Simple favor (engine grounds)
 

So I installed a 1.8 engine in my 90 miata(retaining the 1.6 wiring harness) and it runs okay but i think i may have a grounding issue, it being my megasquirt likes to randomly reset sometimes. Would someone be so kind as to describe to me what the wires look like that are the engine grounds for the 1990 wiring harness (black w/stripe, green w/polka dots,etc...) Or even better a picture would be amazing. ThankS!!:)

an interesting follow-up. the wiring diagrams list a number like G111 next to each ground point and give a vague description of where it might be. is there any chart showing a more specific location? Probably in the FSM?

i take it those are the ground points, thanks Brain, wherever do you find all this delicious info?

he draws it in his spare time

You know that Japanese dude that designed the car, I think it was he in disguise...:vash:

haha. i dunno shit. i just happen to have a Factory Shop Manual.

yeah the numbers are the spots....so under the booster, on the tow hook in the rear, and another spot just below it

Resurrecting this post for the sake of newbs. I took some pictures of what I THINK are some of the ground points mentioned on my NB.

Let me know if I am correct.
https://www.miataturbo.net/attachmen...ound-point-jpg

https://www.miataturbo.net/attachmen...round-back-jpg

Time to zombiefie another ancient thread :P.

I've done a 1.8 swap into my '93 miata and I'm having some problems.
The symptoms are pointing towards a faulty ECU coolant thermosensor.
I've replaced it with a new one and measured the resistance of the old one, which was still fine.

Now I'm thinking I might have messed up some of the grounds.
The wiring schematic indicates the two grounds in the rear go on the back of the engine, but I've wired one to the firewall.
Is the firewall an acceptable place to ground this wire or is it in fact the cause of my problems?

It's a pretty easy thing to check with a multimeter, but if you are worried why not move it?

Anytime you ground something at a different location you create a resistor, the effect is dependent on system. If you ground a fan somewhere, it doesn't care. If you ground an ecu somewhere and the sensor somewhere else you can create a voltage offset.

Thanks for the answer.

I was going to try to bolt the grounds both to the block tomorrow anyway, but I wanted to know if my thinking made any sense at all :P.
Now to find a suitable point to attach them, the 1.8 doesn't have the same mounting points on the rear as the 1.6.

The firewall is not an acceptable ground for critical sensors. The best ground is a sensor return wire from the ECU, followed by engine block and battery.

So it's not a good idea to ground my wideband and extra MS ground at the grounding strap on the head to the firewall?

Is the sensor return wire pin 7 on the mainboard connector or there other pins as well? I'm just curious because I have my aem wideband grounded at the same spot on the engine head as the MS but I am getting a discrepancy of .3 on TS from my wideband gauge. I would like to get the gauges to match up correctly.

For the record, the pictures I posted with the ground points have proved to be solid so far. Post #9