E30 Zone

Yo dudes, could anyone shed a bit of light for me please. Im in the process for repainting my car from brilliant red to graphite grey, i've got the engine back in now and runing but my rev counter has stopped working. Before i stripped the car the rev counter would now and again stop working for a second or three and then just start working again. Since rebuilding the car the guage just hasn't worked, I've checked that it is plugged in and that is all fine i just don't know where to look next. Any help would be great thanks for reading.
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Rev counter is sensitive to a poor connection at the black wire at the main engine loom plug and socket by the fusebox.
Other likely possibility is that your SI board batteries in the instrument cluster have died while disconnected from their life support of the main car battery. Do the Service interval lights work correctly?