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fuse question

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JPSmit

JPSmit

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I've written before about my short in #1 fuse - accessories - horn/ driving light/ cigarette lighter/ interior lights.

purple in

as you know from another post - my horn "post" is out meaning there is no switch working for the horn.

I went to replace the fuse - engine off - and blew a 25 amp fuse just trying to press it into the holder - it was too hot to hold.

Is this normal? should I have disconnected the battery?

ideas?

thanks all
 
Remember, the horn switch is a ground switch, so the problem could still be at the horns.
 
I'm sure it is - especially as after I wrote this I realized (doh) that with the horn relay, the horn is essentially out of the circuit - but to come back to the original question - does blowing the fuse in that manner make sense?
 
Well, it happened, so it has to make sense.

The horn in earlier cars is always powered--I assume it's the same in later ones. The relay doesn't affect this. You pretty clearly have a short in the circuitry between the fuse and the horn. I'd trace the circuit from the fuse to the horn, and see if you can see it. If not, here's a trick: attach a test light (or voltmeter if you have one) to the fuse contacts. If there is a short, it will light up. Now, start disconnecting the power from things--first at the relay, see if it's still shorted (i.e., light stays on), if not, reconnect it and disconnect the next thing down the line, and so on--you can isolate the location by doing this logically.
 
so Steve, if I understand you correctly, there should be no draw across the fuse contacts unless one of the "switches" (horn etc) is "on." The fact that there is current (as evidenced by the fuse blowing) says there is either a short or a ground.

So, if I hook up my voltmeter, and disconnect till I see nothing, I should have isolated the ground

Is this correct?
 
This type of problem can be difficult and expensive to track down as you keep blowing fuses before you can really do any fault isolation. I'd recommend getting an automotive circuit breaker that will snap in in place of the fuse and then you can reset it each time it fails by just removing the battery cable for a second. Try removing one connection at a time from the bad circuit at the fuse block, when the circuit breaker doesn't blow you have isolated the main circuit, not you just have to follow that out and isolate each individual component until you find the short. The circuit breakers should be available at your local auto parts store.
 

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who knew - thanks Bill
 
If the fuse is out of its socket, and everything is turned off, there will be no voltage between the fuse terminals. If something is on, or there is a short, all the battery voltage will be dropped across those terminals. So, you can use this as a short indicator. Cheaper than blowing umpteen fuses.

The resettable breaker that replaces the fuse is another option, and might be a good thing to have, in any case.
 
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