Tachometer reading high

[Evante]

Hi All,

I hope you are all doing well during the COVID crisis.

My tachometer on my 1972 Federal Sprint is reading too high. The car has an electronic points upgrade on the distributor but I do not know what kind. I have included a picture with this post if anyone can recognize the manufacturer.

Does anyone have any ideas about what could be causing this over-reading on the tach, and how to possibly fix the problem? I have looked up old posts but have not found them helpful.

Thank you,

Jay S.

[Evante]

Sorry,
Here is the pic of the distributor.

Thanks again,

Jay S.

[mbell]

This is normally a common issue with electronic ignition modules.

Where does it 12v power connect? The coil?

The tacho works by "counting" current pulse to the coil (1 per spark, 2 per engine rev). The electronic ignition modules can also generate small current pulse. So if the Ignition module is powered "downstream" of the tacho (e.g. from the coil) then the tacho sees pulses from both coil and electronic ignition module. Causing high reading and random behavior.

The first fix to try is to find an alternative switch live feed to power the electronic ignition module. Or you can convert the tacho to RVC but that's significantly more work.

[Evante]

Indeed, the tach is powered with a white wire to the coil.

How do you suggest I find an alternative switch live feed to power the electronic ignition module?

[mbell]

I used the coil power to switch a fused relay, with input power taken from the permanent live of the solenoid. Then connected the 12v (red) power input of the electronic ignition module to that.

[Concrete-crusher]

Actually I have exactly the same issue and electronic module

My tacho is very steady but over reading by around 40%, any chance of a picture or 2 showing the type and location of a relay a d fuse, I might give it a go

Thanks steve

[ericbushby]

I do not know what version of tacho you have, but mine is an RV1.
It works correctly even though the electronic ignition is powered off the coil input terminal and therefore subject to extra pulses on it`s supply line.
I put that down to having replaced the capacitors in the tacho.
Electrolytic capacitors have an expected useful life of 20 years max. mine were 50 years old and therefore could not do their job of smoothing the input power supply,
What I am saying is do not blame the extra noise on the 12 volt input that the electronic ignition introduces, just because the tacho can no longer cope with it.
Also there is an internal potentiometer for final calibration.
Eric in Burnley
1967 S3SE DHC

[mbell]

My tacho is very steady but over reading by around 40%, any chance of a picture or 2 showing the type and location of a relay a d fuse, I might give it a go

Afraid picture is to large to attach...

This relay
https://www.amazon.com/HELLA-003530041- ... B005OV274W

I have +2 that has the solenoid on the bulk head, have a little bracket of the bonnet coach with relay mounted to that. It also runs a few other things, like that are none standard on the car including fuel pump, rev limter, afr gauge. I also fitted a fuel cut off switch in the feed due to it powering the fuel pump.

One thing I found was it doesn't like cheap doses fuses after I reduced it to a 10a. Had problems starting the car because the fuse had come loose. (This fault cost me a parade lap around the Austin f1 race track in the car!)

All that said I think Eric is more likely correct for your issue. You can probably test if it is the ignition module or with simple test lead to power it from another source temporary and see if the tacho becomes accurate or not.