Back of Tach

[archigator]

I have an original Smiths RVI 2419/00 Tachometer on my 1971 Sprint. I have conventional ignition points on my distributor. The tach has not been registering RPMs since I got the car back from my (former) mechanic for repair on some other malady years ago. He swore he did nothing, but it worked before I took the car in...

In examining the tach, I have now noticed a tab on the back that looks suspiciously like a grounding point. Is this tab, shown in the photo below, infact a grounding point? Could this be why my tach is not registering RPMs, or is my problem elsewhere?

Gary
'71 Elan Sprint
Miami, Florida

[Frank Howard]

Gary,

That is a ground connection however Lotus did not use it. Instead, they grounded the tach by grounding one of the two thumbscrews that hold the bracket that holds the tach in position. They used a "daisy chain" black ground wire with several eyelets on it, one of which was connected via the thumbscrew. Hope this helps.

By the way. Nice labeling on the tach wires.

[archigator]

Thanks Frank. I may ground it anyway just for the heck of it. Can't have too much grounding!

With regard to the wire lettering, I print out the label in 5 point Ariel about 6 times, stacked vertically on regular paper. I back the label with doublestick tape and then cut the label out. I wrap the label around the wire and then slide a precut tube section of clear shrink-wrap (got it on eBay), and hit it with the hot air gun. It shrinks down and makes a permanent label... helps me keep track of all that wiring (not my forte). I label each end of the wire as to where it connects on the other end.

Gary
'71 Elan Sprint
Miami, Florida

[neilsjuke]

The white leads that should not be reviseable need to be fitted one way round if the wires could have been changed round by after fit terminals try that you will not damage the unit .
Neil

[archigator]

My Tachometer does not have the wire loop, but instead has a seperate input and output terminal. Are you telling me that if I switched these wires, it might solve my non-reading Tach problem, and I wouldn't fry my Tach in the process?

Thanks,

Gary
'71 Elan Sprint
Miami, Florida

[Frank Howard]

Gary,

Your tach does have the wire loop. You just can't see it because it is internal. The input and output terminals have male and female connectors. If you can rig up a way to switch them, it will not fry your tach but try that only if you have points.

If you have electronic ignition, you are wasting you time fiddling with the RVI tach because you will need to modify the tach as well as the wiring to get the tach to work.

[billwill]

The wire loop is just that! A wire. So reversing the connections won't harm the meter. It carries the current from the contact breaker to the ignition coil. It induces magnetic pulses in the sensor of the tachometer, which are then amplified and averaged to drive the display needle.

On mine the white wire passes twice through an external split loop of springy copper ? which act as a simple transformer & the pickup coil is inside. The Springy copper is prone to bending out a little & failing to contact its other half, so if I take the tacho out, I always give it a bit of polish with fine emery paper & bend it back in to make good contact.

The tacho is fed its power from a simple voltage regulator, which on the original was a vibrating reed thing (yeuch!) which probably accounts for the fact that the tacho needle can wobble back & forward even at constant speed. If the regulator fails you have no power to the tacho & it won't read anything. I fitted a new one to mine a couple of years ago, but as it is a sealed unit I don't know if it is a vibrator or a transistor thingy now. My needle still wobbles! I must check it with an oscilloscope some time.

[alan71]

On my Elan the only thing that is fed from the voltage regulator is the fuel gauge.
The tacho doesn?t need a regulated supply, it?s measuring frequency rather than voltage.

Alan.

[Frank Howard]

The wire loop is just that! A wire. So reversing the connections won't harm the meter. It carries the current from the contact breaker to the ignition coil.

Bill,

It is my understanding that the wire (white) carries 12V from the ignition switch to the ballast resistor where is it reduced before continuing on to the ignition coil.

[alan71]

The wiring diagram shows the voltage regulator on the back of the tacho, although on mine it?s on the speedo.
The 2 terminals next to each other on the regulator are connected together (?U? shaped terminal) so the green wire powering the tacho is 12V from the input and the regulated output goes to the fuel gauge.

Alan.

[billwill]

On my Elan the only thing that is fed from the voltage regulator is the fuel gauge.
The tacho doesn?t need a regulated supply, it?s measuring frequency rather than voltage.

Alan.

IMO: That would be true of a modern digital instrument, but not of a 40 year old analogue instrument. It converts pulses to an average voltage or current to drive the coil of the needle, and its reference is a fixed permanent magnet. So it woulde be affected by change of supply voltage.

The circuit was on a link here somewhereabouts
http://www.lotus-cortina.com/electric/convert.htm

[billwill]

The wire loop is just that! A wire. So reversing the connections won't harm the meter. It carries the current from the contact breaker to the ignition coil.

Bill,

It is my understanding that the wire (white) carries 12V from the ignition switch to the ballast resistor where is it reduced before continuing on to the ignition coil.

Ah, I wrote that off the cuff. The essential ingredient is that the white wire carries the ignition current pulses.

Mine 67 Elan doesn't have a ballast resistor, but on thinking I do recall that the white wire is in circuit between the coil and the ignition switch.

[alan71]

Bill,
I agree, internally the tacho needs fixed amplitude pulses but they are regulated a zener diode.
If you look at the circuit in the link you posted,Q1 switches between supply and 6V below supply, so the pluses to the meter will be 6V even if the supply voltage varies.
There is no need to connect the tacho to the voltage stabiliser. The wiring diagram is a bit confusing here but it?s just showing spare terminal on the input (battery) side of the stabiliser feeding the tacho.

Alan.

[RichardS]

I concur with Alan - on my 1973 Sprint the fuel gauge is the only instrument fed by the voltage stabiliser which is on the back of the speedo.

I am in the process of tidying up my dash wiring and as the original voltage regulator was looking rather tired and I have replaced it with solid state one which are advertised on Ebay and screws nicely into the original space on the back of the speedo.

Incidentally does it matter which way round the fuel gauge is wired in? I am pretty sure mine had the 12v feed from the voltage regulator to the left hand terminal [looking from the back] and the wire from the fuel tank to the right but forgot to make a note!

Richard

[alan71]

Richard,
The fuel gauge relies on the current through it heating a bimetallic strip to move the needle, so it doesn?t matter which way round you connect it.

Alan