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How many amps to fuse the Headlight circuit?  Rating:  Rating
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 Posted: 08-08-2005 05:51 pm
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edward_davis
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Location: Eugene, Oregon USA
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My JH, the orange '75 I bought from Jim Fritts, had an aftermarket fusebox installed by the PPO.  When I first got it, the headlight circuit burned out at one of the crimper connectors between the fusebox and the original wiring harness.  I thought the problem might have been in the fusebox itself, so I removed it and rewired the whole shebang with another set of aftermarket fuse boxes. 

Well, the box had been set up with a 20 amp fuse for the headlights, so I reused it for my new set-up, and it seemed to work fine when I tested it in the garage.  But I was out driving yesterday and turned on the lights, the fuse blew immediately.  Could it be that the lights are drawing more than 20 amps when the alternator is going?  I'll have to do some more experimentation, but I thought I would write in and ask if anyone had an idea how many amps the headlights (with all the marker lights and dashboard lights, etc.) would be expected to draw.  What was the orignial fuse in the 3-fuse Lucas box rated? 

It is nice to have the additional fuses, so that a short in the headlights doesn't cause a bunch of other circuits to fail.  I had a VW with only 5 fuses for a long time, and that was bad enough.  I have the car on 12 circuits, which helps a lot with troubleshooting.


Do I have a short, or just too small a fuse?

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 Posted: 08-08-2005 10:20 pm
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Mark Rosenbaum
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The stock JH didn't use a fuse in the headlights circuit -- unless you consider the wire itself to be the fuse.

According to the shop manual, the steady state power consumption of the various lights in the headlights circuit is:
Headlights       50W   x2 = 100W (high beams)
Side markers     6W   x4 =   24W
Tail lights          21W  x2 =   42W
Instruments      2.2W  x8 =   17.6W (6 gauges, heater console, hazard switch)
License plate       6W  x2 =  12W


The total is 195.6 watts.  Since watts equals volts times current, at the miminum system voltage of 12.0 volts the total current is 16.3 amps.  That means that a fuse rated for a steady state load of 20 amps is just about right.  Therefore, if your fuse blew when you switched on the lights, either there was a (perhaps momentary) short, or the fuse failed due to internal metal fatigue.  And fuse fatigue failures are extremely rare unless the fuse is being used in a circuit with a history of shorts, near-shorts, or overloads.

Normally the only time when it makes a difference whether power for a car's electrical bits is provided by  the alternator, the battery, or both, is when the alternator regulator has failed and the alternator tries to compete with the nearest electrical utility.  This is the sort of rare event that causes wiring harnesses to melt, so blowing fuses would then be a good thing.

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 Posted: 08-30-2005 01:45 am
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edward_davis
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I'm wondering how many JH owners have gone ahead and put in at least an inline fuse for the headlight circuit.  It seems like a good idea, and a simple fix.  Just wire it into the brown wire that goes into the back of the headlight switch...

The whole problem with my lights seems to have stemmed from a short in the license plate lights.  The plastic lens that supports the contacts for the bulbs broke, and the hot lead grounded against the boot lid inside the light housing.  It seems to have happened before, as the lens is smoky and black, and the electical tape around the wires there was melted and fused together.  Also, amongst the parts I inherited from the PO was the original plastic three-point connector for the back of the tailight, melted and burned.  Looks like the license plate light shorted, the switch overheated and began to burn, and the PO cut the circuit and installed the fusebox I inherited.  He fixed the problem on one side, but in my case, the other license plate light broke in the same way and created the same short again.  Except that now, the short just blows the 20 amp fuse and there is no burning.

I fixed the license plate lights with superglue and put a lot of electrical tape on the metal inside the light housing in the bootlid.  This seems to have fixed my problem, but it would be a shame for someone's beloved car to burn up simply because of a short in the little license plate light.

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 Posted: 09-01-2005 06:41 pm
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Mark Rosenbaum
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My car's license plate lights only short out to the trunk lid when I try to remove them with the power on.  I don't have a problem with contacting the inside of the trunk lid, though things do come close.  My response to this was similar to Edwards, but rather than using electrical tape I applied sections of a reflective tape sold to mark fence posts and the like.  It seems to slightly increase the brightness of the license plate lights, too (though this may be purely psychological).

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