RPM counter doesn't work (after engine swap - need help
 

Hi,
I need your brains please.

So I recently did an engine swap to my 79 SC
- old 930.06 out
- new 930.20 in

Motronic and wire harness was fully working fine in donor car.
My car runs as well.

But not the rpm counter :-(

Acording to wire diagram the signal comes from pin 21 of Motronic and goes to pin 2 on rpm counter (black/violett wire).

https://up.picr.de/43245151qd.jpeg

I thoroughly traced that wire and confirm it comes from pin 21 and reaches all the way into the frunk. Made a firm extension from there to rear of cockpit.

I have 3 rpm counters (yes, working, tested):
- a late 78 from my SC
- a Carrera one stamped 87
- a Carrera one stamped 88

My connection with both Carrera rpm counter:
- mass to pin 6 (existing brown wires)
- ingnition plus on pin 5 (existing red/black wire)
- signal/TD to pin 2 (black/violet wire)

My connection with SC rpm counter:
- mass to pin 1 (existing brown wires)
- ingnition plus on pin 3 (existing red/black wire)
- signal/TD to pin 2 (black/violet wire)

No reaction from any of the three counters... where is my "problem"?

Thank you

https://up.picr.de/43245152sy.jpeg

https://up.picr.de/43245153ax.jpeg

Your Instagram shows a working tachometer. What was the culprit in the end?

Yes. Works now.

Wiering was unlogic. PO did something hidden to my understanding.

Never trust wiering you didn’t do yourself, haha