Crank sensor taps for piggyback fuel module issue

I currently am running into a strange issue. I have a Mazdaspeed 3 that will be using port fuel in tandem with direct injection, and I'm tapping the crank and cam signals as well as MAP. Cam and MAP seem to be reading fine, however when I tap the crank wire off of the ECU, it creates a no start scenario. Starts just fine when disconnected. What I've tried:

-Checking for the wire grounding out

-correct wire on the ECU side

-piggyback module isn't grounding that signal wire out

Any insight would be extremely helpful!

Is it inductive or hall type? There may not be enough resistance between the two ecu inputs to get a clean voltage profile at low rpm if it's inductive. If there are ground offsets between the two ecus that could be problematic too.

Are the signal grounds of the two ecus tied together? Do both ecus earth at a similar point with adequate wiring?

If you are only running port injectors off the piggy back you could use the factory ignition outputs via additional transistors or small solid state relays to generate a low resolution square crank trigger pattern good enough for port injection by the piggyback.

I believe it's a hall type sensor. Reads ferrous teeth on the crank gear. I checked the resistance between grounds of the piggyback and where I believe the ECU is grounded at (measures at almost absolute 0 within margin of error). However that's not the first time I've heard somebody mention that piggyback modules that are reading extremely sensitive sensor material like this need to be grounded on the same ECU ground.

Could you elaborate a bit on ground offsets and what those mean specifically? New term in my book.

Actually, I apologize. It is a reluctor style and has a 5v, ground and a shield.

The reluctor is basically just a small coil generating a voltage & current with the change in magnetic field of the trigger passing. At high rpm they are really good because the fast flux results in high voltage shifts but at crank speed they are relatively weak. Hall sensors are basically a compromise to take that signal within the sensor and output a more solid 5V square wave which is easier to use at crank/idle rpm.

It could be either of an earth offset or not high enough internal trigger circuit resistance (and hence not seeing trigger voltage threshold) of the parallel ECUs that is causing your problem.

May be worth investing in a USB oscilloscope to chase the isdue or accepting a lower resolution triggering arrangement as I suggested. You probably want to get as detailed information about the trigger threshold of your factory ecu and piggy back to know your target for consistent starts.

A new/high CCA battery and larger starter power & engine earth may even get you there too. Minimising wiring lenght to the piggy back may make a marginal difference too.