Grounding issue

Thorin

CarAudio.com Newbie
May 11, 2022
5
0
I have a nice bare metal body ground in trunk reading 0.2 Ohms compared to the battery terminal. When I turn on the ignition, the resistance in the trunk jumps to 40 Ohms when compared to the battery.

I also checked from battery to the front seat bolt (not seat belt), and same thing. Key off 0.2 ohms, key on 40 ohms.

When I check between the battery and engine block, the resistance does not change with the ignition on.

This is driving me really crazy. I hope someone can help.
 

Thorin

CarAudio.com Newbie
May 11, 2022
5
0
  • Thread Starter Thread Starter
  • #5
It's a 2016 Ford Fusion, so should be considered unibody, I guess. In the meantime I pulled the battery, cleaned terminals and clamps and reassembled. The body ground was actually screwed into the shock tower wall as shown in the upper right of this photo.

39053


I removed it and they hadn't even removed the paint, so I reassembled with bare metal. The wire in the upper left is from the battery post and it appears that is what goes direct to the body ground (it goes into a big harness, so not 100% sure). Not shown, but I also attached a second #6AWG from the battery post to the body ground with star washers.

Now in the trunk I get 0.1 ohms with the key off and about 13-14 ohms with the key in accessory. Something still isn't right.

I'm also getting about 9 ohms between the battery positive and the grounding bolt shown above. That's like 10" from the battery. I just don't get it.

Checked voltage between terminals and got 11.77V key off and 11.42 key on. In the trunk from the battery post to the trunk ground it was 11.77V off and 11.41 key on. So about .35V drop with accessories on.

Any ideas what could be screwing up my ground? Should I just say **** it and run a big ground wire from the engine compartment to the trunk? It's a real pain in this car, so I'd love to avoid that option.
 

1aespinoza

Junior Member
May 22, 2013
2,067
634
also getting about 9 ohms between the battery positive and the grounding bolt shown above.
You meant battery negative? If yes, then more scrubbing is in order. If you have 14 Ohms out back then subtract 9 Ohms from this connection you'd be at 5 Ohms, maybe less. That big ground in the harness may be going straight to the starter. I still advise a ground point from alt bracket to that tower bolt.
 

Thorin

CarAudio.com Newbie
May 11, 2022
5
0
  • Thread Starter Thread Starter
  • #7
Yes, sorry, battery negative. There are two big wires in that harness. Hard to tell where they go, but the smaller is about the same size as the one to the tower bolt. I'll check the wire to/from the alternator tomorrow. I've had it with this for today.
 

Thorin

CarAudio.com Newbie
May 11, 2022
5
0
  • Thread Starter Thread Starter
  • #8
Ok, one last thought for tonight. If the ground goes from the battery to block and block to body, my jumper from the battery to body should have resolved that reaistance change, right? I hate electricity.
 

1aespinoza

Junior Member
May 22, 2013
2,067
634
Ok, one last thought for tonight. If the ground goes from the battery to block and block to body, my jumper from the battery to body should have resolved that reaistance change, right? I hate electricity.
Yes. That 9 Ohms resistance is awkward, being that it is only a 10 inch span.
 

metalheadjoe

Unapologetic prick
10+ year member
Oct 21, 2007
1,595
855
Ohmmeters measure resistance by measuring voltage and comparing the voltage to a known resistance. They cannot accurately measure resistance on an energized circuit.
 
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