Last year, I found a replacement tach for my 1987 944S.
There is nothing wrong with the original tach, but I found one that integrates a fuel consumption gauge




Today, I mounted the new instrument in my cluster, and connected everything again.
When I started the car, the consumption gauge works, but the tach is dead.
My first thought was to transplant my original tach to the new instrument, but unfortunately that appears to be impossible. The printed circuit boards are totally different, and what is even more amazing is that the "new" tach has 4 connections instead of 3 on the original tach.

This is the original tach:





And this is the "new" combined instrument:







I don't know from which exact type of 944 this new instrument came from, and I can't seem to find any information at all about schematics or testing.

So now I'm looking for any info about this instrument, how I could troubleshoot and test it, or maybe even repair it.
Any help would be greatly appreciated!