Got a sort of result! Lots of reading the maintenance manual gave me the idea that there was some sort of selection process to differentiate between an RIV sensor and a KG (injector line ) sensor, and it was a short between RIV/KG select line and KG low volts which is also fed volts cc via a pot. devider
So it recognised the device, a simple spark inductive pickup, consisting of a ferrite loop and coil of about 0.3 ohms...

I had to test it on my mower! (I don't drive a gas car and the Honda is awaiting parts,) it is reading RPM x 2 I believe!

Bcak to the drawing board! I might just try putting the inductive loop on the KG (inj line) input next!