Alternator fuse - Blown - interesting

[Ean Austral]

Gday All,


About 6 weeks ago I installed a inline fuse on my alternator to see if it could combat the problem of a faulty alternator killing the battery and leaving the car dead in its tracks, in some case's stranded in some dangerous positions.


Since doing the mod , I have thru much discussion about if it would work ended up with an 80 amp fuse , which for 6 weeks has been fine until last night. I have had the car offroad, and spent 10 days camping whilst having the 80 amp fuse , so the alternator has had to work to re-charge both batteries.


So the red battery light comes on yesterday , my wife drove the car home and then did another trip to the shops and back , so I am guessing its driven 40 or so ks since the light came on , and it would have had the air con on , radio cranking and no attention taken to reduce power draw on a not charging battery.


So far I have checked the battery , its at 12.2 volts, so the battery is ok. today I will start to investigate why the fuse blew , whether it is a fault in the alternator , which my reason for fitting the fuse in the first place , or , if when I have run the new cable I have not done it right and the cable has shorted against the car.


will be interesting.


Cheers Ean

[Graeme]

I recall that someone drove a D3 in WA for days without an operating alternator, recharging the battery overnight at each motel. Non-critical systems were shut-down but for just a few short runs I'm not surprised that the battery has held-up.

Its possible that the regulator has failed rather than suffered from failed diodes or a shorted cable.

Edit: I should read what's written more carefully...the fuse has blown.

[Ean Austral]

You should Graeme , its the idea I stole off you.


Cheers Ean

[SBD4]

Ean, if the alternator itself is capable 150A, then you need a fuse of at least that or greater to prevent the fuse from blowing during normal usage. It is possible that the car has met a condition of normal operation that has required more than the 80A fuse can handle.

[Ean Austral]

Ean, if the alternator itself is capable 150A, then you need a fuse of at least that or greater to prevent the fuse from blowing during normal usage. It is possible that the car has met a condition of normal operation that has required more than the 80A fuse can handle.

I don't think so as the car has run for 6 weeks , its been on the gibb river road fully loaded with fridge/freezer and all the other things that the car gets loaded with whilst camping , I have driven at night with everything running and its still held up, plus , the battery at 12.2v tells me its not trying to charge a battery with a problem , on the Gibb the battery was down to 11.9v several times after a couple of days stopped.


I did this to try and combat the problem that seems to be more and more common on the D3 where the alternator shorts and delivers a big current back to the battery, which has killed batteries and left cars dead in the middle of intersections. The car is built with 1 cable that goes to the starter then to the alternator and the only protection is a 500amp fuse which never seems to blow.


Best case is that the alternator has a fault and the system did as its designed , as there has been much discussion about whether a fuse would work , or , if the battery would be killed before it could blow, as they handle double their rating for up to 2 minutes, hence why I am at 80 amps.


Worse case is I never shielded the new cable correctly and its rubbed thru and is arcing out somewhere after its time offroad.


It could be a waste of my time and effort but it could be an answer to a problem that has happened to several D3 owners on this forum.


Cheers Ean

[bee utey]

Fuses selected to carry close to their design load get hot and eventually fail due to vibration. I've seen it plenty of times, it can take a few months sometimes but it's almost a certainty. I'd be using a 100 to 150 amp thermal circuit breaker myself, manual or self resetting.

[drivesafe]

Hi Ean and thats an interesting result so far.

I think you will probably find bee_utey is on the right track as to what caused the failure, but your use of the 80 amp fuse is as high as you can go to be able to get the protection you are after.

So very interested to see further test/operation with the 80 amp fuse.

Hi Sean, and have a read of this thread, it will explain in detail why Ean is using the 80 amp fuse and not a higher rated one.

http://www.aulro.com/afvb/projects-t...-cable-d3.html

[Graeme]

If no shorts anywhere then perhaps try an 80A re-settable CB. My D4's 120A CB hasn't tripped yet but it may be a little high.

[Ean Austral]

Well it looks like bee utey was right, cant find any issues and charging fine with new 80amp fuse in it. I will keep an eye on it and if it keeps blowing i will look at a resettable C/B once i have to install my last spare fuse.

Cheers Ean

[drivesafe]

Hi Ean, not sure you will find an 80 amp Manual Resettable Circuit breaker abut 75 amp versions are available and should be close enough for what you want them for.

BTW, your test while not conclusive, does demonstrate that there is a problem with the way alternators fail on Land Rovers and in most cases, do not give an indication of the failure.

Also, with the protection you have setup, ( depending on how the alternator fails ), if your alternator does fail, you could, if the battery is fully charged at the time, get about 2 hours driving time before the battery is flattened.

However, this still may not apply if the alternator is faulty and not charging the battery for some time before the alternator totally fails.