Keep Blowing Fuses

I am replacing my running/clearance lights on my 1976 Class C, but I keep blowing fuses. I can't seem to find out where the short is. Any ideas?

It could also be caused by a bad wire contact, not necessarily a direct short. Poor wire contact (often a bad ground) causes the amp draw to increase. If it increases too much, it'll blow the fuse. It can be tough to find the source of the bad contact, but I'd start by pulling the clearance lights and looking at how well the wires are connected to the fixture. In a 1976 rig, odds are you're going to find a fair amount of corrosion on wherever wires are connected to anything.

Kev

I did a temporary fix by disconnecting the marker lights. The fuse was also connected to the back running lights as well.

Now that I disconnected all the marker lights, the fuse is not blowing.

I figure that as long as there are reflectors on the light covers, I should be OK.

I don't know of any laws that require marker lights to be operational on RVs.

KD7ONE said:

I don't know of any laws that require marker lights to be operational on RVs.

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It's definitely required in Connecticut. But unless you're doing something outrageous, RVs are pretty invisible to law enforcement.

Kevin Means said:

It could also be caused by a bad wire contact, not necessarily a direct short. Poor wire contact (often a bad ground) causes the amp draw to increase. If it increases too much, it'll blow the fuse. It can be tough to find the source of the bad contact, but I'd start by pulling the clearance lights and looking at how well the wires are connected to the fixture. In a 1976 rig, odds are you're going to find a fair amount of corrosion on wherever wires are connected to anything.

Kev

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Not so. A "poor wire contact" would be defined as one with a greater resistance than normal.  Ohm's law states that current is equal to voltage divided by resistance. Therefore, a high resistance contact would result in decreased, rather than increased current.

Joel

HappyWanderer said:

It's definitely required in Connecticut. But unless you're doing something outrageous, RVs are pretty invisible to law enforcement.

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Same in NH.

The legal requirements for marker lights are based on vehicle  But as size & weight, not private vs commercial or RV vs truck.  But chances of being ticketed for lack of them are probably slim.

You should isolate the marker light system in pieces, e.g. try the front only vs the rear only to narrow down where the short is. Then proceed from there to narrow down further.

Gary RV_Wizard said:

The legal requirements for marker lights are based on vehicle  But as size & weight, not private vs commercial or RV vs truck.  But chances of being ticketed for lack of them are probably slim.

You should isolate the marker light system in pieces, e.g. try the front only vs the rear only to narrow down where the short is. Then proceed from there to narrow down further.

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You mean front first, then back? Then individual lights from there?

Another thing you might consider is upgrading to LED marker lights. I do that whenever I replace a lamp. Less current draw.

how big is the fuse you are blowing?  Marker lights burn about  I can not easily find the specs on Marker Light BUlbs but I'm guessing around 1/2 to 3/4 amp a pop and you have a LOT of marker lights.

Great Horned Owl said:

Not so. A "poor wire contact" would be defined as one with a greater resistance than normal.  Ohm's law states that current is equal to voltage divided by resistance. Therefore, a high resistance contact would result in decreased, rather than increased current.

Joel

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I stand corrected.

While you're tracing the short, put a 12 volt incandescent light bulb in place of the fuse that's blowing.  It will glow dimly under normal conditions (depending on how many other light bulbs are on the circuit) then glow brightly while limiting the current to a safe value when the short is present.

Considering the number of bulbs in a marker light circuit, something like a sealed beam headlight (50 watts or so) should work.  It will barely glow when the marker lights are operating normally but will go to full brightness under a short.

The two parallel pins on a 3 pin sealed beam bulb will fit into a standard extension cord and activate the high beam filament.  Obviously don't plug the other end of the extension cord into 120 volts unless you're into fireworks. 

You could even put long leads on it and place it outside where you can watch it while wiggling the wires.