I need help with this really really baffling issue,

The engine idles at 900 rpm with ELR connected on pump, when I test the voltage, I get 12V solid and it drops as soon as I accelerate the engine. This behaviour in the manual says that the rpm sensor is bad - replaced with a spare - no dice. The engine still idles at 900 rpm whether hot or cold

The manual says to check rpm sensor (my tach works perfectly)
the 4 plug temp sensor works
the wiring harness is 4 years old (date stamp) on harness

Is my ELR module bad - and where should I be looking for one.