Additional wiring in a 4WD

+ and - to the batt = less voltage drop ,using the body as the neg comes with its own set of problems such as rust/paint ,loose bolt/screw , best to batt 2nd best to engine block least to body.

While the earth return wasn't such a big problem in older, pre electronics vehicles, it cetainly could be in the newer computer driven models.
The manufacturers carefully select a few strategically placed earth connections for the auxiliary wiring, to avoid stray earth currents and galvanic reactions.
With things like another stereo under the dash, the earth return could connect to the nearest one that's used for the dash wiring.
For high powered add ons its definitely a better idea to use two seperate wires. Twin core wiring comes in reasonable sizes and its like running a single wire to install it.

You are right about the fuses.

Klaus

If you use twin sheathed cable you gain red and black together in a protective cover
available in 3, 4, 5 and 6mm squared cable
Cheers
Charlie

Hi Terryfirma, first off, I’m surprised someone has not yet posted how they just use the body for the earth and have done so for years and never had a problem.

I would like to know how anyone would know if they had a problem or not because as has been pointed out above, there are so many things that can cause problems that don’t necessarily show up straight away but can cause major problems down the line if you use the body as the earth return for anything that uses more than 1 or 2 amps max.

In a worst situation, when using the body as an earth return, you can get a resistive joint in the vehicle that can be so bad that it eventually causes a fire.

If you ask a professional installer of electrical equipment, they will tell you they always run both the positive and the negative wire to the point where the new accessory is to be installed.

Willem, correct me here but what you were saying is that “ all earth wiring goes to the body near the battery “ and the bit you left out was that you have the earth from the battery bolted to the same point of the body, is this correct.

Terryfirma, this is a good way to get around the problem of trying to hook up all your earth wires to the negative terminal of the battery. Simply mount a bolt through the body, run a cable from the battery negative terminal to the bolt and then run all the other earth return wires to the same bolt.

The only thing I would suggest is that you work out what the current requirements will be for each wire you are going to run and make sure you use cable that is at least twice the current rate. This will reduce the voltage drop that will occur and the use a fuse with a current rate about 50% larger the the intended current requirement for the accessory. The reason for the larger fuse is to stop the fuse melting.

If you are going to power a driving light that draws 10 amp and you use a 10 amp fuse, you will not blow the fuse but in a very short time you will melt it.

This is an example of what I mean, You want to power a fridge that draws 10 amps. Use cable that is rated at at least 20 amps and fit a 15 amp fuse. This will give you a safe and efficient install.

You can use this as a very basic formular to workout each wire circuit you are going to install.

Cheers.

Here are results from ACTUAL TESTS in an NP Pajero.

If you are putting a power connector in the back of the vehicle and you plan to use wire to the battery for the Earth/Black side, then you will need to use AT LEAST 10mm OD 4G (21mm sq) to get LESS voltage drop than using the bodywork.

Using 6mmOD 8G (7.5mm sq) cable I measured 0.3 volts drop at 10 amps. Between Battery Negative and the bodywork at the rear I measured 0.13 volts drop at the same current - nearly 1/3rd.

In the days of screwed and rivetted bodies, there was a valid reason not to use the body for Earth Return - in today's spot or seam welded bodies you have a huge conductor from front to back - why not use it.

Yes, if the Battery Earth lead breaks there will be damage to anything that is directly earthed to the body - but if you check a modern vehicle that is just about every bit if electrical equipment in it.

Yes, if a fuel line breaks fuel will spray everywhere - but no-one puts a jacket around every fuel line - just in case.

Yes, if a tail shaft breaks it can cause serious injury - but when did you last see a tail shaft restraint on a car / 4WD - just in case.

In a monocoque construction the strength of the body is dependant totally on the metal-to-metal welds - which also provides the most reliable form of electrical connection.

"For someone adding wiring to a vehicle, there is no way of knowing whether these earth points can handle any form of heavy load. "

- a good point. There is lots of metal under the Dashboard that is not connected or poorly connected electrically to the bodywork.

It may appear to give an connection but then be intermittent when moving or with aging. I was thinking only of solid points like seat mount bolts.