The OBC on my 1990 has throw a wobbly.. Oddly when you crank the car the display is OK until the motor fires then the top half of the digital display is blank, the lower part OK.??

I've disconnected the battery for half an hour and re connected and that made no impact.

 

Could it be a bad display? Is it blank, showing the lower half of the characters? Is the backlight working? Try shining a light at it to see if the LCD portion is working...

I have a spare OBC Tach (has backlighting issues) that I could help you out with if you need it.

 

back light works but it only shows the bottom half?

I'll rip it out tomorrow see if anything is playing up

 

IT sounds like a circuit board issue.. I know the clocks have had issues with soldered joints.. but the OBC tach doesn't have the same kind of access from the rear. You may have to be careful getting the case open, IIRC, there were no screws or any kind of user-openable fasteners back there...