No revs on the rev counter

[james-t]

I have a series 2 1970 with a rev counter that does not even flicker when engine is rev'd. I presumed the instrument was faulty so sent it to Speedy cables for restoration and repair, (nine weeks later) it was duly returned .Now installed and connected still not a flicker. I have checked all the wiring to the coil and fuse even connecting a separate earth to the instrument but nothing. The engine starts and runs ok and the points are set correctly.Unless any one can give me a possible solution it will be back to speedy cables for them to check that the instrument functions. (something i am not keen to do as i may not see it again for another 9 weeks). Any clues or help would be greatly received.
James-t

[christopher storey]

James : the S2 tacho ( assuming it is an original one ) is an impulse instrument which works by counting the ignition pulses. It sense these by the two coils of white wire which are looped at the back of the instrument and which lead from the ignition switch to the coil and indeed provide the feed to the coil . However, the moving coil mechanism which drives the pointer is powered by a switched 12 volts green wire from one of the fuses . If this wire is not connected you will get no reading . To test it, take a temporary wire from any 12 volt source in the let down flap of the centre of the dashboard ; if it then works , you have your answer and you need to look for the disconnected green wire

If it has been altered to some other system, then without knowing what the system is it is hard to help you. Perhaps you can give us full details of any ignition mods which have been made because if your car has been converted to electronic ignition it is quite common for the sensing tacho no longer to work , and I am afraid it is then necessary to substitute a suitable printed circuit board and associated components for the original electrics in the tacho

I hope this helps

[james-t]

Thank you for your input on this problem. I will test the power source on the green wire at the fuse.I have tested the green wire for continuity on a meter but not for volts . The ignition system is completely standard ie points ,condenser and a Lucas sports coil. The rev counter should have been refurbished by speedy cables as standard ignition as requested. I will test the green/power supply and re post with results.

[christopher storey]

James : I should also have said that the tacho is very earth sensitive . You will need to check not only that it is earthed to the back of the panel ( usually by a black wire with a hoop connector which attaches to one of the posts which hold the clamps with the knurled nuts at the back ) which you seem to have checked already , but also that the panel itself is satisfactoily earthed to the frame of the car - this is by no means always the case as the plastic covering sometimes interferes

[Patrick NICOLAS]

Having had a S2, I seem to remember that the fuse that powers the tacho also supplies the cooling fan. If my memory is correct, this failure must be taken seriously.

[PeterCrespin]

James : the S2 tacho ( assuming it is an original one ) is an impulse instrument which works by counting the ignition pulses.

Not so Chris - you're thinking of the S3 tach (he said, handing his friend a large fig leaf).

The S2 instrument uses the loops in the white feed wire to induce a reading in the instrument proportional to the current (not pulses) flowing along it to the coil. It doesn't count the switching effect of points or electronic ignition like the S3 and modern tachs.

A consequence of this is that without the tach being properly in circuit there is no feed to the coil during normal ignition switching, although for ballast cars the engine will fire during cranking IIRC. Hence the problem is normally non-running engine rather than a non-functioning tach.

In this case, if the tach is good it is not correctly wired. The fact that the engine runs without it suggests there has been some jiggery-pokery inflicted on the car's wiring which will need to be amended to get correct tach function.

Pete

[christopher storey]

I stand by what I said . The S2 tacho senses the ignition pulses . It is an RVI 4610 instrument on my S2 and I have completely rebuilt mine with new capacitors and resistors . It works, I agree, by assessing the modulation of current, rather than the later instruments which assess the variation in voltage, but the principle is identical : it counts pulses occasioned by the low tension circuit being interrupted by the opening of the points . This is one of the reasons why these instruments often cease to read when an electronic system is substituted with a totally different triggering mechanism for the low tension circuit

[PeterCrespin]

Fair enough. I just think that impulse tach is a term classically used for the 'normal'direct connection voltage-sensing type that can be bought anywhere for practically any car. they are two simple two-connection type fitted to Jags from the S3 E and contemporary XJ onwards. The S2 type used briefly between S1 and S3 works indirectly by induction.

You're right that it's the energy pulses flowing to the coil that are responsible for inducing the transformer current measured by the indirect S2 type, I've not seen those referred to as impulse tachs. Otherwise you'd have the same name for two distinct types of instrument, which is where confusion can arise.

A minor point though, so I'll happily cede to your greater practical rebuilding experience, where I've simply transferred the guts of an impulse tach into an early AC tach casing.

Pete

[christopher storey]

Peter : ( and anyone else who may be interested ) one of the minor advantages of the electronic rather than the AC generator type tachometers is that as you crank the engine over you can actually see the pulses as the needle jumps very slightly. If you are not getting a start, that at least tells you that the low tension side is working and that you should not need to grovel around trying to see if the points are ok !

[james-t]

Gentlemen, thank you again for your suggestions.Here is the update.The rev counter still not working.I have run two separate earth wires to the back of the tacho. I have 12 volts on the green power cable from the fuse to the connecting plug and out of it. The engine starts and runs. The white wires with either a red or black trace are connected to their respective white wire into the connecting plug. I have spoken with speedy cables and they confirm that the rev counter was set up for standard ignition. They have agreed to test the unit whilst i wait and will be visiting them Wednesday - unless some inspiration comes from such knowledgeable member of the forum.Thanks again.