E30 instrument circuit board adapted for E30 M3

It is not uncommon for E30 BMWs to develop intermittent malfunction of dashboard instruments, in my case involving the coolant temperature gauge and the on board computer (OBC).  The more common faults to investigate should involve the mounting stud of the temp gauge, and improving the solder surface that acts as a ground for the gauge.  It is also wise the check the continuity of the traces on the board with a multimeter and resolder all the connections to the appropriate connectors.

When all this has failed to bring improvement, it may be time to try replacing the entire instrument motherboard.  In my case, a new E30 M3 motherboard is hard to find. and then is expensive.  The ordinary E30 motherboard is nearly identical and cheaper, except in providing a socket for the ABS warning lamp.

Look at the lower right, and you will see a blank hole in the 3rd socket from the right.  This is an E30 board that I got on Ebay.

The job is to make the round hole in the middle into the same shape as the others.  I used a piece of blank PC board and traced and cut a hole using this board as a template.

This piece was then epoxied to the back of the E30 board.

Copper foilwas burnished over the contact areas for the bulb.  This is a craft store item used for stained glass.

 

Solder is applied.

Jumper wires are added to the connectors at C1 (blue) 15 and 23.
 

I used the original back which has the cutout for the tachometer.  It is possible that the original faults with the M3 board were fixable, but I tried everything I could think of and saved this for last.  Before you run out and buy another OBC, you should try this first.