Wire burnt out near battery on MQ Patrol

[Heathx4]

Had a bloody flat battery today so tried to jump start. Connected cables and wires near battery began smoking. Disconnected cables and found one wire between two, two wire plugs (so only a section 100mm long) had burnt through. No idea what it does, or why it smoked. Resistance between ground and past burnt bit of wire is around 5 ohms. Think it is a job for an auto elec? Anyone happen to have wiring diagram for L28 equipped MQ? What about starting/getting to auto elec?

[murcod]

The wiring could have overheated from resistance somewhere in the cable (eg. a bad connection, or old wiring.)

5 Ohms is too much for the ground cable- you shouldn't be able to pick up any resistance on a normal multimeter. You can buy universal fit battery earth leads from places like Supercheap- could be worth seeing if something like that will work.

The bit that burnt out could have been a fusible link. Some cars have a smaller diameter wire close to the battery to act as a fuse.

[Heathx4]

The wiring could have overheated from resistance somewhere in the cable (eg. a bad connection, or old wiring.)

I hope that's what it is, but I'm thinking that that wouldn't explain why the battery went dead in the first place, and it seems to great a coincidence for the two events (dead battery, burning wire) to happen around the same time. So I'd like to think I just replace the crappy bit of wire and plugs, and away I go, but maybe there was a short to ground on the other side of the burnt wire somewhere

5 Ohms is too much for the ground cable- you shouldn't be able to pick up any resistance on a normal multimeter.

Oops, I didn't explain that properly it seems (I was in a big rush - the building power was being cut in <5 mins!). I mean the resistance between the negative terminal on the battery, and the side of the burnt wire away from the positive terminal. Essentially I was measuring whatever it was that was drawing the current that burnt the wire. Diagram:

-ve BATTERY +ve ---plug A-----burnt wire----plug B-----------unknown

After the wire burnt between plug A and plug B, I measured the resistance between the -ve terminal, and the burnt wire side of plug B. I was measuring the resistance whatever the wire use to run to, and I don't think 5 ohms is so low that it should burn out the wire - the wire wasn't that thin, but perhaps corroded and old.

The bit that burnt out could have been a fusible link. Some cars have a smaller diameter wire close to the battery to act as a fuse.

I was hoping that might be the case. Very handy to know that this has been done in the past - I couldn't see any other reason why there would be this short bit of wire between two plugs, right near the battery. That makes me feel a little bit better, if it was essentially a fuse (a bit of a dramatic fuse, compared to the little glass cylinders though!).

Thanks muchly for your help. I'm still not sure what I'll do - I think I could probably replace that bit of wire and things may well run fine. However, that's not solving the root cause and I'm inclined to get an auto elec to have a squiz. Be nice if I could drive until then though...

[Tas_Dean]

Heath, are you certain you connected the jumper cables correctly?
From what you have said, it sounds like it is the fusible link that has burnt, but you need to ask why!
IF you connected the jump leads reversed, you could have blown the regulator for the alternator, hence creating a short to earth, and smoking of the fusible link.

It may be coincidence also, as the regulator may have died, flattened the battery, and when you put the jump leads on the fusible link has blown.

In either circumstance, there is a fault in the vehicle as the fusible link has blown. The first place I would look is the alternator.
If you want to fault find yourself (thus saving $$$ with an auto electrician), start by disconnecting appliances until the resistance as you have measured, gets a lot higher. I'd disconnect all the wires from the alternator first. If the resistance goes high, you've found the problem.

Do not use a random piece of wire as a fusible link. You can buy fusible links in various amperage ranges from an auto electrical distributor, or probably even from repco etc. Generally, the size of the fusible link will match the maximum output from the alternator. Do not replace the link before you find the problem.

Cheers, Dean

[Heathx4]

Wrap-up:

Car jumped started fine with burnt out wire disconnected, and all electronics worked - once the car was on! But battery wasn't charging unless burnt wire was held together. No more heat or sparks from wire though.

Anyway, was still a bit unsure, so drove to Newcastle Auto Elecs who charged $98 including $24 for a new fusable link, $3 for another wire to run straight from +ve terminal to alternator. They reckon there was a fair voltage drop over the existing wire, and that coupled with the dodgy old fusable link was enough to drain the battery and cause the burn out. Bit pricey, but nice to know it's been done properly.

[Tas_Dean]

Well done Heath, you've worked it out! The price also isn't too bad! Take off the cost of the links and wire, and it's only $60, and that would be about the going rate per hour for an auto-electrician, with a 1 hour minimum charge. A lot of tradesmen now have a minimum charge, so a twenty minute job will cost you an hour!

Cheers, Dean