Anyone know which (Unused) pin on the ICU can be used for the tach? I have the replacement tach coming today and want to try that signal...
Pin 7 - if you have an OEM wiring harness there is currently nothing connected to Pin 7.
BUT - Not all OEM BMW marked ICUs implement Pin 7, nor do a number of copiers. However I can state categorically that the ORIGINAL fitted ICU to all bikes 1981 through 1984 does implement Pin 7. Copies manufactured by (amongst others) Werle, Siemens and a large number of nameless European and Chinese manufacturers might, or might not.
It gets worse. the "grandfather" of all these devices is the Bosch 0 227 100 124 (which will work perfectly in our BMWs by the way). But the same unit manufactured by Bosch USA does not implement pin 7.
Late model "high power" ICUs (own irremovable heat sink and "slanted" mount) - I don't know.
I have a selection of BMW, Bosch, European copies, Chinese copies and unknown origin copies (Endurolast). To date I have spent time working out the timing differences between them and why that might be so, I have never spent any time working out which ones fully implement the tach pin.
Given that nobody I know of has ever had to use the tach pin on the ICU to get a signal for their aftermarket tachometer, and given the fun and games you will have just to access that pin (and the easiest way I can think of is to find a European car wreck that used the same module and cut the connector out of the wiring harness and then fabricate a "remover" to extract one of the sockets which you then place in your BMW ICU socket) I would say find a better tachometer, or at least one from your current manufacturer that isn't faulty.