Tachometer problem

[S2Andy]

Hi Gents. My S2 rev counter has started misbehaving All is well for a few miles then it starts reading wildly. After a while it will settle back down only to start misbehaving again for no apparent reason. I've checked the obvious earths, coil connections and fuse and the engine itself is running superbly but all to no avail. Sometimes I feel that other electrical items coming in exacerbates the problem eg engine fan.
Is it likely to be the instrument itself? Whilst its entertaining to see 8000rpm I don't believe a Cologne will do that. Thanks Andy

[GreenV8S]

There are two common ways for the rev counter to work - voltage sensing or current sensing. Current sensing would usually be very reliable but is much less common in modern cars in my experience. Voltage sensing involves the tacho picking the ignition pulses out of the electrical noise on the ignition LT circuit. Anything that changes the noise or voltage characteristics of the ignition LT circuit could cause the tacho to get it wrong. Since you already checked the obvious ignition components and since the engine itself is apparently running well, I'd be looking for issues outside of the ignition system.

Check the alternator output voltage and battery voltage. If these are too low or too high, that might upset the tacho. Check for voltage drop between the ignition output at the switch, and at the coil. The 'big yellow connector' in the steering column is notorious for breaking down with age and might be causing a voltage drop which is confusing the tacho.

[phillpot]

A Cologne will do 8000 rpm........ but only once
.

[S2Andy]

Thanks for the comments. After searching the internet going to check out Battery and Alternator tomorrow.Yellow connector already sorted.

[Barry S1]

Mine just stops when the car gets hot and starts again when it all cools down, I thought it was normal

[phillpot]

GreenV8S said:

There are two common ways for the rev counter to work - voltage sensing or current sensing. Current sensing would usually be very reliable but is much less common in modern cars in my experience.

Hand written onto the wiring diagram in my Steve Heath book is "current sensing" , my car is a 1990 early S3, think I came to that conclusion after reading a thread on here some years ago. Can't recall the writer but I must have been convinced he knew what he was talking about!

I could of course be wrong

[S2Andy]

Thanks for all the comments. I disconnected everything and put it back together and it seems to work. Hey ho.