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interior light short

4521 Views 29 Replies 3 Participants Last post by  johndfw
Hey folks

Anyone know how to track down a short at the interior light cir.?
I replaced the fuse and it blew once I got going.

Any help would be appreciated
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Here's a little trick I've used countless times to find shorts in fused circuits:

Since the fuse blows and you know you have a dead short, start by pulling the fuse and connect one side of the ohmeter to the shorted side (not power) of the fused circuit. Clip the other lead to a good ground and you should get 0 ohms or very low resistance (this verifies your shorted condition without having to use a fuse). If the ohmeter has a tone to verify ) ohms, start wiggling your wires in the lighting circuits, documenting everything as you progress. For interior lighting shorts, look carefully around the door switches and wiring of the harness where it goes between the door and car for signs of chaffing. Cycle the doors while watching the ohmeter, etc. Keep at it until you isolate the short. The ohmeter is a great tool, but your eyes may find it just as easily! Good luck!
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Let's stick with the original fault which may or may not have anything to do with a previous radio replacement. With the radio disconnected, does the lighting problem go away?
Okay, let's start with the basics; this problem HAS to be driving (no pun intended) you nuts already. . . When you pop in a new fuse, idoes it blow instantly? If not, does activating some portion of the lighting circuit cause the fuse to blow? --Headlights, door switch closing, etc. If the fuse blows instantly, if you place a multi-meter on the shorted side (not the power side, or you get to buy new fuses for your multi-meter!) of the fuse holder (--just to play it safe and avoid measuring the wrong test point, make sure the fuse is removed, too!) what kind of resistance reading are you seeing? --This is to check for a dead short (0 ohms) as opposed to low resistance. Okay. . . it's time to play twenty quetions now: What work has been done on the vehicle lately? When did you first notice the problem? Was it intermittent, or did it occur and never go away? If it did appear and take up residence, what were you doing when you first noticed it? Report back and we'll continue!

--No problems, just solutions awaiting discovery! CC
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Pulling bulbs might not be the best course of action, as bulbs (and LEDs) don't generally short. They open. A decent multi-meter need not be expensive and they're great for all kinds of projects and repairs. A meter is to electrical guys what a hammer is to carpenters or an IED is to terrorists. I highly recommend obtaining one and having someone teach you how to use it; it isn't difficult to use once someone shows you how. Whoops, just fell off my soapbox! Let's get back to the problem at hand.

Here's one (of many) routes you can take, based on the test equipment you currently are using. Connect your lighted tester, and verify the short is present. Find the brightest flashlight you can get your hands on and get ready to use the second best piece of test equipment in your arsenal: The good old, MARK 1/Mod 1 eyeball. Take detailed notes as you do perform this procedure to prevent repetition and find your way back if the symptoms change while you progress toward finding the fault.
Open the doors, and cycle the switches that activate the interior lights. Shake the harness that exits the doors and enters the car near the hinges. If you can remove the door panels, go ahead. What we're looking for is chaffing of a wiring harness on a repetitively (doors opening and closing regularly) moving surface. If you can get in the door panels, slowly and carefully inspect the harnesses for any signs of wear and tear, especially where the harness contacts any metal surface as it routes through the vehicle. A short can be impossibly small physically, but still able to cause enough problems to ensure you pull most of your hair out while troubleshooting, so use that super-bright light you've got to eyeball the harnesses while rolling them around to look at all sides.
This is basically the procedure for the entire lighting system you need to perform. Trunk lids (another moving surface with a harness to chaff) are another culprit. Go slow and eliminate as much as possible, then report your findings. Look for bent leads on switch contacts, broken/cut wires, etc. If the trouble light goes out while you're shaking harni (plural for harnessesessesss in geek-speak) you're there buddy!

Don't despair, there are many ways to eat this elephant, but the basic tactic is always: One bite at a time! Go man Go!!!
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To answer your question, I don't have a lighted circuit continuity tester. I'm a multi-meter kind of guy with 25 years' exp with electronics. I've spent more hours than I care to remember chasing elusive problems that make you want to seek therapy by the time the fault is isolated --especially with shorts and opens, while working on a wide variety of systems, from submarine weapons systems to spacecraft. A continuity tester will work for this problem, but a multimeter gives you more info, like resistance to ground. Any tool is better than no tool when you're working on a problem like this!
Ahhhh. . . some more info. The mystery deepens. . . I don't want to derail your thought train, but the flickering dome light might be a symptom and not the cause. If you placed a new fuse in the circuit and you had partial operation for five seconds, that is a good clue. A hard short would usually blow instantly. (I'm using generally and usually becuase there are no hard and fast rules when troubleshooting. One must always keep an open mind to avoid tunnel vision.) A slow-blo fuse might work for a VERY short time. You might try this, just 'cause it's quick: Open all doors and the trunk and install a new fuse. (Just in case a chaffed harness is contacting the frame.) Keep us posted!

Nice to hear you've made substancial progress John. I'm not convinced the problem is resolved though . . . just dormant. I don't mean to rain on your parade, It's just that I've seen phantom faults dissapear, only to show up when I least need or expect them. So . . . with that said, I would zero in on your door/trunk harni and give them a very slow, methodical visual inspection, paying close attention for a small, black speck (could be as small as a pencil lead) on the vehicle's frame in the vicinity of where the harnesses come closest to the frame when the particular door is closed. Look for a small, black or whitish discoloration on the harnesses, or tiny pinch marks in the insulation. Use the brightest light available to you. There's also the other side of the argument that has credence: "If it works, don't mess with it." The choice is of course, yours. Good luck with this and don't hesitate to return to this forum for any help or advice from the excellent members who frequent these areas.

Cheers and Happy Motoring! CC
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