Tachometer noisy

[Tintin]

I'm posting this in "Engine" because I can't see an "Electrical" section for this model.

My tachometer (impulse drive from the SW side of the coil) has developed an annoying ticking sound. Has anybody had one of these apart to see if they are repairable?

Thanks.

Robbie

[BrianG]

When you say a 'ticking' sound is this related to varying the engine rpm when the car is stationary? I say this because the problem may not be due to the electronic drive (unless this has some kind of fault) but due to road vibration rattling the innards of the tacho.

Essentially these tacho movements are typically a 240-degree moving coil d.c. milli-ammeter, and the jewelled pivot bearings may have become loose with time allowing the moving coil armature, which carries the pointer, to rattle.
Normally 'Smiths' type instruments (speedo or tacho) are only dismantle-able by first removing the chrome bezel ring around the front,then removing the rear screws and then withdrawing the entire mechanism and the glass through the front. Unless you have the special swageing tool to undo the bezel or re-fit it (it slides around under the rim to lift it or rolls it over on refitting), it will look a mess if you try and remove it with a screwdriver etc. If you can remove it you may be able to adjust the bearing pivots carefully with a watchmaker's screwdriver to remove excessive end-float and stop the rattle.

Might be best to get an exchange unit?

[Tintin]

The noise is present with the car stationary at idle and up to about 1500 rpm when general engine noise begins to drown it out; on release of throttle the noise stops as revs fall back to 750 and then returns after 15 seconds or so. The ticking is rapid - several clicks a second - as if generated by a rapid opening and closing of an electrical circuit. I will investigate further for looseness and perhaps run it with the instrument removed form its position in the dashboard to try and identify it more precisely. From what you say Brian, repair sounds a specialist sort of job and I see on Google there is an outfit in Wales who carry out instrument repairs - might send it off to them if its not a simple fix.

Thanks.

Robbie

[BrianG]

The noise is present with the car stationary at idle and up to about 1500 rpm when general engine noise begins to drown it out; on release of throttle the noise stops as revs fall back to 750 and then returns after 15 seconds or so. The ticking is rapid - several clicks a second - as if generated by a rapid opening and closing of an electrical circuit.

Robbie,
That suggests that the meter pivots have a bit of play in them. The circuit inside most of these is not very sophisticated and the pulse of current to the meter at low rpm can sometimes set the coil/pointer movement rattling. (The meter mechanical damping should smooth out the individual circuit pulses at low rpm, but can sometimes still show a small 'fluttering' of the pointer at idle speed.) If you remove the tacho and shake it fore and aft it will probably make a similar noise to confirm this diagnosis.

A specialist repair should be able to improve the situation.

Regards, Brian

[Tintin]

Thanks Brian - that's most interesting and very helpful.

Robbie