Ground Wire in trunk, attached to antenna. What is it grounding? '97 Z3 Roadster

[Euphanasia]

I have a voltage drain that I've traced to the ground wire in the trunk. It pulls 8V, and 2 μA, so not a huge deal, but I'm concerned since it's drawing current when everything else in the car is turned off and the head unit is pulled. Any ideas?

[rf900rkw]

It, uh, grounds the antenna assembly.


20 milliamps is a totally acceptable drain in a modern car. Why are you worried about 2 microamps; 1/10000th less? How in the hell ldid you even measure that?

[Euphanasia]

OK--I guess I wasn't clear enough. The antenna assembly bolts directly to the fender and there is a ground eyelet attached to the coax cable, so it's grounded even without this particular wire. The wire in question comes in from outside of the trunk, so I haven't been able to trace it. If it's just a redundant ground for the antenna assembly then why would it carry any current at all? And why would it have current with the car turned off?

I'm not sure about the accuracy of my voltmeter, but I do know that there's enough current there to make visible sparks when I touch the ground wire to ground. If it's doing something--grounding something in the car that draws current with the ignition off, I'd like to know what it's grounding. If it isn't grounding something then I have an electrical short and I certainly want to know about that.

[pwhitt]

depending on your meter, it could be noise. you can get micro amps of current if you measure between the ends of a long piece of pipe. the car is relatively large and you might even see current resulting from sunlight heating large pieces relative to others... 8 microamps is utterly negligable.

you say "it pulls 8V" - which means at some point you measured a voltage on the line. i'm assuming you disconnected it, then measured 8 volts between the lead and ground. you can get measurements like that resulting from all manner of things, especially considering it's only going to present 16 micro watts of power - that is essentially zero.

what do you mean when you say you have a "voltage drain?" your battery is dying? i promise you that 16 micro watts isn't a problem... if you see hundreds of mA, you've found something.

[Dakar Ole]

An antenna circuit must be grounded (black wire) and the powered booster circuit (brown and white wires) must be grounded independently.

[Euphanasia]

The battery is indeed dying when the car sits. There are real, visible sparks when I touch the terminal to ground. So my guess would be that my voltmeter is measuring it incorrectly, or I'm not reading it correctly. I'm getting visible sparks and consistent, measurable current when the only thing that is connected is the ground wire. Why would a ground wire spark?

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An antenna circuit must be grounded (black wire) and the powered booster circuit (brown and white wires) must be grounded independently.

Understood. So if I were to trace that brown wire, where would it terminate?

[Vintage42]

... The antenna assembly bolts directly to the fender and there is a ground eyelet attached to the coax cable, so it's grounded even without this particular wire...

Do you have more wires than this? If the photo link is bad, it came from this thread:
https://www.zroadster.net/forum/viewtopic.php?t=8613



The photo link, and the photos in the thread itself, are not reliable, so I have to grab and post the photo myself.

[Euphanasia]

Do you have more wires than this?

Mine is exactly like that. Well, yours is a lot cleaner...

[Vintage42]

Then you do not have any extra wires to the antenna base in the trunk. Do you have an aftermarket radio that might be connected to the antenna wiring up in the console?

[Dakar Ole]

The battery is indeed dying when the car sits. There are real, visible sparks when I touch the terminal to ground. So my guess would be that my voltmeter is measuring it incorrectly, or I'm not reading it correctly. I'm getting visible sparks and consistent, measurable current when the only thing that is connected is the ground wire. Why would a ground wire spark?

- - - Updated - - -



Understood. So if I were to trace that brown wire, where would it terminate?

I would guess to the stereo ground to turn the booster on. Is your HU from the factory?

The black wire has continuity with the coaxial ground so it might be for safety or interference.

[rf900rkw]

It takes some very expensive equipment to even read in the microamp range. If it's sparkling, you are talking tens of milliamps or more.


The wire you speak of is not mounting on the antenna. It is mounted to the unibody, sharing the mount point with an antenna support bracket. By the wiring diagram, it is ground X13006.


ground-x13006.pdf

[rf900rkw]

After a quick look, I'd say there is a good chance you are chasing your own tail. The trunk light grounds there, along with most of the back of the car. If you haven't disabled the light, you will show about 200mA draw. You can slap Ohm's Law against that see if it matches your 8V, if you want.

[Dakar Ole]

My antenna base is out right now awaiting a grommet so I touched the brown wire from the brown/white bundle to ground with ignition off and there was no spark at all. It is an all factory system.

[Euphanasia]

It takes some very expensive equipment to even read in the microamp range. If it's sparkling, you are talking tens of milliamps or more.


The wire you speak of is not mounting on the antenna. It is mounted to the unibody, sharing the mount point with an antenna support bracket. By the wiring diagram, it is ground X13006.


ground-x13006.pdf

Aha!! Thanks, That's VERY helpful. At the very least, this will give me some guidance regarding pulling fuses to see what's causing this. The trunk light is working, so it clearly has ground without this wire, but it's possible that it's not a very solid ground. Thanks for looking that up. That's a lot for one small brown wire to do!!

[rf900rkw]

Ground loops are fun. With that ground disconnected, the trunk light will still light up somewhat, seeking a ground path through the front parking lights.

[Euphanasia]

Ground loops are fun. With that ground disconnected, the trunk light will still light up somewhat, seeking a ground path through the front parking lights.

And that is indeed what was happening. I have an LED bulb, and it's very bright out, so it was hard to distinguish between "lit" and "sort of lit." Thanks for all the help.

[rf900rkw]

If the trunk light is an LED bulb, then the current draw specs I gave are out the window.