Choosing an easy to get to earth point on S Class Coupe

Submariner Gold

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Berkshire United Kingdom
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Vicovation MF3
As this is just a semi temp install maybe for 6 months.
I really dont want to pull the extremely tight fitting trim apart.

I have found 2 easy to get to areas.
1. Is a bolt , that is holding the centre console down under the cupholder compartment.
2. Is behind the rear centre armrest.

They both seem to bolt into what looks like aluminium / magnesium alloy.

Q.1. Are these alloys good enough earth points? (note there is not much steel on this car eg front wings, bonnet, boot, doors, rear sub frame, floor and front subframe and roof are all some kind of magnesium/ally alloy or straight aluminium
Q2 if I run a cable from the negative battery post to a given selected part and there is very little resistance on a multimeter is that safe to assume thats an OK earth point, for my new USB charger point powered by the cigar lighter circuit?
 
Cars don't have earth points, they have ground (or negative) points. Any metal part is OK.
 
Q.1. Are these alloys good enough earth points? (note there is not much steel on this car eg front wings, bonnet, boot, doors, rear sub frame, floor and front subframe and roof are all some kind of magnesium/ally alloy or straight aluminium
Q2 if I run a cable from the negative battery post to a given selected part and there is very little resistance on a multimeter is that safe to assume thats an OK earth point, for my new USB charger point powered by the cigar lighter circuit?
It doesn't matter if there is a bit of resistance, it will just mean that the charger doesn't get full battery volts, it will still work fine as long as there is a circuit.

Aluminium is often used for cables these days as it is cheaper than copper, I don't think there is anything to worry about other than getting a good connection that won't corrode, should be no trouble as long as the bolt is tight.
 
The ground for my 12 V expansion in the back of my car i get from one of the bolts that hold the back seat in place.

Making a hard wire in my friends car i drilled a hole in the metal beside the fuse box and then soldered the ground cable to that
 
I am confused when you say "Q2 if I run a cable from the negative battery post to a given selected part and there is very little resistance on a multimeter...... You should be getting the same reading as a dead short. (as in both probes touching each other). Any thing less would indicate a poor connection.
 
I am confused when you say "Q2 if I run a cable from the negative battery post to a given selected part and there is very little resistance on a multimeter...... You should be getting the same reading as a dead short. (as in both probes touching each other). Any thing less would indicate a poor connection.
Yes, i didnt explain that very well
In the normal position for resistance I was looking for a dead short i.e.
one the Fluke meter you can overide the auto setting, And measure a minute resistance i.e. the tiniest resistance of the 6
meter wire to get from the front to the back. Basically I got the same reading as just measuring the wire, i.e no resistance.
It was a real pig pulling the back panel off, its secured by compression into a groove. I was convinced it was going to crack. But the Mercedes worksheet was right you just pull like hell!
4EC8E313-3AF1-48B4-9EA2-78869537DE70.jpeg
I found this spare theaded post securing the metal frame of the fuse box to the chassis. So popped on the ring and a 6mm Nylock nut.
So sorted. :)



43A8BFCE-56F9-4DA4-88EB-1C81DAFAE1D5.jpeg

Sadly had to abort putting the USB charger port and external battery in the armrest box. The cables are just too thick.
I think it will have to all go in the rear footwell. Not ideal but then despite being a massive car there really is sod all leg room in the back of these cars, so it's never used.
 
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That looks like normal steel to me, not aluminium, the bare stuff being zinc plated. The body is all pressed and spot welded, not how you build with aluminium.
 
That looks like normal steel to me, not aluminium, the bare stuff being zinc plated. The body is all pressed and spot welded, not how you build with aluminium.

Yes you are right. This bit is a galvanised and painted steel formed plate part to hold the seat back in place. It bolts onto a really heavy duty alloy chassis. But as it has some other earth points on the right, it produces a good earth or sorry 0V ground. :)
 
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