Why did the fuse blow?

[Terry Field]

Went out on a short blat yesterday and on return home plugged in the CTEC trickle charger via the cigarette lighter as usual, but nothing happened. Turned out that the 25amp fuse that houses the charger and the horn had blown.

Fuse replaced and all OK. Just curious as to what might have caused the fuse to blow.

[John Vine]

Could just be age.

When you say "houses", how is your charger connected? Does it take its power from the horn fuse, perhaps via a piggy-back?

Does the horn work now or does it blow the fuse? 

JV

[Jonathan Kay]

Same questions as John.

And is your horn earth switched or live switched?

Have you got a wiring diagram and a multimeter?

Any recent work on the car?

Jonathan

[revilla]

It's odd you should mention that, because twice now I've had my main 30A ECU / engine loom fuse blow when trying to crank the engine without disconnecting my CTEK (by accident). My CTEK is the little 0.5A version and connected directly across the battery (via a 15A fuse that didn't blow). It made absolutely no sense to me at all and I kind of put it down to just being a coincidence, but in nine years of ownership and regular use its never blown that fuse without the CTEK plugged in. I still can't offer any sensible suggestion as to why your fuse or mine blew!

[Roger Ford]

Revilla, while it's true that car electrics rarely seem to conform to norms such as Ohm's law, I really can't see that CTEK charger being related to the fuse blowing.

However, I would encourage you to get a stock of 30A fuses in and see if we can find any new science - perhaps Revilla's law could be discovered here?

[revilla]

@Roger Ford - Neither can I really!

I think was more of a case of sod's law than Revilla's law, although there are those who might argue they're the same thing

If I do win a Nobel for Physics Beyond the Fusebox I'll be sure to credit you for pointing me in the right direction.

[Terry Field]

By 'eck!

I thought I had asked a simple question

I have no idea about 'piggy backs', 'earth switched; or 'live switched'

I have a wiring diagram and a multimeter.

The only thing I have done recently is change the light bulbs.

Apart from that, everything has worked just fine since I bought the car a year ago, so I don't thing there is a fundamental wiring error.

I was just a bit curious, but having changed the fuse, and everything is working again, I am a very happy chappy

[Nigel Fox]

I assume you have a 15A fuse feeding the cig lighter and the horn.

Obviously the 12V supply downstream of the fuse has been connected to earth thus blowing the fuse.

If the horn is working normally, I would suspect that the circuit has been earthed during the operation of the cig lighter plug and socket. I would have a close look at both components, they are often of poor quality.

[Terry Field]

Nigel, thanks,

That makes more sense to me. the socket and plug are, as you say, quite poor quality

[John Vine]

Re #7: I have no idea about 'piggy backs', 'earth switched; or 'live switched'

In case it helps....

This article explains what a piggyback is and how to use one.

There are two ways you could connect a horn:

Earth-switched:  12v supply from ignition switch to horn (possibly via a relay). Wire runs from the horn earth terminal to the horn button.  The circuit completes (switches to earth) when you press the button. 

Live-switched:  12v supply to the horn button.  Wire runs from the button to the horn live terminal. When you press the button, power is supplied to the horn. The horn is already earthed to the chassis.

AFAIK, 7s have earth-switched horns.

JV

[Terry Field]

Many thanks John - I will study and learn