Tacho on the fritz?

[davo5]

Hi Guys,

1991 NA6, completely OEM, no mods, no engine warning light (if it has one).

Occasionally whilst the car is idling, the RPM will flicker to VERY low, like 150 - 200rpm. But as hard as i try to listen, the engine makes no noticeable change in sound or NVH, its like its still humming away at 800 / 900rpm, so given occurrence = low, severity = low, its a problem i have been mostly ignoring.

What raised my concerns was tonight whilst driving home at around a very constant 70kph ~3000rpm, the tacho went for a blip, to 3500rpm for a second or so, the engine didn't hesitate, lurch or backfire, but the tacho just went briefly nuts. I've been out for a drive since to try and replicate the fault but i can't get it to do it again.

Is it common for the the tacho to fail in older MX5's and flicker from time to time irrespective of correct engine operation? Or is this indicative of something more serious, like immanent failure of the crank angle sensor?

[Garry]

I have read that a bad earth can do this. I think the particular earth point they mentioned was the one on the throttle body. Maybe you could have a check of all the earth points and look for bad contacts or loose wires?

If there is no change in engine performance when the fault occurs it would indicate that the fault is in the tacho wiring somewhere.

[Matty]

If it was a sensor issue you would definitely feel the car lugging/surging/stuttering.

[daffyflyer]

mine often flicks up and down a bit too, always has.

And sometimes when its idling at the lights it flicks to 0 and everything goes quiet.... oh. thats the cams