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Old 6th June 2006, 19:49   #1
DRC
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Default Wierd wiring schemes

I have searched the site and did not quite find anything relavent in any threads. Not sure How effective was the search.

It all started saturday night when My BIL mislead into a traffic jam. I noticed quite a few guys were getting too close to my tail and screaching to halt. And soon I found out my break lights were not working.

Sunday morning, I decided t fix the problem myself as it was a SUNDAY, likelyhood of getting a mechanic was less and I was adventurous.

i checked the fuse, its alright. Pressed everything to rule out loose connection from fuse, switch, connectors, etc... nothing happened. Then I decided to check the voltages. could not find my multimeter (when was the last time I used a multi meter.....) so went and bought a new one for 1000 bucks. When I checked the voltage at the fuse no voltage at all...
Hmm Kirchoff's law is wrong . Later I started doubting if the instrument is really properly grounded? Looked around and could not find a proper ground with out having to scrape paint. Finally I decided to use ciggy lighter's ground. While trying to hook the prod, I inadverdantly shorted the +ve, and.... kaput the fuse went off.

Surprise 1: Ciggy lighter fuse is a 30A rating, Why on earth?
Surprise 2: This fuse is shared between, lighter and AC blower , so surprise 1 is answered with a new surprise.

3: upon keen look, there are lot many shared fuses with wierd combination of loads. Ciggy lighter and its illumination falls under different fuses..
Some of the relays mentioned as "reserved" in the manual, are being used for unknown reasons and appearantly getting hot.

Surprise 4: After I replaced the fuse (borrowed from rear window defogger) break light issue mysteriously got solved. Honest, I dont know still what was the problem.

Now that the problem got sorted out, I am not seeking a solution. But please post your views about sharing a safety device like FUSE amond various loads like this. Is it becase they can save a lotta money? A fuse to end customer cost 7Rs. and a mis-hap/dissatisfaction could cost much dearer... what U guys think

Is this habit of cutting corners/wires/fuses a practice in other cars too. I heard in Ikon, wiper motor fuse is shared with alternator field.

Your views and examples please
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Old 6th June 2006, 21:49   #2
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i've seen newer cars share fuses for couple of devices. the older ones have mostly single fuses for each function / device. if u would compare the features new to old cars then the amount of new features sure would require to have fuse sharing or the fuse box would have look like a big DB box of some shopping mall. also u r right the mishaps are far too costly.

seen brand new Mercs catch fire and nothing could be done - short circuit.
just a couple of days back saw a new MB140 van catch fire and destroyed. luckily in most of these mishaps i have seen no one was injured.

Just a note ::: never leave ur kids in a car with the Engine AC ON and go to do the 10 minute shopping. Can prove to be very fatal.

just chk my topic "rear lights not working" just incase u have a problem of this kind in future.

Last edited by 2fast4u : 6th June 2006 at 21:54.
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Old 7th June 2006, 01:06   #3
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Yes, In a midsegmenter itself there could be quite a few functions and result in a huge fuse box IF the design calls for all of them in one place. But that is not the case. In fact it is not true either.

Apart from a regular fuse box, there are fuses everywhere.

What I have not understood is that what teh manufacturers are trying to save on, Space? fuses? $? complexity? Servisability? or they are just trying to make Jokers of customers?

Imagine An unsupecting sutomers complains about a non-functional fan, and the service guys CONvince him that the lighter is also faulty. Fix it with a fuse and charge for both Don't you see this possibility, which can be reality in India at the least.
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Old 7th June 2006, 02:13   #4
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Quote:
Originally Posted by DRC
Surprise 1: Ciggy lighter fuse is a 30A rating, Why on earth?
Surprise 2: This fuse is shared between, lighter and AC blower , so surprise 1 is answered with a new surprise.
a cigarette lighter has a heating element which needs to give out a lot of heat to 'light' your cigarette in the shortest time possible. the more the amount of current drawn by the heating element in a short time, the faster it's temperature is going to increase.

your car battery will probably be rated to give out a constant current of @ 25Amps (not the Cold Start current which is much higher). if the fuse is rated at anything lesser, it's just going to blow up.

as regards the fuse sharing between aircon & lighter - a car aircon consumes @ 50W of power. preferably, an aircon should be hooked up in a system that produces >=350W of power with atleast 50W spared for the aircon.

simple calculation - suppose your ciggarette lighter produces 12Vdc x 12A = 144W. you still have a lot of power left over for the AC to consume - so don't worry about it too much.

Quote:
Originally Posted by DRC
3: upon keen look, there are lot many shared fuses with wierd combination of loads. Ciggy lighter and its illumination falls under different fuses..
Some of the relays mentioned as "reserved" in the manual, are being used for unknown reasons and appearantly getting hot.
if you check, your audio system & aircon will never be on the same fuse as car amplifiers continuously draw a large amount of current to produce their required power (400Wrms, 1000Wrms, etc.). other smaller devices eg. cabin lights, door lights, power mirrors, etc. can be shared as these devices are only operated intermittently.


it's common practice to share fuses among similarly rated devices. if one were to install a seperate fuse for every device, troubleshooting would be a nightmare. there would be a lot more wires & connections running around just for fuses - just doesn't make sense - from an electrical point of view.
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Old 7th June 2006, 12:43   #5
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Aah78, good to see your reply...however, I have a few things to differ with you.

I dont think you need large currents to heat an element. Ex, a Tungsten filament lamp heats its element to 1800 degs in less than a millisecond with very few milli amps. In my guess, the lighter should not take more than 20 watts for the heat it produces.

I am getting convineced that it is a common practice to share the fuses. But why? I again dis agree here that shared fuses will make the debigging complex. In fact they make it simpler. You still have only one fuse to look for if the function fails. No it does not save on any wiring. you still need to draw one wire per function after teh fuse. and can't save wire before the fuse either, because it's already one thick wire. All it saves is a one stupid 7 rupees fuse
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