Twitchy tacho.

[rmd24]

Had my S4 for a week and a bit. Nearly all the initial niggles sorted, but suffering from twitchy tacho syndrome. When the engine is first started the tacho operates normally for about ten minutes or so but then becomes increasingly erratic swinging over the whole scale. Amazingly a touch on the brake pedal stabilises it but as soon as the pedal is released it starts to swing again. Other tricks it performs are a little jump when the headlights are turned on, and a swing over the whole scale when the ignition is first switched on.
Obviously an electrical glitch somewhere but I don't really know where to look.
Any help would be much appreciated.
Roger.

[Frank Howard]

Roger,

Do you have electronic ignition?

[rmd24]

Nope. Coil and distributor and RVI tacho.

[alexblack13]

Hi Guys,
Hey Roger, My one has developed a similar 'life of its own'.. Completely random. I am suspecting a duff connection somewhere within the bowels of the instrument. This morning I had the car out and it stopped completely. I tapped the face of it and away it went for the day... haven't seen it since!! Only kidding... But it stayed working OK for the day. It will stop again when heading off on my hol's on Wed' though. G/teed!!

I think the fact that a 'tap' with a finger gets it working again suggests a bad con' either in or at the rev-counter itself..

Just my luck..The weathers not to be the best either.

Alex.....

[adigra]

Mine was all over the place as well, but ?52 at a specialist for a rebuild and refurb/paint, sorted it out (with a warranty as well).

Adi

[alexblack13]

Mine is scheduled for overhaul during this winter. The instruments are in nice nick and to get to the revcounter I will be taking out the speedo 1st. I don't think it need touching although I am tempted.....

Hey Roger. I think I would recommend having yours checked / serviced..

Ready for the off Adi...

Alex.......

[Frank Howard]

Nope. Coil and distributor and RVI tacho.

Roger,

Having a coil, distributor, and an RVI tachometer does not necessarily mean you don't have electronic ignition. Let me ask the question a different way. Do you have contact breaker points?

[deebee]

rmd24 - The circuit board connections may be the issue with your twitchy tacho.

The tachometer in my 68 DHC went its own way recently by deciding to read about double the actual revs. The 'double' reading occurred after I replaced a dead electric fuel pump so that change became the first suspect. To add to the mix of unknowns, the tacho began to pulse when the indicator was turned on after the replacement pump was installed. The pulsing fluctuation increased substantially with an increase in revs. So much so that at an indicated 4000 rpm the tacho oscillates between 3000 and 5000 rpm. I have a standard coil and distributor.

Much testing and another replacement pump and some rewiring proved that the fuel pump was not the cause and an auto electrician reckoned that the electrics were OK. Earth points to the chassis were all made good.

I eventually had the tacho checked by an instrument repairer who re-soldered a number of connections on the circuit board. This fixed the 'double' reading - but I still have the pulsing when the indicator is used. (Any ideas for this issue anyone?)

[billwill]

I would guess that the supply voltage to the tacho is reducing when the indicators are on, so check the bits of the circuit that are common to both & clean any contacts & joints on the shared circuits. Especially check the earth circuits.
, just tightening them is not enough, you need to undo them, clean them of crud & oil & then retighten them.

[Mr.Gale]

Another item I would check is the pick-up connector on the back of the tachometer. I would remove the little brass nut, pull the "U" shaped connector with the block that has the loop or wire through it and clean the connector with fine sandpaper and/or contact cleaner. You should also clean the connectors on the back of the tach.

Gale

[StressCraxx]

On the back of the tach, there is a little device with a power wire going to it. It is the voltage stabilizer. It works well for a while until it gets warm, then starts to get flaky. It is a small points/voltage regulator. I believe the output is 10 volts to the instruments.

There are new solid state ones for sale on fleaBay. They are also available from the usual brit car specialists.

Regards,
Dan Wise

[rmd24]

Thanks for the ideas guys, I'll go for the easier ones first. The question arising from this is how to get to the back of the instrument. The speedo is obviously accessible but the valance under the dash conceals the tacho. Is this easy to remove?
Thanks,
Roger.
P.S. I have had a long line of Lotussssesss, starting with a 7, Lotus Cortina, Europa and an Elise but this is my first Elan, hence all questions!

[69S4]

P.S. I have had a long line of Lotussssesss, starting with a 7, Lotus Cortina, Europa and an Elise but this is my first Elan, hence all questions!

Sounds like you like a challenge. Trying to keep these things going tends to put all of life's other problems into perspective!

The under dash valances are held on by a few recessed screws. If you look closely at the moulding you'll either see a few small holes (4-5mm) that you can put a small screwdriver through or some small rubber bungs covering the holes. The screws go into the edge of the dash.

After you've removed the moulding it's a lot easier to get to the tacho although I've found it easier to take the speedo out as well so you can get access through the speedo hole. It's probably overkill to remove the steering wheel but it does make the bodily contortions needed to get your hand up behind the dash a bit easier.

[billwill]

On the back of the tach, there is a little device with a power wire going to it. It is the voltage stabilizer. It works well for a while until it gets warm, then starts to get flaky. It is a small points/voltage regulator. I believe the output is 10 volts to the instruments.

There are new solid state ones for sale on fleaBay. They are also available from the usual brit car specialists.

Regards,
Dan Wise

The original stabilizer is a vibrating reed thing, highly unsuitable for feeding power to an electronic device, so I have never been sure if the power to the tacho passes through it. It's fine for slow instruments like thermal bi-metallic fuel gauge indicators though.

The wiring diagram in the manual is very hard to follow, so I've never checked that thoroughly, though I'm pretty sure that mine after being rewired is not feeding the tacho through that stabilizer.

A modern solid state stabilizer would be much more sensible there.

[simonknee]

That voltage stabiliser is only for the fuel gauge and other gauges that have voltage based sender. Check out the classic smiths wiring diagram. http://www.lotuselan.net/uploads/smiths ... wiring.jpg All lotus looms and diagrams should conform to this but the drawings are not easy to follow.

The reason that it is on the back of the tach is just that it is a handy ground for the casing of the unit.

A word of warning is that it is possible with some looms to connect things wrong and have this unit powering stuff it shouldn't. If you suspect that you or a DPO might have done then you should disconnect the wires from the voltage stabiliser and ensure that only the appropriate instruments stop working.

The fault is probably a dry joint in the tacho. Open it up and reflow all the solder points you can get at. You may be able to spot the dry joint as you go by wiggling the legs of each component and looking for movement in the solder joint. Treat it carefully. Please use a nice weller iron or similar and not one of those nasty gun shaped things.