Okay I'm stumped. I've been battling grounding issues for the past 2 weeks in my 1995 Miata. I have a AEM EMS4 and AEM smart coils all wired up. Now I have a few issues:

1: I think I've found a short somewhere in my METER fuse circuit. O2 sensor Black/Yellow and Black wire do not show open circuit, instead it shows a low resistance value. I don't believe this is normal but I'm not entirely sure. Logic would say that power and ground should have infinite resistance/open circuit. Any ideas?

2: Something is feeding voltage to my sensor ground. I BELIEVE that I have completely isolated my sensor ground from chassis ground. For sensor grounds I'm using the Black/Blue and Black/Light Green wires. These are pins 2C and 2D on the ECU. B/LG runs from ECU to front of engine, serves as a ground for CAS. This wire used to be linked with a clasp terminal to two Black wires. I unlinked this wire to isolate it from chassis ground. Black/Blue runs from ECU to TPS, EGR (map wired in), Coolant temp, MAF, O2 sensor. This wire never touches ground so it's also isolated. I also have the coil pack sensor ground wire grounded to the sensor ground.

Now, when I turn on the car I get .5V through the sensor ground, and all of my sensors read high. When I ground the sensor grounds to chassis my sensors go back to normal. This would be great except that the sensor grounds can't be grounded to chassis. When the car was running this way I was getting a bunch of noise in my sensors causing the car to misfire. I'm not sure if problem 1 and problem 2 are related, and I have no idea where to look now. When I start unplugging things, coil packs, fps, map sensor, the voltage drops. It kind of makes sense, if there is a set amount of current backflowing somewhere, then as I decrease resistance, voltage decreases. Connecting everything back would increase resistance and increase voltage?

Anyway maybe I'm chasing down the wrong hole right now, if someone can point me in the right direction that would be great! I'm about out of ideas...

Also my AEM EMS4 is grounded using the chassis 2A and 2B. I was considering grounding the EMS directly to the battery instead.

 

Is this a new install or was it working fine and then crapped out all of a sudden? I had something similar happen with a pinched O2 wire but it's possible a sensor could have failed too. Try disconnecting each sensor one at a time and see if that .5v goes away with one of them. My problem was a dead short so the engine fuse popped immediately. I haven't messed with an NA8 harness but the same principles apply.

 

Sensors being the crank and cam sensors? I have a similar car (1994) and I'm having all sorts of timing errors.

EMS-4 Series 2, as well.

 

Every sensor on that bus.

 

What are the sensors on that bus?

 

I don't know. I don't have a wiring diagram for that year. On my car, every sensor under the hood (IIRC) is on the same ground bus.

 

This is a new install. I'm also wondering if I have a short as well. I pulled every single fuse in the fuse boxes except for MAIN, disconnected ECU, dash and then i checked resistance between power cable and ground. It wasn't an open circuit, but it was reading some super high resistance value in the Mega Ohm range... Then when I pull the main fuse the power cable to ground became open circuit as i thought it should be...I'm not sure what to make of it. Shorts usually don't read giant mega ohms??

That's exactly what I'm getting. I'm thinking it's a power surging through the sensor ground. That can happen if you have your sensor ground grounded to chassis. Although, stock cars seem to all have their sensor grounds grounded to chassis or block...

All sensors are grounded through the Black/Blue or Black/light green i believe...

 

Alright, sounds like we're having the same issue... or something extremely similar. Are you doing a VVT swap as well or are you just moving to the NB sensors?

What type of wiring are you using for the crank and cam sensors? Straight? Twisted pair? Shielded?

 

Check that you have all of the sensors wired correctly, look for ground and Vref being swapped at the connector. Also with the ignition on but the engine off open EMS tuner and unplug sensors one by one while watch the sensor read outs.