xsvboost

I have an turbosmart eboost2 boost controller and tapped into the ecu rpm signal wire. The eboost is picking up the correct rpm readings but it appears that the signal is then not picked up by the factory tach. Having unplugged the eboost from the spliced connection the signal is restored to the factory tach, it's apparent that it's one or the other despite the solid wire connection. Any assistance would be appreciated.