12 replies to this topic

[GigaDave]

I've finally managed to replace the radiator, and in the process had to replace the water pump too. Everything was going well (a little too well) and when it came to firing her up - nothing. Nada . Now due to previous experience we were able to suss that the fusable links that go from the loom onto the starter motor had corroded (as the same old electrical systems had failed - interior light, immobiliser, fog light stereo). We checked the fuse box and all the fuses are fine, it's only when we opened the wiring up it became apparent that all the wires had turned into green powder!

Now the question - what rating are these fusable links? There are 3 of them in total. Past the links the wiring is all fine, it's just the links that have gone. I have various wiring diagrams but for all I know I could be looking at a map of China.

In case it makes a difference the car is a 95/96 SPi Cooper.

Help me Mini geniuses - you're my only hope!

[pikey7]

What do you mean by "fuseable link"?

Are you on about the bullet connectors?

[GigaDave]

They look like a 6" or 7" length of wire with an eyelet at the end. As far as I can see they all bolt on to the top of the starter motor and are usually held inside of an insulated length of 'hose'. I'll try and get a pic when I get home from work...

[dklawson]

Having a Mk1, my car doesn't have fuseable links, however, most of my modern vehicles have them. I was unaware that there were different ratings available.

When I have bought replacement battery cables these were often included with the terminal that clamps onto the battery post. You may or may not have the same parts available. I've never seen ratings on the ones included with new battery cables.

[GigaDave]

Excellent, cheers. I was told that different ratings are available and that the most likely rating would be 30 amps... If anyone knows for sure I'd greatly appreciate it!

[sparkle]

Hi there,
cant help with your question because i searched the forum for the same info. i've totally stripped down my loom so that the bare minimum is in the engine bay, moved all the various fuses and relays to a nice box in the glove box, re routed the wiring thats left into a slim conduit, then i decided the big ass wires with the fusable links would have to go too. i'm gonna take them straight from the battery inside the car so they're not seen but wanted to replace the links with modern day fuses but dont know what rating! its holding me up now. very annoying.

[mk1leg]

a little tip when renewing spades or bullet apply a dab of Vasaline to them this will stop water penertration and stop going green..............

[dklawson]

While probably not a record, this has got to be close.

I can't remember in recent times when I last saw a 5+ year old thread resurrected.

[sparkle]

I know this is a very old tread but i'm back,- this time with answers! Long winded answers but definitive.
As i said before, i've cut out the wires in question so i took them to work and in true MythBusters style, we tested them to destruction.
Step 1: The best guess on the internet for the fusable links was 30 amps so i stripped the big brown loom wires to determine the rating of the cable its supposed to be protecting. They are 44 strand which are rated at 28-30 amps. (Seems way too low in my opinion)
Step 2: We made up a circuit of bulbs with an ammeter in there and hooked it up to a good battery.
Step 3: We kept adding bulbs and checking the ammeter until we ran out of bulbs at 43 amps.
Step 4: We ditched the bulbs and went for the dead short across the battery. We did put in a 60 amp midi fuse so as not to blow up the battery (didn't fancy getting the sack today). The ammeter went to 70 amps, and then it blew...the midi fuse.
Step 5: Replaced the midi fuse with an 80 amp strip fuse. 86 amps, a bit of smoke and the strip fuse blew.
Step 6: Getting silly now. Doubled up the strip fuses. Touched 90 amps, lots of smoke, some sparks and after a few seconds the link eventually blew. Horray.(the '28-30 amp' cable still intact!??)
Concusion: You are never gonna overload the wiring by running eveything at the same time, not even if you lit it up like a xmas tree. But in the case of a dead short, you'll get a bit of smoke from the engine bay and lose all power rather than a full on loom melt down and the inevitable burnt out shell at the side of the road. I'll be replacing them with 60 amp midi fuse and the apropriate wiring.

[sparkle]

Just thought i'd add
That experiment was on ONE link. There are THREE in the loom.

[Dan]

Did you test the time taken to fail at a particular current? Time is a rating for each fuses type as well as how much current they handle. Fuses fail, that is the point of them but what many people don't realise is that a fuse will always fail eventually. If you left one running even a very low current for tens of thousands of hours it would eventually fail. Using a 60 amp fuse might well cause you some problems, there's a reason the car uses fusible links here. A 60 amp Mid-Val will fail instantly (under 5 seconds) at 120 amps, and will hold a continuous current of 60 amps (meaning it will fail in so many thousands of hours plus). At any current between those values it will fail after a certain time on an exponential scale. When you turn on your injection car, several high current devices are dumped on the battery at the same time. The starter solenoid, PTC heater, fuel pump, injector, immobiliser etc. will all hit it together and some of these things are high inrush devices. If you have the lights and wipers on at the time you turn the key I could easily imagine the startup current going over 60 amps for a short burst. The fusible links may well be 30 amp rated, and if the wiring is indeed 44 strand that's quite likely (incidentally that's not undersized for the standard car because there are two in parallel and there is only a 55 amp alternator fitted. Also the SPi loom uses thinwall cable which is rated 33 amps at 44 strands) but they are slow blow. A spike of a few milliseconds duration is no danger to the wiring unless it many thousands of amps like a lightning strike or something. But a large spike even if very short could well take out an instant fuse and that's why the car has slow blow fusible links rather than instant blow fuses. Just like in your house, the wiring in the building and the people in it are protected by the instant blow stuff in the consumer unit. But the electric company is protected from your installation by a slow blow company fuse in the supply.

[Sprocket]

The fuse links protect the wiring from a dead short If you work out the math, you can easilly see that the current draw with a dead short is 10's of 1000's of amps instantanious(Volts / Resistance = Amps, so 12 / 0.001 = 12000 amps). Midi fuses are designed to blow quickly with a dead short, the sort of short you were exposing them to while the Hypalon fuse link is just a smaller gauge wire than the standard wiring after the links, with a special Hypalon insulation.

Not exactly the best way to test the rating of these fuse links, but then why does that matter when they are designed to protect the wiring from a dead short, its just a weaker link compared to the standard wiring after it. Would you rather have the wiring melt and fuse at any random point along the loom and result in this?

[sparkle]

There are things i know and it looks like there are some things i dont know.
I know the rating of a fuse is the load it would be happy to draw all day long, and the blow rating is generally double this.
I also know the same is true of cable ratings.
I know that any device will have a start up spike, be it a bulb, motor, etc.

My car is a 94 sprite so i dont know if this would be thin wall cable or not but it's on a carb so no injection or any of that stuff to worry about. I've also de-commisioned a lot of stuff like heated rear screen, heater, interior lights, so there will be less draw than standard.
As you say there are three of these fuses in parallel so thats spreading the load three ways. I'm sure i'll never draw that much power even if everything spiked at the same time. So the fuses i put in are in case of the dead short senario, in which case i'd rather have instant blow than slow blow, Wouldn't you? or not? Knowlege is power and i dont want to be giving people the wrong info.
Cheers