Why you should remove the negative battery terminal before doing ANYTHING!!!!

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Tony Hwang wrote:


Think "fuse" in this case.... unless it was one of those Chinese-made fuses from Harbor Freight... ;-)
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You're kidding right?
--
gburnore at DataBasix dot Com
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Gary L. Burnore wrote:

Probably you don't even know basic Ohm's law or Kirchoff's or Tevnin Norton's theorem, etc. If you want to debate, email me directly. Let's compare our CV first.
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Tony Hwang wrote:

tony, this is usenet - "credentials" don't mean shit. just stick to the facts.
fact 1 - batteries don't "surge". period. some [reactive] peripherals can do it, unless protected. in cars, they all are. dome lights are not reactive.
fact 2 - ohmic behavior dictates that battery voltage drops as current increases, not the other way around.
fact 3 - ecu's are "idiot proofed". per #1, a dome light has no reaction that could possibly cause a problem, even if the ecu was unprotected.
conclusion: either address reality or move on.
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No but a direct short could cause a problem in the item shorted. In the case of some vehicles that could be the BCM if they use it for things like an interior light timer/dimmer (close door and dome light stays on for a bit) Shorting that circuit cause problems.

Yes BUT take a chunk of wire and toss it across the battery. Measure the battery voltage. It likely won't cause a voltage drop as it heats up and melts.

VERY WRONG. ECUs are easy to kill if you are not paying attention. My SOP in the body shop is to pull the ECU on anything that rolls in for panel work. One good zap from a welder can kill the ECU without being near it.

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Steve W.
Near Cooperstown, New York
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Yes,it will.That battery has internal resistance,and a short will draw a lot of current and make for a heavy V drop.(the same happens every time you start your car;there are specs for battery Vdrop while cranking.)
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Jim Yanik
jyanik
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Jim Yanik wrote:

Internal resistance is very small value. Against dead short it's not a big factor. Cranking is not a dead short situation. Once I dropped a pair of plier across heavy industrial battery terminals. The plier litterally evaporated.
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Tony Hwang wrote:

what is it then? state the number of ohms. typical honda civic battery.

it is when it's the largest resistance in the circuit!

and?
sure. and how much did the voltage drop?
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Steve W. wrote:

rubbish.
"likely"??? dude, it /definitely/ does! grade school science class.

eh? welding is a whole different ball game to shorting a fuse protected light circuit. welding is done at voltages and currents in excess of those seen in vehicle operation, and is accompanies by all kinds of radio frequency noise, both from the spark and the welder, depending on welder type! yes, it's good to disconnect the ecu while welding. no, shorting a light circuit doesn't replicate those conditions.

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Guys, guys... go get a schematic and find out if the dome light is in the same circut as the ECU. Even if it is, I don't think the changing of the bulb is what caused the problem. I do think that whatever caused the light to function intermittantly MIGHT have cused the ECU problem. Either way, if the light was working sometimes, that would tell me that there was nothing wrong with the bulb. Funny thing about light bulbs... they either work, or they don't. The filament is either complete, or it's not. It doesn't go back once it's blown. Okay, so if there's a short, it would be either in the fixture, or in the wires going to it. Furthermore, That's what fuses are for! I've done it many many times. Shorting things out and blowing fuses and replacing them again. The fuse opens the circut in the event of a short before ANYTHING gets damaged (unless the fuse is either disregarded, or replaced with a fuse of a higher amperage). And furthermore, since we're on the topic of fuses, this is the whole reason that things like dome lights and tail lights and headlights and radios and ESPECIALLY ECU's are all on thier own circuts with thier own fuses. Kind of insurance, if one thing blows a fuse, you don't lose everything at once. I do agree with the titlehead of this issue, being "disconnect the negitive before working". But this rule is mostly in place because sometimes people like to play with the wires and accidently drop the exposed end of a hot wire onto the bare frame of body of the vehicle. The result can be a blown fuse at minimum, or a small fire at most (I've been there, too). However, I regress (and conclude), the dome light bulb cannot be the reason for ECU problems. ECU's may or may not be "idiot proofed," but they are not on the same circut as the dome light, the dome light cannot cause "power surges" and niether can anything else on a vehicle, and even if anything were to happen, the fuses would have caught it. Case Closed. Go get a new ECU, and re-wire your dome light. quit griping about crap that is not relevant. The more you gripe, the more other people despise you, and the job still is not done.
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On Tue, 13 Nov 2007 04:33:54 +0000, fury45iii wrote:

Yeah, but we're talking a Nissan here! I've seenthings in Nissans I've never seen in other cars.
And, he said the glass was broken. How it even lit without burning out is a mystery!
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First you said that the bulb was in pieces, not that the glass was broken. Was it really broken? I have seen several times that the glass part of a bulb can separate from its socket but still hanging by the leads it may still work. And I have also seen that the ends of a broken lamp filament can dangle together and get get kind of welded together, so it works again, but usually not for long. What may kill an ECU is the very very short transients in voltage when a cirquit somewhere in the system is abruptly opened or closed, especially when it is high current (a short) and/or repeated many times. If he made a momentarily short cirquit while changing the bulb that MAY be enough. That is why with modern cars we are advised not to use jumper cables without transient suppressors.
Asbjrn
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A welder and a dome light are vastly different things. An arc welder generates a HUGE reactive voltage spike when striking the arc, and that spike can wander all over the entire vehicle and fry sensitive electronics and not-so-sensitive things, too, like alternator diodes. Standard procedure there is to disconnect the battery whenever doing any welding on the thing. Shorting a dome light will NOT generate any sort of spike. Period. You need a coil to generate spikes, coils like those found in starters, alternator rotors, ignition coils. You might as well try to generate a spike by disconecting and reconnecting the battery.
Dan
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Dan_Thomas snipped-for-privacy@yahoo.com wrote:

Lamp filament is a tiny coil, LOL!
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Tony Hwang wrote:

with an air core and virtually no inductance. if you have instruments that can measure any voltage spike you get from a coiled bulb filament, i'd love to see your readings.
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I do.
BUT, you don't need one. If you have an 1141 lamp with 50 loops of 1/16" (that's estimated by eye here) about 1/2" long, you can use Wheeler's Formula:
50^2 * (0.065)^2 10.5 --------------- = ----- = 0.27 microhenries 18 * 0.065 + 40 * 0.5 28.0
That's a lot less than the hundreds of henries that an arc welder ballast will have, and it's probably small enough to be compensated for by the capacitance of the wiring, even. But it's enough that you could measure it carefully with a scope and a pulse generator, even though it's actually going to be swamped at any reasonable voltage by the nonlinearity when the filament heats up and its resistance increases. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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Scott Dorsey wrote:

excellent. and it's /well/ below any level that could /possibly/ impact the ecu!

indeed.
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jim beam wrote:

You seem to forget that the dome light is 'live' all the time using a ground as a switch.
They can have the glass fall out of the base and the loose internal connecting wire(s) can and will cause a short circuit. This happens all the time with bulbs such as the 1157 in taillights.
Short circuits can do nasty things to computers like blow the input diode 'before' the fuse has a chance to blow.
It's not like the designers of modern vehicles are just BSing when they say to unhook the negative before working on them.....
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 'New' frame in the works for '08
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Hello:
This might seem odd to you but, newer cars use bulbs that are a specific impedance. A 2157 is the same candle power as an 1157, but the impedance is higher. That's why if you use a 2157 in place of a 3057 for argument's sake.
You'll get a check engine light in an OBD2 compliant vehicle.
RK
wrote:

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On about 92 and up:
The dome light is connected to the Body Control Module.
RK
wrote:

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