Aftermarket rev counter electrical gremlins

[lewist123]

Hi all

For many years I have had an aftermarket rev counter in the Estelle. It was wired to the points side of the coil and works perfectly. Recently it has stopped working and so I purchased a similar one from Ebay. When if failed, the needle stayed at 0rpm.

The new rev counter looks the same, but is a bit more 'modern' and likely has more modern circuitry inside based on how you set the number of cylinders and its startup procedure.

I have wired it up and this one also doesn't work (it could of course the faulty as well). It just goes straight to 8k RPM and says there.

I have checked all the wiring and its all good.

I have a frequency mode on my multimeter, and when probing the coil I get a reading of 160hz - which equates to 4,800RPM (based on 2 coil pulses per revolution). Thats a bit odd as the car is ticking over at circa 800RPM.

My multimeter might be overly sensative and triggering on additional noise and back EMF events, or perhaps something else is going on.

Would a failing condenser perhaps result in a very noisy signal and either fail to register on my old one, or over-read on the new one? The car appears to run fine at the moment, revs OK etc. It is being a bit rubbish at starting from stone cold at the moment but I have new carbs on it which I am still setting up so my current thinking is the cold start is probably carb related rather than ignition related.

Any thoughts or suggestions? In the absence of any solid ideas, I will just parts cannon it and replace the condensor and get another rev counter (they are dead cheap on ebay and amazon, but I do want to keep with this style of unit for various reasons).

[ChasR]

I'd be wary of cheap rev counters if they are the cheapy Cheap items.

I had one on my 2CV, with 2 different ignition setups on it, and the behavior remained with the rev counter. The last owner sung its praises, I thought it was cheap junk. In essence, it registered the rpm vaguely correctly, until it got to 4krpm, at which point, if the counter was to be believed, it swung all of the way to 8krpm, almost as if my 2CV had VTEC. The sound of the engine said otherwise, as did my later ignition setup.

I had the following ignition setups on it.

-Points with the condensor gone, and a Gammatronix ignition box in its place ; the box effectivly made the points a switch, and not load dependant
-123 Ignition setup, which in essence, eradicated the above.

Your Hz reading could be down to noise. Have you go access to an RPM meter on a multimeter, to validate for a signal?

[lewist123]

Just a quick update

I played around a bit more today and found quite a lot of high frequency interfearance/transients on the signal line from the points which I thought might have been confusing the new rev counter.

I knocked up a resistor/capacitor low pass filter and put that between the points and the rev counter and it started working correctly.

I opened up the old broken rev counter and found that it had an internal low pass filter in it. I suspect the newer version doesn't have one and so it was reading the interfearance as points pulses and overreading by so much it was just pegged at 8k rpm.

On the slight odd chance someone finds this in the future, I used a 500 ohm resistor and a 1uF 50v electrolitic capacitor. The resistor goes in line with the signal wire and the capacitor goes between the rev counter side of the resistor and ground (make sure you get the polarity correct).

These values give a cuttoff freqency of 300hz, which equates to 9k rpm on a 4 cylinder engine. Any freqencies over 9k rpm are blocked which is fine for my engine.

L