Aftermarket Head Unit Grounding
 

I'm in the process of installing an aftermarket Android based head unit into my 1991 to use as my "gauge pod" (see this thread: https://www.miataturbo.net/megasquir...device-100788/). Previously, my dad had someone install an aftermarket single din head unit sometime back in the early '90s, and this is what I found they had done for a ground:
https://cimg9.ibsrv.net/gimg/www.mia...583ff2bb02.jpg


Last night when I was using a multimeter to double check the wires, everything was fine, but this morning after installing my patch harness, my head unit wouldn't turn on. When I checked, I was reading .4v difference between the battery and ground at my wires. Upon checking again, everything was fine. Well, that's a bad ground.

I've never really wired this before, and I'm guessing what's shown in the picture is not the correct spot for a ground for a head unit. Where am I supposed to pull in the ground? It's not part of the harness I have. I only have a single connector, and the ground is supposed to be found in the connector I don't have.

It's been a while since I was inside the dashboard of an NA, however I believe that is the OEM ground point.

Unscrew it, clean everything up, and see what that gets you.

Cleaned it up around the screw, checked voltage (looked good @ 12.6 without engine on), and hooked everything up again.

Head unit would not turn on.

Unhooked patch harness, checked voltage, and it was ~1v.

Checked versus a clean spot on that metal, could never get it to read over 1.2v.

How is this piece of metal connected to other pieces of metal and other grounds? This still doesn't seem like a solid grounding spot for pretty much any load.

From an academic standpoint it's not great, but it's usually "good enough."

As a broad generalization, the goal is to get a piece of metal into contact with the body of the car. The MK-1 Eyeball is a useful tool here.

As said, factory ground for the stereo is just to use the dash frame.

personally i'd replace those godawful cheap crimps, and the twist nut thing, with a soldered join and some heat shrink. (or at least use a decent crimp)
Those cheapo crimps often loosen up and give crap connections, mainly because they're made from the cheapest crap chinese metal, and work by just being smashed flat rather than actually grabbing the wire properly.



Regarding the dash frame and how it connects to the ground, it's attached directly to the vehicle bodywork at multiple points.
The two screws you see there go right into the trans tunnel, and from memory the upper part of the frame (near the radio) connects to it, and in turn connects to the main dash bar, which is bolted to the bodywork at each end, just near the outer air vents.

Well crap, if it"s supposed to be grounded and it's not, that's making me worried about grounding Gremlins. How do I go about ensuring this piece is correctly grounded? Should I just tie it to the ECU frame?

I cant see how it wouldn't be grounded.
It's a piece of metal, attached by metal screws, directly to the body of the car.

Have you checked it's actually not grounded, and not just your multimeter, or the test leads on it, that are acting up and not working?






*bear in mind, i'm not an electrician by any means, i've literally set a multimeter on fire, and melted another one*

it's possible the problem lies in my positive side, but if that's the case I think I'd be really screwed and be having tons of electrical problems. I'm going to start with cutting out the old 5a fuse box (the black box in the pic). The fuse wasn't blown so I had kept it, but if it's not that, it sounds like I have a major electrical issue.

The fact it seems intermittent says wiring.

Have you been checking voltage/ground directly from the OEM plug?
or from the weird fusebox (that you dont really need for a radio install, they usually have their own fuse on the head unit. Plus the OEM harness is fused)

Personally i'd just remove the non-factory wiring and start fresh from the OEM plug.

Resolved! It was the old shitty fuse box that I should have removed in the first place. To clear up any confusion in case someone sees this thread: I was WRONG about the grounding. It's fine. It was that crappy fuse box (black box) in the picture.