tacho error...

My tacho reads around 900 rpm with key out and ignition switch at off.
When i turn to battery on pre ignition point it jumps to around 1400 rpm.
When engine idles it reads around 2100 rpm.

Are these tachos adjustable in anyway? Is it pulses or voltage driven?
Was possibly interfered with recently prior to car purchase when the speed had to be changed to kmh from mph though can't swear on it.

what car is it as i think the old motos are cable driven from gearbox and modern are pulse from one of the many sensors ( someone will be along in a bit that knows)

you need more salsa..

1998 disco v8
not highly critical unless i get cruise control working and then i can cut out at 5000 rpm which would be more like 3500

where does the signal come from on a v8..... is it from the ignition coil or from the alternator like on a diesel. tonce you know where it comes from you can isilate the source to determine whether its a fault in the electrickery in the dash or summit going wrong in the ignition or alternator. the diesels use a signal from the alternator so disconnecting it is a simple way to isolate the fault. but your petrol motor may use the ignition to provid the signal.

thanks ricc, time to learn how to navigate RAVE!

ricc, looks like you were on the money.
I believe it is the alt that sends the ac signal to the tacho.
I guess check the alt works correctly first, should be a matter of running the engine and checking batter terminal volts pick up post ignition.
It does seem to work linear with acceleration so I am unclear what could cause the offset and unless it is some coil on the tach itself i may be chasing earth connections all over europe!

result!
it ain't the alt...12.6 volts key out and marginally over 14 idle.
can't see it being belts as it's not erratic when running so slip or slack for the mo i am ruling out and the belts seem ok, not sure of tensioner etc but that's a post check just for having it right.
strange is the bounce on the tacho when keyed to power pre ignition, seems a dc current is effecting something i have to assume is normally driven by an ac voltage. gawd, magnetics!

had the cluster off and can't see any smoking guns.
so i have sourced the old mph cluster to check an other tacho and see if it is simply the tacho coil which is my best hunch for now.
related to post on fuel consumption next will be to actually check idle, need to figure out how to use a timing light and set retard etc.
Whilst taking the old cluster off realised that whoever last took it off made a real mess of it, pulled it up and off without removing steering wheel so screw hold area broken and tabs in the back shattered. My first purchase will be the instrument bezel...anyone know where to source a tan coloured instrument bezel, no need for switches etc. just the actual facia piece. Also looks like some serious post works done, looks like SRS or ABS added after manufacture??? New yellow loom running to the bumper sensor.