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#35 of 1159 electrical shortage???
Jul 20, 2006 (12:48 pm)
Hi, I'm new to this forum and website. And I'm not going to start off well, because I have a problem!! I have a 1996 Toyota Camry station wagon, and yesterday when I got in the car, the clock, the radio, and the overhead dome light didn't work. I wouldn't have cared as much if I wasn't driving 4 hours home from Maine, but I figured it was a fuse, and I'd just change it at home. I got the new fuse for it, but as soon as I put the new fuse in, that fuse blew too. So the guy at the autoparts store said that I probably had an electrical shortage that had something to do with THAT fuse in the fusebox. I know I can't really get off complaining because those are the only three things in the car that I don't really need, but I'd still like to fix this. Any ideas as to whether or not I can do this myself and how? Or, how much do you think it would cost in the shop? Has this happened to anyone? Also, when we opened up the fusebox in the engine compartment, the dome/clock/radio were all using a 25 mini fuse, when it relaly called for a 20 mini fuse. Could that be related?
#36 of 1159 Re: electrical shortage??? [scitsailor]
by MrShift@Edmunds HOST
Jul 20, 2006 (1:15 pm)
If you have a short, it wouldn't matter if you used a 100 amp fuse, they'd all probably blow.
dome lights are notorious for short circuits...but...since a "short" circuit means that a wire is grounding "short of" the destination, you'd have to trace the wire all the way from the fuse box to the dome light.
You might start by looking at the switch in the door that works the dome light...remove that switch and tape it up so that the wires can't touch anything. Let the switch dangle in outer space and put in a new fuse.
If that doesn't work, drop the dome light out of the headliner and do the same thing. Make sure no wires are touching and just let the dome light dangle up there---if either of those cures the problem you know the short was either at the door switch or the dome light area.
The radio rarely causes such a hard short like this unless it was one of those home-grown installations done with tape and wires twisted together. You might look under there and see if you see a lot of homemade wiring. Factory wiring is neat and tidy.
As for the clock, that might be hard to pop out of the dash to check.
Last area would be BEHIND the fuse box, where you'd have to unscrew the entire fuse box and inspect behind it, in the area of the fuse that is blowing.
Always exercise caution when fooling with wiring. I always like to have someone nearby who has loosed the positive battery cable so that it's just a slip fit on the battery. If something starts to smoke, you yell and they yank.
#37 of 1159 Re: electrical shortage??? [Mr_Shiftright]
Jul 20, 2006 (2:02 pm)
"...I always like to have someone nearby who has loosed the positive battery cable so that it's just a slip fit on the battery..."
Unless Toyota is different from everyone else selling cars in North America, that's wrong. Whenever disconnecting an automobile battery in most production cars over the last 50 years, always pull the negative (-) battery cable from its terminal first. Pulling the positive (+) cable first is apt to spark, with the potential to set off a hydrogen gas explosion at the battery and send pieces and hot sulfuric acid flying in all directions. (The acid and battery shrapnel aren't at all conducive to continued eyeball function, but they can sure be a dandy motive for learning Braille...)
#38 of 1159 Re: electrical shortage??? [Mr_Shiftright]
Jul 20, 2006 (2:19 pm)
where abouts in the door is the dome switch?
#39 of 1159 Re: electrical shortage??? [Mr_Shiftright]
Jul 20, 2006 (2:27 pm)
shortage is something experienced when there's too little of something.
you mean electrical "short" or short-circuit.
i agree with most of what shifty wrote. except me personally, i would have the negative leg of the battery loose and the one to be disconnected by a bystander.
think about replacing a battery: the very first terminal to pull is the negative. why?
1). If you are using a metal tool without insulated grips on the positive terminal first (i.e. with the negative strap still attached), you are creating a closed circuit with your body if another part of your body is in touch with the metal body of the vehicle. it's not voltage necessarily that can defib the heart, but current, and chances are the path you've created passes in close proximity to the heart. if the current draw is just right, i think you can have an interesting experience.
2). if you have a battery which is de-gassing, shorting the positive with the negative (say the tool slips and touches part of the frame) will cause a nice short almost directly, but in close proximity to the battery. there's likely to be sparking, and where there's spark and degassing... possibility of explosion.
(2) is one of the reasons why when you jump a car, you connect positive battery terminals first, and then one leg of the negative jumper cable to the good battery, and the other end of the negative jumper cable (i think i have this correct) to some exposed piece of metal on the car your trying to jump far enough away from the battery that any sparking will be unlikely to cause a problem if there is degassing.
now some may say, but wait when I'm replacing an old battery with a new one, and i install the positive first, then the negative, if i'm physically in contact with the negative at the time i am also touching the body of the car, won't there be current flow? i think the answer to this is yes and no. yes there will be current flow, but more current will flow through the path of least resistance, that being the terminal of the battery and cable clamp, and not through the vehicle body, your body, to the cable clamp.
shifty - did i get this one correct?
one edmunds tee-shirt extra large pleese.
ok, just exercise caution with the battery.
