from the coil so that's next on the list, thanks, good suggestion.
I'm starting to think this might some form of EMI/RFI but I'll have to wait a day or two before I can investigate further.

[itchyfeet]

Try a different 12V and earth too, just to rule it out, deffo not taking 12V from your leisure batt are you?

[ripley]

Cheers, will do. 12V is straight from G5 terminal on back of the fuse/relay panel so not leisure battery.

[syncroandy]

The tach output from an ignition coil is very noisy by nature with spikes of 100+ volts, normally a tach will have filters on the input to take care of this but maybe this tach filter circuit isn't up to the job or perhaps the coil is at fault. Is the tach explicitly rated for direct connection to an igntion coil ? If you've been sold it for this purpose perhaps query it with the seller in the first instance ?

[Jawin89]

I had the same problem with mine and after a lot of head scratching and google searching i installed a 10k ohm resistor on the feed wire from the coil and it cured it.

[marlinowner]

https://www.vdo-gauges.com/media/instru ... meters.pdf" onclick="window.open(this.href);return false;

[ripley]

@syncroandy - wiring diagrams the tacho came with show direct connection to the coil. It was purchased from DemonTweeks (Brickwerks were out of stock), don't know what to expect from their customer service but could be worth a shot.

@Jawin89 & marlinowner - Thanks for those suggestions, I had read that an in line diode or resistor can be used to 'smooth out' the signal but my knowledge of such electrickery is too poor to work out that kind of thing so cheers both for pointing me in the right direction.

I'll let you know how I get on over the weekend.

[itchyfeet]

I had the same problem with mine and after a lot of head scratching and google searching i installed a 10k ohm resistor on the feed wire from the coil and it cured it.

in series or to earth?

I went for a cheap veethree 52mm gauge and it works just fine

[syncroandy]

Google for 'tach filter circuit' will show a few simple options to try.

[Jawin89]

It was installed inline close to the coil. Spade connectors were crimped onto the ends and connected that way.

[ripley]

Well, I finally got chance to have another look at this and I'm pleased to report the problem solved, the solution being Jawin89s' suggestion of a 10k resistor (0.5W) fitted in line on the signal wire although I fitted it at the tacho end rather than the coil end.
Thanks everyone for the input.