Confused lighting issue

[cob427sc]

I am nearing completion on my 818 that I purchased partially completed from another builder. It uses a 2004 donor but has an Iwire electric harness. When I turn on the parking lights everything works fine but when turning the switch on for the headlights the headlights work fine but the fuse for the parking lights/tail lights/plate light blows! Electrical has always been my weak area and this has me stumped. I've tried disconnecting various lights (and testing them independently) but still can't find the problem. Could it possibly be that the light switch itself has a short inside? any and all suggestions welcome.

[RPGs818SNA]

The light switch is unlikely to be the problem, as it simply grounds the low side of the lights to make them shine. If it grounded the wrong wire, it would just make the wrong light shine. A more likely cause is a connection going from the wires connecting the tail light fuse to the tail lights to the wires going from the headlights to the headlight switch. That would cause your symptoms. Here's a simplified drawing.

Blowing fuse.jpg

In order to track down the misconnection, I’d try to cobble together something to replace the fuse that is blowing with a 12v bulb, turn on the headlights which should light the bulb, and then follow the fuse wire to the tail light bulbs to find the problem. That won’t make it easy, but might make it doable.

Good luck,
RPG

[cob427sc]

I'll give this a try. I wasn't clear on this but it affects all of the tail/front/sidemarker/license plate lights. They all run on the same fuse and go off when it blows. I have tried disconnecting lights to see if I can find a bad bulb/fixture but haven't found the problem. What confuses me is that they all work fine when only the light switch is in the parking light position. The fuse blows when the headlights are turned on. If there was a short in the parking light circuit it would blow the fuse when switched in that position.

[RPGs818SNA]

If you had a bad bulb or fixture, the fuse would blow in the parking position. If you have a short from the parking power wire to the top of the headlight switch, the parking lights would work fine if the headlight switch is open. But when it closes, it will blow the parking fuse as current passes through the parking fuse and short and headlight switch to ground. There may be other problems that would also cause this behavior, but this seems the most likely. Good luck tracking it down.