Errant tachometer

[FGN59]

Hi all,

I’ve recently established that the tachometer in my late 1968 280SL (manual gearbox) is off, by roughly 25%. It exaggerates the actual revs of the engine by 25%: if the engine is working at 800rpms, the tacho will display 1.000, if the engine is at 2.000 rpms it will display 2.500 rpms.

Does anyone know if there is an obvious fix for this, like a missing or wrong or superfluous gear in the linkage between the engine and the tachometer?

[ja17]

It would have to be a problem with the instrument itself. Everything else is fairly direct.

[WRe]

Hi,
I second Joe's guess and would open the tacho, disassemble, clean and grease/lubricate it or ask Ingo Spenner from Pagodentreff.de.
...WRe

[FGN59]

Thank you both. I will follow your suggestions and let you know what transpires.

[Pawel66]

Maybe just the hair spring is weak or lose...

[FGN59]

Are you trying to take advantage of the weak spot I have developed recently and make fun of me? What is this hair spring thing?   

[Pawel66]

 
Probably I used the wrong term. I meant the spiral like spring that I think (not sure) may be there inside. It creates resistance for the hand axle and also is responsible for return of the hand back to peg.

I speculate 100%. This spring may not even be there. I know in general mechanical speedos and tachos are often made this way.

[FGN59]

Ok, thanks for the explanation, I think I understand what you mean / describe.

What puzzles me is the apparently constant ’overclocking’ proportion of 25% (about), which doesn’t seem to come from a weak, or random, or loose mechanism, but more like a wrong gear or electrical resistance or winding generating a constant effect. I think I’ll try to contact Ingo Spenner or try to find out how the tachometer works before doing anything.

[Pawel66]

I think you might have received a good advise to open it and clean up and see what happens.

Personally - I would just send it to a specialist. Opening it and cleaning up may or may not help. To check it, you need to re-install it - lots of work, risk of scratches of dash, etc. I do not know what are the odds of fixing it by cleaning it up, not sure if it would be worth it. Not to mention you can break it more....

I had the speedo stuck at 120km/h (looked funny in the garage) and the tacho squeeking. Took them both to a specialist, not wanting to risk more damage or double work.