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Mborras.
I see we are chasing similar things from my other thread. I also see this is an old thread, but useful.

I call the tachiometric relay the "fuel pump relay" or "drive relay". On my 84 spider, the black/white wire on that relay is on pin 31b and is the tachy signal from coil (-) . Do not ground that wire. The ignition ECU switches that wire to ground during cranking/driving. That same coil terminal (-) should also have a shielded yellow wire that is the tachy signal to the FI ECU pin 1. This yellow wire carries the tachy signal that is massaged by the FI ECU to provide the FI pulse. (Tomorrow, I will spend some time figuring out if I get the tachy signal back at the ECU.)

In re-reading two-point-o's post, I believe he spliced and hard grounded the black wires on pin 85 of the main relay (which is also connected to pin 31 on the fuel pump relay). My schematic has those black wires grounded through the FI ECU. If those black wires were not grounded, the FI ECU would not get power, nor would the injectors get their 12V supply (via the main relay), and the fuel pumps wouldn't give their initial pre-crank kick. So, I think there would have been bigger issues if those black wires weren't actually grounded through the ECU. I do not see how two-point-o would have measured 12V at the injectors if these wires were not grounded, since the main relay would not have been energized. I also don't know if we're all talking about the same car/jetronic. Do you have a schematic?
 

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My understanding is that pins 16 and 17 are two of three grounds needed for the FI ECU. The other is pin 5. Yes, those wires ordinarily go to a ring terminal that is bolted at the AAV mount. I'm most interested if that is your issue!
For me, I have continuity and grounds are good, which is a good thing, but not-so-good in that I have to look elsewhere.
Good luck. I'm waiting with bated breath.
 
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