Thread: Separate Alternator cable for D3

Done about 100ks and so far no issues , will be testing it out this weekend when I put it under some load with the fridge and a few other things.


I have read the D3 alternator is rated at 150 amps so I think 175amp fuse should be ample , but time will tell.


Cheers Ean

Hi Ean and sorry to be a wet blanket but when my TDV8 RR alternator went, it did not cause a total dead short, it just had a continuos 155 amp overload.

This will not blow your fuse but will still flatten the battery in a very short time. In as little as a few minutes if the battery is in a low state when the overload occurs.

The problem is not just the 155 amps being draw from battery by the alternator failure but there is also the additional 40+ amps still being draw by the vehicle's own electrical needs.

So when the alternator goes, there is at least a 200 amp load being applied to the battery, and no battery, regardless of size, can tolerate that sort of load for very long.

I am looking at a way to indicate that there is an abnormally low battery state but it's a way off yet.

Most vehicles will give some form of warning, usually via the battery light in the dash, but for some reason, most Land Rover alternator failures do not give any advanced warning and this is why they end up being such a dangerous failure.

Originally Posted by drivesafe

Hi Ean and sorry to be a wet blanket but when my TDV8 RR alternator went, it did not cause a total dead short, it just had a continuos 155 amp overload.

This will not blow your fuse but will still flatten the battery in a very short time. In as little as a few minutes if the battery is in a low state when the overload occurs.

The problem is not just the 155 amps being draw from battery by the alternator failure but there is also the additional 40+ amps still being draw by the vehicle's own electrical needs.

So when the alternator goes, there is at least a 200 amp load being applied to the battery, and no battery, regardless of size, can tolerate that sort of load for very long.

I am looking at a way to indicate that there is an abnormally low battery state but it's a way off yet.

Most vehicles will give some form of warning, usually via the battery light in the dash, but for some reason, most Land Rover alternator failures do not give any advanced warning and this is why they end up being such a dangerous failure.

No worries Tim,


its good to get another side of it, especially from someone in the trade. My simple brain tells me that the 175 amp fuse is to high , I would expect that 80% of the alternators capacity would be a more accurate size fuse.


Do you have an idea of what the amp output from the alternator under normal operation would be. If the Alternator is 150 amp would it normally run at 80amp and under extreme load be 120amp or what do you think.


I might invest in a few different size fuses and see if maybe a 125amp fuse keeps things running and if it blows its time to see whats going on.


I am still happy that I have done this, its just a matter of the right size fuse so if things do go wrong I get some warning not just a dead car in the middle of the road.


Cheers Ean

Perhaps a 150A or even a 125A fuse would be more appropriate for a 150A alternator. I might try a 175A for my 3.0's 180A alternator.

Originally Posted by Graeme

Perhaps a 150A or even a 125A fuse would be more appropriate for a 150A alternator. I might try a 175A for my 3.0's 180A alternator.

My thoughts as well Graeme. You would think any short above that will blow the fuse and atleast let you know its time to investigate.




Cheers Ean

Hi Ean and Graeme, unfortunately, a fuse is not the answer to the problem.

All automotive grade fuses are designed to carry their marked current for 4 hours.

But to achieve this, a fuse must be able to carry a higher current for a shorter period of time.

The amount of additional current that an AUTOMOTIVE grade fuse carries is set down in specific tolerances.

For example, the same automotive fuse must be able to carry 135% of its marked rate for around 30 minutes, and much higher currents for shorter time periods.

So your 125 amps fuse will carry around 170 amps for 30 minutes.

There is a reason for these higher than marked currents. When a DC motor is first started, it can cause a near dead short load of a micro second.

As such, all automotive fuses are designed to carry many times their marked rate for a few seconds, to compensate for such startup events.

Again, using my own experience, where my 140 amp alternator had turned into a 155 amp load, a 125 amp fuse would not blow for at least 35 to 40 minutes and no battery will tolerate the 200+ amp load for 15 minutes, and that is if the battery was in good condition and in a fully charged state, before the this sort of load was applied to it.

The best solution to give a warning with the way Land Rover alternators fail, is to have a current monitor that can detect both the current load, but more importantly, the direction the current is travelling.

So while the alternator is working as it should, as a power supply, and the current is travelling to the battery, all is well.

But if the alternator turns into a load and current starts to be drawn from the battery, you would get a warning in the cab.

This type of device would be easy to setup but is just too expensive.

I must try to find out what size fuse is fitted to the Territory and also its alternator's rated output. However the reason for the Territory's fuse could be to prevent a fire if the alternator lead shorts to ground rather than the protect the battery against short-circuiting Zener diodes in the alternator.

I might have to try a 100A fuse to see if it blows with normal use.

Littelfuse, who make the Mega fuse, state that at 200% of the rated load the Mega fuse will survive between 1 and 15 seconds. At 135% of the rated load the fuse will survive between 2 and 30 minutes, for which even the minimum time is too long. Hence it would appear that a fuse rated at half the alternator's rated output would be appropriate, ie 90A for a 180A 3.0 D4 alternator or 70A for a 140A D3 2.7 alternator. I might try a 100A fuse, being the closest without going too low. However whilst winching a higher rated fuse should be used due to the longer time at high current draw.