E30 Zone

help my wifes car, a 320i convertible 1988 on an "E" has developed a few probs. One is when you turn off the engine the tacho flicks up and stays on 1000 rpm with a "click" under the bonnet ,like a solenoid, the gauge which tells you the fuel economy also moves as far to left as is poss when you turn on the ignition the tacho moves to 0 and the guage moves to the right about 10 mm.

The tacho reading when off is a known fault which often affects instrument clusters fitted with SI boards using Lithium batteries. It doesn't affect the gauges when running, or cause any other problems.
There are a few things under the bonnet that should click when you turn the engine off.
Ignition on and engine not running : You are not using any fuel, so economy gauge should be fully to the left.