#40 of 1159 Re: electrical shortage??? [user777]
Jul 20, 2006 (2:54 pm)
Shortage actually works in this sense if you think about it, because there is a shortage of electricity getting to the things I need the electricity to get to. Ie: there is none.
#41 of 1159 Re: electrical shortage??? [scitsailor]
Jul 20, 2006 (3:16 pm)
when you have a short with very little resistance, you have maximal current flow. Voltage = Current * Resistance. The volts across the battery is 12, so as resistance decreases, current increases.
this is why a fuse rated for a given current blows.
so - you're in a situation of excess and shortage at the same time i guess.
agreed, the current is flowing in the circuit which has the short, and not as much (relative) in the circuits that are well behaved.
shify's idea to pull the over-head dome assembly is a good one. if you have a custom, non-OEM radio/amp, I'd start there myself.
#42 of 1159 Re: electrical shortage??? [user777]
Jul 20, 2006 (4:34 pm)
I had a short once in an old vette, found in the wiring harness the flexed between the car and the drivers door everytime it was opened. Years of opening or closing caused the failure.
Personally however with your limited symptoms, I'd look at the dome light first (easiest), unless someone messed with the factory radio.
You may want to look at the electrical schematics for that year/car (see the 'other' web site), to make sure there aren't items on that circuit that you don't realize aren't working (like a lighter).
#43 of 1159 Re: electrical shortage??? [user777]
by MrShift@Edmunds HOST
Jul 22, 2006 (9:25 am)
RE: Short circuit:
Maybe I'm trying to outsmart myself here.
My reasoning was not to take OFF the positive cable, just to loosen it a bit so you COULD yank it off.
Now why the positive? Well, yes, conventional wisdom says to take off the negative and this is probably good advice. But you know, either one works in an emergency. Maybe BOTH should be loosened, to give the stand-by person TWO changes to interrupt an electrical fire.
My understand of "short" is that the current falls "short" of the target, that is, it completes a circuit well before the circuit that was supposed to be completed. After all, the dome light is the end of the line, so a live wire touching ground before the dome light has completed a "short circuit" back to the battery.
I really don't believe the term is related to "short" as in "lesser amount". It's "short" as in distance, not volume.
ANYWAY-- I'm always nervous about giving electrical advice out to anyone without supervision, but if they use common sense and have a back-up plan (like two people working on it), they should be okay.
Nigel Shiftright once told me, with singed finger pointed in the air: "Destruction happens at ten times the rate of construction".
#44 of 1159 Re: electrical shortage??? [Mr_Shiftright]
Jul 22, 2006 (10:37 am)
if the yanking is done without contact to the body of the vehicle and the terminal (i.e. someone grabs the insulation), which wire you pull is gonna be immaterial to the interruption of power.
but, you'll have to re-attach that positive at some point won't you? then you'll have the issue with possible shock.
short: well that is essentially a path to ground with little intermediate resistance. what is the resistance of a length of wire? there is a formula, but for most wire used in a car, it is pretty negligable.
where there is small resistance, more current must flow, hence a fuse rated for X amps will blow when sinking more than X amps. remember volts = current * resistance.
a fuse in a car, or in a house is rated such that it is the weakest point in the circuit, right? you want the fuse to blow when it senses too much current flowing. the heating which will result will cause the fuse to fail.
if the fuse were rated larger than the current carrying capability of the wire, or metal conducting parts in the destination device, guess where the heating is going to occur and where the circuit will open.
you never want to "over-amp" a fuse or a circuit breaker in your home for the given wiring. you want the fuse or ckt bkr to trip and protect the wiring and the device.
in a car: no vehicle fires please.
as to fall's short of the target, in general this statement is true, however the target (like the overhead dome light) can be the target. i understand what you are saying, the real target is the bulb in the overhead dome light. no argument there.
as far as loosening both clamps... i'm not going to give you advice shifty. you're the auto guy. me though? i wouldn't do it to either terminal. i mean to say there is a time to remove the negative and work on a circuit, and there is a time to re-connect... but to leave loose? nope. i can't see the advantage, and only see a disadvantage. let's say you got a current drain and you are tracking it down, and lets say you leave 1 or 2 loose clamps. guess where there is going to be resistive heating (happens where the amount of conductor / contact is smallest)? just like the sizing of the fuse (generally set by thickness of fuse material). guess also where there is gonna be potential arcing?
if you have a battery out-gassing...
well. maybe *i'm* over analyzing the problem now.