Tacho under reading

[pptom]

In the process of swapping to crank fired ignition, so i've a trigger wheel and sensor on the crank. A side effect of this is that i have a digital readout of engine revs and its alot different to what the rvi tacho is showing me. 3200 revs on the readout is showing as 5000 on the tacho, so while running in the engine using 5k as my rev ceiling i'm actually nowhere near. Its a way different beast using the proper revs!
I've a lucas sports coil, standard points dizzy and converted to negative earth during the cars extensive rebuild , swapped the contacts in the tach case and reversed the induction coil to enable negative earth.
It doesn't jump around at all, very stable, i'm assuming its gone way out of calibration, but is there something more simple amiss?
Tom

[Concrete-crusher]

Hi Tom , it will be interesting to see what others say , I have exactly the same problem after converting my tacho to negative earth. It?s very steady but over reads by around 50%

I assumed it was just an internal component getting old

Steve

[mbell]

One option is to fit a spydia board to your tacho. This has various voltage inputs and you could possibly connect it directly to what ever is controlling your spark.

I've fitted a rev limit on my car with 12v output for the tacho which I've connected to the spydia board I fitted. Works well but I needs a bit more calibration.

[pptom]

Going to have to go spidya eventually to work with the new ignition, but i'm in no rush to spend any more money this month!
Just want it sorted for running purposes as i'm currently using the distributor.

[ericbushby]

Hi Tom and Steve,
I suggest that you establish a figure you can trust as a starting point.
1000 rpm should give you 17 MPH in top gear on a standard car with 3.77 diff.
I found the easiest way to check speed accurately is with a Head up Display (HUD) app on a phone.
It works on GPS.
This gives you a base to start from.
The RV1 Tach has a calibration potentiometer on the circuit board.
The most likely components to fail with age are- The 100mf electrolytic capacitor, which has a life expectancy of 20 years max. and the other two capacitors. They are easy to replace and should make the tach reliable again at very little cost.
Best of luck
Eric in Burnley
1967 S3SE DHC

[pptom]

Thanks Eric

[billwill]

I think Eric has told you all you need, except perhaps to mention that there should be a tiny hole for a screwdriver on the back of the Smiths tacho, for you to adjust the calibration from outside.

The notes with the Spydia kit contain instructions on how to make up a square wave generator at mains or 2x mains frequency to use as a calibration source (I think).

But if you are SURE that your digital readout is accurate you can just use that and set the Smiths tacho to match. but be VERY SURE.

~~~~

I've tried out some of the android apps on my phone which claim to be flashing tachos, but I've not found one that I would rely on the readout.

[mark030358]

Or you can buy a square wave generator off eBay for about ?6, which is much easier.

[Hawksfield]

Hi


As a starting standard a 165/13/80 tyre with 3.77diff in top gear gives 18.44 per 1000 rpm

[pptom]

Thanks, checked last night and 1k revs on my crank position sensor seems to be just under 20mph so i'm doubly sure its correct.
For any interested its a nodiz system so similar to a megajolt, but has a Bluetooth output to turn your phone into a digital dash with revs, advance angle etc. Runs off a qed crank pulley / trigger wheel and standard ford hardware such as coil pack / crank sensor.
Can also add temp sensor outputs which will be handy in the future as i'm seeing 120?c when running on the original dial, although i also have 60?c ambient temperature, so either the gauge or sensor is dodgy.
Had a few set up issues, so i currently also have the dizzy and wiring in place until i'm 100% confident.

[mbell]

The notes with the Spydia kit contain instructions on how to make up a square wave generator at mains or 2x mains frequency to use as a calibration source (I think).

Mine came with a 3.5mm headphone cable that you plug into a device and connect to the low voltage connection. There a few sound file from their site that have different frequencies square waves, so you can calibrate across a few different points.