E30 convertible instrument cluster relay

[Roweygym]

Hi guys .have had some mayor issues with the instrument cluster but have Just changed si batteries and everything appears to be working okay but rev counter is either in correct position or showing higher revs when still ticking over . This was not working at all prior to replacing the batteries. Have a donor rev counter so may just swop it over and see if it reacts the same . Need to check resistance on fuel senders (dual floats ) as needle is bouncing everywhere as usual . Have cleaned earth nuts so all good there . Question i have is the black relay block sitting against the firewall to the right of the steering column the relay that controls the instrument cluster ? If it goes faulty what would I be looking at as an indication ? ?

[Brianmoooore]

The large black box has just two tiny relays inside of it, in one corner of the circuit board, and should only be there if you have an OBC in place of the analogue clock.
One relay is for the CODE alarm horn and the other is for the CODE immobiliser.
It has absolutely nothing to do with the instrument cluster.
To check your fuel level sensors, ensure you have less than half a tank on board, then pull off one of the plugs and short it out with a wire link. Drive the car like this, and see if the gauge is stable, albeit giving an incorrect reading.
Remove the wire link, push the plug back on, pull the other sender plug off, short that with the wire link, and drive the car.
The faulty sender is the one that gives a stable reading when its plug is shorted out.