v8ian
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Oct 19, 2023 17:31:35 GMT
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I am a touch troubled about my oil pressure light at the moment, the way I see it is the light is a switched feed, so 12v into the bulb and the bulb is then earthed via a oil pressure switch, for some reason, I wired the light thru a 7.5a fuse, while not doing anything electrical like turning things on, or fiddling, after about 5 mins the fuse blew? bulb is still good, changed the fuse for a 10a, after about 8 mins the fuse blew again, ?? this is a dedicated circuit, my limited maths says 12v feed, 2watt bulb, the ampage should be 0.66a so why would the fuse blow? a high resistance in the switch?
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Atmo V8 Power . No slicks , No gas + No bits missing . Doing it in style.
Austin A35van, very different------- but still doing it in style, going to be a funmoble
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jimi
Club Retro Rides Member
Posts: 1,816 
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Oct 19, 2023 18:18:22 GMT
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Higher resistance = lower current, Ohm's Law
If your blowing a fuse then the current on it is excessive, don't put in a bigger fuse, that's asking for trouble.
@12v a 2w bulb will pull .166A, at running voltage say 14v the the current drops to .143A.
If your blowing a 7.5A fuse with a 2w bulb then something is wrong with the wiring, you have some kind of short to earth or there is something else in the circuit causing excessive load, You need to check the wiring.
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Black is not a colour ! .... Its the absence of colour
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jimi
Club Retro Rides Member
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Oct 19, 2023 18:19:22 GMT
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What car ? Is the wiring standard ?
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Black is not a colour ! .... Its the absence of colour
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v8ian
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Oct 19, 2023 18:42:15 GMT
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Not the wiring is not standard, I know putting the bigger fuse in is a bad idea, what confuses me is if the circuit is pulling 10a, the bulb surely @0.166a will act as a fuse in its own right and why does the fuse take so long to blow, if it was a dead short the fuse should blow almost instantly.
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Atmo V8 Power . No slicks , No gas + No bits missing . Doing it in style.
Austin A35van, very different------- but still doing it in style, going to be a funmoble
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jimi
Club Retro Rides Member
Posts: 1,816
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Oct 19, 2023 19:49:55 GMT
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The current isn't necessarily passing through the lamp. What happens if you remove the lamp (leave it out) and replace the fuse ?
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Black is not a colour ! .... Its the absence of colour
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v8ian
Posted a lot
Posts: 3,758
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Ill throw a wild one out here, the wire from the from the switched live is paired off to run to four destinations, oil pressure, tach, speedo and alternator/charge/exciter wire, the speedo, tach and alternator are not connected, just run to their destinations, The wild question being can a flasher relay slowly ramp up and flash, and could this be the reason for the long wait before the fuse blows, Because this wiring is a real cobble together with 50years of mods, some good, some bad, could I have picked up an unsuitable feed from the fuse box, (2 glass fuses) the fuse blowing is s supplementary one I have added. IE something to do with the flasher, (I know a fuse into a fuse is not good either) Ill pull the flasher relay later and switch on the ignition, at least I can say I have done something  |
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Atmo V8 Power . No slicks , No gas + No bits missing . Doing it in style.
Austin A35van, very different------- but still doing it in style, going to be a funmoble
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Ahh, right, in that case check the loom for damage. You may even have to unwrap the loom to check that another wire has had an excessive load through it and melted the insulation of itself and surrounding wires, and it's picked up a leakage to earth.
Colin
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v8ian
Posted a lot
Posts: 3,758
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Oct 20, 2023 13:25:24 GMT
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Ahh, right, in that case check the loom for damage. You may even have to unwrap the loom to check that another wire has had an excessive load through it and melted the insulation of itself and surrounding wires, and it's picked up a leakage to earth. Colin Mmmmmm that has stirred a thought, going to check something tomorrow, the idiot might be me, lets hope so, DOH |
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Atmo V8 Power . No slicks , No gas + No bits missing . Doing it in style.
Austin A35van, very different------- but still doing it in style, going to be a funmoble
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Oct 20, 2023 13:59:18 GMT
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Ahh, right, in that case check the loom for damage. You may even have to unwrap the loom to check that another wire has had an excessive load through it and melted the insulation of itself and surrounding wires, and it's picked up a leakage to earth. Colin Mmmmmm that has stirred a thought, going to check something tomorrow, the idiot might be me, lets hope so, DOH Surely not.  |
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v8ian
Posted a lot
Posts: 3,758
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Oct 20, 2023 15:28:47 GMT
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Mmmmmm that has stirred a thought, going to check something tomorrow, the idiot might be me, lets hope so, DOH Surely not. Well, perhaps I should look before I post....... OOOPS, the answer was, you might have an earth fault, Mmmmm that might just be staring be in face, there just happens to be a bundle of black earth wires staring me in the face with a ring terminal not attached to anything but air, screw this to a earth post, fit a 5a fuse, turn on the power light on, give it 5 mins, the light is still on go in, have a cuppa, cake and a chat, and 20 mins later, the light is still on, perhaps I Might have this sorted, |
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Last Edit: Oct 20, 2023 15:33:32 GMT by v8ian
Atmo V8 Power . No slicks , No gas + No bits missing . Doing it in style.
Austin A35van, very different------- but still doing it in style, going to be a funmoble
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jimi
Club Retro Rides Member
Posts: 1,816
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Oct 20, 2023 18:21:53 GMT
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 At the end of the day you got it sorted, that's what counts Sometimes you can't see the wood for the trees, happens to everyone now and then 👍
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Black is not a colour ! .... Its the absence of colour
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At the end of the day you got it sorted, that's what counts