93 Miata PNP2 comms issue, suspect grounds
 

I have a 1993 1.6l Miata set up with a MS PNP2 and an AEM UEGO 30-0300 oxygen sensor.

BLUF: I am having communication problems between my laptop and the PNP2, where it kicks out comms. Disconnecting the cable and reconnecting will start it again, and then it kicks out again. I contacted EFI Analytics with a log file and they suggest I have a sensor grounding issue that is causing a spike that upsets the RS232 chip. Disconnecting lets it reset. Any advice as to where to wire what connection will be appreciated!

Additional Information:

• Using a shielded RS232 cable and a separate RS232 to USB adapter. Note, I also have a RS232-USB adapter and long USB cable that does the same thing.
• The PNP2 is from DIYAutotune and is connected to the existing Miata wiring harness. The case of the PNP2 is not intentionally grounded, but it is attached to metal brackets screwed to the bolts in the floor for the stock Miata ECU cover plate.
• Current AEM wiring as shown below

https://cimg2.ibsrv.net/gimg/www.mia...e1952009b4.jpg

I suspect I need to separate the AEM O2 power and signal grounds, and should probably run a separate ground to the PNP2 (connecting the Sensor ground).
BUT! maybe one of you has fought this gremlin and can offer advice. Thanks in advance!

******* update *******
I ran two wires through the firewall and over to the MS PNP2. I could connect the

• brown wire - 0-5 negative to pin 22 Signal Ground
  AND
• pin 21 oxygen sensor input (bypassing the other O2 sensor input using the Miata wiring harness)
  OR
• pin 5 Chassis ground to the firewall near the megasquirt OR up by the engine block\
  OR
• run a third wire and connect all three

Forgot to mention before, the car is sort of drivable - not awful, not great. When the computer connection is working, I can see the O2 readings matching the installed AEM O2 gauge, but was fine tuning and working things like idle droop and the air conditioner boost. Unfortunately, the computer connection is too unreliable to continue tuning.

Sensor ground = ECU ground
heater ground = some chassis bolt

Yes you DO NOT connect both sensor ground and heater ground at the same point.
https://forum.miata.net/vb/showthread.php?t=697517

*edit* power should come from a dedicated switched 12v source with 6A Fuse. I wouldn't use the aux connector.