Overcounting rev counter diagnosis

[ESM]

I've been looking at my huge folder of log files from zillions of cars to see if I was right to think this data was particularly noisy. Now I'm not so sure. It's at the upper end but most I've looked at (that have a sustained period at 4000rpm or so) show noise on the rpm data that isn't wildly different. Back to the drawing board!

[steve411]

On my way up to Curborough on Sat I was sitting for long periods on the motorway at 4000rpm. After about 90mins, I noticed that I was doing 4500 but still at the same original speed. Stopped for breaky soon after and when I got going again, it wasn't long before the rev counter had climbed from the "correct"? 4000 to 4500 then 5k and eventually 6k without ever changing speed significantly. Sometimes it did drop back down to about the right reading.

I've done a search on here and come across threads that suggest 1) a bad earth or 2) a duff Alternator.
Problem is I'm a numpty at this sort of thing. What exactly do I need to check where to find out which if either of these things is the problem. Any other ideas welcome too.

Hi my name is Stavros from Athens and i am an S2 owner. I have the same problem with my rev counter.
Before checking and changing things could you tell me how you resolve the problem?
Thank you in advance!

[MPx]

Hi Stavros - I'm afraid its one of those issues never solved - I seem to specialise in them! I'm happy to report its a relatively rare occurrence for me. Typically happens on a motorway at a fast cruise. Should be reading 4-4.5k and I suddenly realise its suggesting 5 or 6k. Always drops back to a correct reading in the end and I've learned to ignore it! In my investigations I have been told about the (to me !) overly complex driver of the Tacho - something along the lines of converting a sine wave to a square wave or vice versa from one of the sensors. I'm not knowledgeable enough to understand it and so far no-one else has been able to explain it to me, so I still don't really know how it should work. But since its fundamentally electrical and not mechanical, it is likely to be a poor connection or earth somewhere - such that the voltage presented to the instrument is not correct. If you find out anything more, please share.

[steve411]

Hi Stavros - I'm afraid its one of those issues never solved - I seem to specialise in them! I'm happy to report its a relatively rare occurrence for me. Typically happens on a motorway at a fast cruise. Should be reading 4-4.5k and I suddenly realise its suggesting 5 or 6k. Always drops back to a correct reading in the end and I've learned to ignore it! In my investigations I have been told about the (to me !) overly complex driver of the Tacho - something along the lines of converting a sine wave to a square wave or vice versa from one of the sensors. I'm not knowledgeable enough to understand it and so far no-one else has been able to explain it to me, so I still don't really know how it should work. But since its fundamentally electrical and not mechanical, it is likely to be a poor connection or earth somewhere - such that the voltage presented to the instrument is not correct. If you find out anything more, please share.

Thank you very much for your answer! I am looking the electrical manual and all the previous related post.At the beginning i was thinking the coils, later faulty Tacho, but after all the posts i am thinking like you said a poor connection or earth somewhere. For example maybe an open ground circuit CKT YR11 K(reference sign ground), which "create electrical noise" (i read about this in ΕΜJ.5), could cause that symptoms. If i find anything more i will share here. Thank you again!