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Basic fuse question


skillinp

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Hi!

I was having some trouble with my left headlight being really dim compared with the right hand side (yet according to my multimeter, the connection was still pulling 12V), so I looked around and found it might be a fuse problem or a ground problem.

I looked here:

http://www.classiczcars.com/forums/electrical-s30/4684-fuse-box-73z.html

Found the fuse that I needed to check, the upper right hand one.

I pulled it and just for kicks, turned the lights on. Nothing changed.

I still had the dim left hand light.

Um wat?

Put the fuse back in after dusting if off with my fingers, the light is now 100%

But I think that the fact that the left hand light still worked, albeit at only partial strength, must indicate something else as being wrong. How could the light work at all without the fuse? Should I be checking for crossed wires or something having melted?

Thanks!

Paul

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Bad grounds would be my first suspicion. One of the grounds on the front end of the car is corroded and the current is finding the path of least resistance. Going through the other light to ground is easier than going through the corroded ground. Just a thought...

Charles

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You're backfeeding through the headlight. The car switches between high and low beam by changing which headlight element is grounded. (That's the high low beam switch.)

When you have the switch on low beam, current flows through the good fuse and down to the headlight bulb. There it splits in two. Most of the current will flow through the right low beam filament, to the high/low beam switch and back to ground. A little bit of current will flow through the right high beam filament, on to the left high beam filament, to the left low beam filament, to the high/low beam switch and back to ground.

Now to break out the EE math...

We can calculate the amount of current that should be flowing versus the amount flowing when there is a backfeed.

Let's use the stock ratings for the bulbs (50W high and 40W low) and assume the ratings are for a 12VDC system.

Current is Power/Voltage, so with good fuses, you're pulling about 3.3A on low beam.

We can estimate the resistance of the low beam filament at 3.6 Ohms and the high beam at 2.88 Ohms.

This means we have 3.6 Ohms going through the low beam on the right and 9.36 Ohms for the backfeed.

Since Current is also Voltage/Resistance, we estimate 1.28A going through the backfeed.

Power is equal to the square of the Current times Resistance, so this is the power consumption for each filament in the circuit:

right/left high beam = 4.7W

left low beam = 5.9W

post-5413-14150825287741_thumb.jpg

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You're backfeeding through the headlight. The car switches between high and low beam by changing which headlight element is grounded. (That's the high low beam switch.)

When you have the switch on low beam, current flows through the good fuse and down to the headlight bulb. There it splits in two. Most of the current will flow through the right low beam filament, to the high/low beam switch and back to ground. A little bit of current will flow through the right high beam filament, on to the left high beam filament, to the left low beam filament, to the high/low beam switch and back to ground.

Now to break out the EE math...

We can calculate the amount of current that should be flowing versus the amount flowing when there is a backfeed.

Let's use the stock ratings for the bulbs (50W high and 40W low) and assume the ratings are for a 12VDC system.

Current is Power/Voltage, so with good fuses, you're pulling about 3.3A on low beam.

We can estimate the resistance of the low beam filament at 3.6 Ohms and the high beam at 2.88 Ohms.

This means we have 3.6 Ohms going through the low beam on the right and 9.36 Ohms for the backfeed.

Since Current is also Voltage/Resistance, we estimate 1.28A going through the backfeed.

Power is equal to the square of the Current times Resistance, so this is the power consumption for each filament in the circuit:

right/left high beam = 4.7W

left low beam = 5.9W

Man, EE was definitely one of the tougher classes for me in college, and it's still intimidating! However, I think I understand you here. Is that anything to be worried about, do you think? The part in EE they never explained (annoyingly) was what the real-world consequences would be for various scenarios, like this one.

Thanks!

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Eons ago, back in the 70s, when I was racing and street driving my 240 Z I had the exact problem with my headlights. Being a very good customer of the local Datsun dealer, I went in to ask the experienced shop foreman about why one headlight was not as bright as the other. He said to change the fuse for that light. It fixed the problem. His explanation was that the fuses break down over time and, although not totally blown, they are not 100%. The fuse change worked and I never had that problem again.

Charlie

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While fuses can degrade, it's also wise to check for a short. Pull the fuse and disconnect the headlight from the engine wiring harness. Use an ohmmeter to measure from the inside post of the fuse holder to ground. It should show an open line between those two points. If so, replace the fuse and go, and while you're at it, replace the other fuses, too. They are probably just as old.

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although not totally blown, they are not 100%.

As an electrical engineer I've got to call b.s. on this "theory"

A fuse may have corrosion on the outside that adds resistance to the circuit, and dims the lights, but the fuse itself is either 0% or 100%. No cigar.

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As an electrical engineer I've got to call b.s. on this "theory"

A fuse may have corrosion on the outside that adds resistance to the circuit, and dims the lights, but the fuse itself is either 0% or 100%. No cigar.

The fuses are not perfectly sealed. They can develop corrosion inside. Also, I have seen fuses heat up enough without blowing that the connection between the fuse material and the end cap failed.

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I had the mindset that fuses were always either open or a dead short and so confident with that that when I checked a fuse with an ohmmeter if I saw any movement of the ohmmeter needle I assumed I was about to read a dead short and would disconnect the leads without actually getting an accurate reading. Then once upon a time I found a fuse that had partially blown open (or had corroded?) but I thought it was good because the meter needle would start to swing but in fact it was no good because I didn't wait for the needle to finish swinging to get an accurate reading which would have shown resistance and not a reading near zero ohms. I only saw that once, but I remind myself of that lesson from time to time, that electricity is always smarter than I am.

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  • 2 weeks later...
Hi!

I was having some trouble with my left headlight being really dim compared with the right hand side (yet according to my multimeter, the connection was still pulling 12V), so I looked around and found it might be a fuse problem or a ground problem.

I looked here:

http://www.classiczcars.com/forums/electrical-s30/4684-fuse-box-73z.html

Found the fuse that I needed to check, the upper right hand one.

I pulled it and just for kicks, turned the lights on. Nothing changed.

I still had the dim left hand light.

Um wat?

Put the fuse back in after dusting if off with my fingers, the light is now 100%

But I think that the fact that the left hand light still worked, albeit at only partial strength, must indicate something else as being wrong. How could the light work at all without the fuse? Should I be checking for crossed wires or something having melted?

Thanks!

Paul

Perhaps a little late, but I just discovered something which could have led to your problem. Background: A couple of months ago I had a dim headlight, pulled fuse, still on but dim. Cleaned connectors, blew out the fuse box area, got'em to work.

A week ago I had a total breakdown 50 miles from home. Suspected electrical, suspected fusebox and was close enough to right. While it didn't cause the stoppage, when pulling the fusebox for replacement and inspecting the wiring harness, I discovered a small amount of melting in the body harness across the headlight wires. Several years ago I had high-beam switch meltdown, which I know suspect may have caused the melt years before and it was just chance that they came together to short, and chance that my cleaning moved them far enough apart to not short. Just something to check if you haven't solved it.

Chris

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