Rev Counter inconsistent

[MartinH]

I am running Aldon Ignitor ignition, a standard coil bought from euro car parts and an alternator. My car runs fine nearly all of the time with this set up. However, on a longer journey I typically have approx 4k RPM at 70 MPH at the start of the trip but, as the day goes on the revs indicated, for the same same speed, creep up so that after an hour or so I am seeing 4.5k-5k RPM at 70 MPH. The needle is not bouncing around.

Can anyone shed any light on what may be happening here? It doesn't effect how the car runs and I'm not sure it really is revving any harder it just reads higher.

Thanks
Martin

[rgh0]

Sounds like a heat effect on aging tacho electronics. Potentially a resistor or something somewhere changing it value as it heats up slowly affecting the calibration

cheers
Rohan

[EPA]

I think this a failing capacitor in the tachometer I replaced the internal module for one of the Spiyda units when mine did this.Its not too hard to do and Spyida are very helpful.
I have seen an article that identifies the capacitor but can't remember where.

Ed

[billwill]

The Spiyda cantact details are in the following topic:

lotus-electrical-f38/tachometer-electronic-ignition-problem-t39820.html

[englishmaninwales]

That explains mine too!
Thanks

[prezoom]

Mine starts off high, and as time passes, drops about 500 rpm. May not be correct, but it makes me happier when flogging down the interstate.

[ericbushby]

There are only two capacitors in it.
One is 0.47uf 250v. and the other is 0.25uf 150v. for which the nearest preferred value for modern ones is 0.22uf.
the voltage is not critical as the circuit works on 12 volts.
I think anything above 50 volts would be OK.
Whilst there, I would also change the transistors. Mine had just two PNP germanium type in.
Again not critical.
There are two things difficult to do.
First is releasing the glass from its seal. I had to apply release agent for several days to avoid breaking it.
The component changes are easy.
It then has to be calibrated with the built in variable resistor so you will need another means of measuring the revs, some strobes have that feature.
I found a good write up at http://www.lotus-cortina.com/electric/convert.htm and there will be others.
Hope this helps
Eric in Burnley
1967 S3SE DHC
Edit, according to the drawing from Lotus Cortina there is a 100uf cap. which should have failed by now.
They have a working life and need to be replaced. I forgot having changed that, but I would not have left it in.

[bob_rich]

Hi Martin

It depends which tacho you have. On my 1974 +2S130 I had the RVI tacho with the internal current loop.
and it can usually be recognised by the fact that there are two wires exiting the case as the current loop is internal and exits on these two wires. This only uses 1 transistor.

As the tacho warms up the coil on the meter movement warms up and so for a given voltage applied to the coil the current will drop caused by the temperature coefficient of the copper windings on the meter coil. To compensate for this a PTC ( positive temperature coefficient ) thermistor is fitted. Unfortunately I do not have any info on the PTC thermistor.

In my (limited) experience the most usual failure is the capacitor across the base and emitter of the single transistor
it is a 2.5uF electrolytic and changing this is usually beneficial. A modern fit that would be more reliable would be a 2.5uF Tantalum bead rated at 16V.

A benefit of the internal RVI tacho is that the reading is insensitive to the battery voltage. A couple I have repaired
showed little variation of RPM over a battery range of 11 to 16V.

I have attached the circuit of the RVI internal loop taco for info

Hope this helps best of luck

Bob

[ericbushby]

Now you see, I didn`t know about that.
My tacho. is also marked RV1 and yet has two transistors, one 100uf electrolytic and the `plug in type of current transformer on the back. Voltage stability is apparently done with a zener diode.
In my experience all electrolytic capacitors on old equipment should be replaced, because if you don`t do it now, you will have to soon. The electrolyte dries out and the value can change or become unreliable.
By the way, if you look at the write up which I have suggested above they have described moving a `diode`.
This is actually a 150 ohm resistor R7. otherwise the instructions seem OK. The main theme is about conversion to negative earth, but it gives a lot of general useful information.
Eric in Burnley
1967 S3SE DHC

[billwill]

There are as far as I know TWO different circuits for the RVI tachometer.

I think Bob's circuit above is the later one.

But the earlier one is also not prone to car voltage variation as it has its own (9volt ? ) stabalizer inside in the form of a zener diode.

[MartinH]

Thanks to all for the usual stream of advice and recommendations.
Delighted to hear it is not a major issue and seemingly relatively easily fixed. Maybe one for next winter.

Thanks

Martin