so here we go guys, let see who can guess how many amps ( current consumption ) travels down my additional earth cable that so many cynics feel isn't needed ?

I thought I would do some tests the other day when it was -2 degrees out side. please bare in mind that landrover have replaced the original earth cables so I was interested to see if the cable would be redundant , but I have always believed that the earth cables are a poor design !

A [b]So who can guess the nearest, to AMP consumption down a cable that technically shouldn't be needed ?

B so who can guess the nearest amps returning back up the cable whilst the engine was running !

For a head start we are looking at figures between 0-600 amps , so could be 0 or just a few or even 600 amps !

I have actually videoed the ammeter with my mobile phone at cranking so when we have good number of guess's & i can work out how to post the video I will.[/b]

Last edited by oldgeezer on 31st Jan 2013 6:28 pm. Edited 1 time in total

Approximately the same as travels down your main earth cable. And if you add a third cable then approximately the same current will flow down all three. That's the way electricity works!

^^^ I agree.

4?

22 Al C

Subbing as I'm interested in this...

Cant guess the answer but I'm sure it will be interesting....

Terran Terran



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knows

599 my guess Former Mod/Member, with the most post & Chicken George Arch nemesis

I = V/R if i remember correctly

need more info to answer... but as a rough guess ...

I reckon that cranking the engine over will draw circa 100 to 150 amps. - splitting your existing earth cable would roughly half that so about 60A(ish)

At idle ... depends on long after the cranking your measuring, and also what else is running...

lights - 15 to 20A
blowers - 10A ( assuming no ceramic heater )
Engine stuff ( ECU, pumps, injectors, power steering ) - 15A
Other ECU's depending on the car 10A
plus 20 - 30A to recharge the battery??

so very rough guestimate is in the region of 80 to 90A that the alternator is pumping out in the timeframe not long after starting the car and letting it idle, with about upto 30A of that going back up to the battery ... so about 15A back up your new cable?

I've poked a little fun at 'oldgeezer' in the past regarding his extra earth wire, but I have no doubt of the consequences if the starter motor or any other components earth return path has an electrical resistance.

The ideal should be 0 ohms and if the original earth wire does not meet that criteria then volt drop will occur and heat will result. Adding an extra earth wire in parallel and if its resistance is lower than the original will command a better earth return resulting in a higher current path.

Until we know the resistance we cannot guess the amps, I have'nt ever doubted you oldgeezer as your argument is perfectly valid if a resistance of LR's earth cable is present, or rubberised engine/transmission mountings etc are affecting a good return path to the body

EY is absolutely right, of course. I'm merely pointing out that if the resistances of the earth cables are as they should be then each cable will carry an equal share of the current. However many cables there are.

Note also that if the original earth wire were to fail then all the current would travel via the 'spare', so it's important that, even though it's only a spare, the second wire is capable of taking the full start-up current, both in terms of its own capacity, and that of whatever it is anchored to.

One does have to be careful with things like this, as a failure of the original earth can result in current travelling via a path it was never intended to take, and damaging things as a result.

Don't forget the engine earth strap is also part of the alternator charge circuit and if not up to scratch can affect battery charging.
Fitting an additional earth strap from the engine is an extremely good idea on any car over a few years old, I always do it.
You just have to remember the dubious way the engine on a Discovery 300tdi was earthed and the endless starting and charge problems it caused on cars over a certain age.

pab wrote:

One does have to be careful with things like this, as a failure of the original earth can result in current travelling via a path it was never intended to take, and damaging things as a result.

Wholeheartily agree with 'pab', stray earth returns and electronics do not mix well.
While I have never had a problem like 'Oldgeezer's' it would pay everyone to at least check and clean their earth connection both on the battery and the bodywork, so long as this resistance is lower than any other earth return it will use it and not some 'piddly' earth wire under the dash that has a better connection but grossly undersized.

When the better weather come in I think I'll remove mine and give it a clean

some interesting comments so far , i will put you out of your misery soon.

I haven't given a number as yet, so I suppose I ought...

The current during cranking is the interesting one. IIRC the starter motor is rated at 2kw. If it were actually producing 2kw (unlikely) the total current required would be 166 amps. Split between the two cables that would be 83 amps per cable. But, as I say, it's unlikely to be producing the full 2kw, which will reduce the current. But we also have to account for the other things which will be drawing current at the same time. So I'll take a stab at something around 65-75 amps down the 'spare' cable whilst cranking. (Of course if the glow plugs are energised at the same time the figure will be even higher, so I'll stipulate no glow plugs!)

As for once the engine is running, I really have no idea as there are too many variables, and it will change over time as those variables change.

EDIT: If the glow plugs are on (and all 4 are working!) it would add something like 25 amps to the spare cable current, taking it up to 90-100 amps. I think.