Tachometer reading full scale intermittently

[Doddsy]

The tachometer on my Series 3 OTS has developed an intermittent fault where it goes to full scale (7000 rpm) and stays there. It has returned back to normal so far after several minutes. Tapping the tachometer doesn’t seem to effect this.

Has anyone experienced this before and does it sound more like an internal problem or a wiring issue? Would an open connection cause this? My car has the Reopus ignition system fitted.

The tachometer hasn’t ever been out in the 40 years that I have had the car so probably due a service.

[cactusman]

If the car runs normally it is a fault within the tacho....it's driven by a pulse from the ign and essentially the tacho electronics converts the pulse train to a voltage that is pulse frequency dependent. The actual gauge is just a big glamorous volt meter! I'd say a service would fix....speedograph Richfield will sort it out....