Internal lights fuse problem

My father has a 1994 E34/M50 that keeps blowing the fuse for the internal lights. It seemed as if the problem was with the switch for the trunk lid, but it doesn't appear that's the case. Apparently
there's a short somewhere, but we're having trouble tracking it down. Here's what has happened:
He would go to the gym, open the trunk, take out his bag and all was well. The trunk light would work. Close the trunk, go into the gym, come out, open the trunk and the light wasn't working. Fuse blown. This has happened several times. Yeah, we know, if the fuse keeps blowing, then something's doing it for a reason. But he's been letting this happen to try to diagnose the problem. (He's not stupid with this as he's an electrical engineer by trade.) He's tried moving around the wire harness around the trunk hinge to reproduce the problem, but no dice.
Since the problem seemed to be associated with opening and closing the trunk he disconnected the trunk switch for the light to see if the problem was with the switch itself. Fuse still blows. So that's not the problem.
The latest experiment involved locking the car and then pulling on the driver's door to turn on the interior lights. (Trunk switch is still disconnected.) Lights turn on then eventually fade out as they're supposed to. He does this three or four times and things work fine. The next time he does it, the lights don't turn on. Fuse is blown.
So the problem is elsewhere. He has a Bentley manual and has the wiring diagrams to refer to, but he doesn't know where to start looking. Does anybody know where the problem lies?
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so in short it's activated by the very slightest movement of the car............gonna be a bugggger to track that one.
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Normally, the switch just completes a ground circuit so any fault in the switch shouldn't result in a fuse blowing.

I'd be inclined to look for a wiring fault where the loom goes to the boot lid - insulation frayed through causing a short somewhere.
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Dave Plowman (News) wrote:

Yeah, he has looked there and he hasn't seen anything positive. It's my understanding that the manual doesn't show where the wiring is positioned and I have a feeling that it's not exactly easy to reach. While the Bentley is the best third party manual available, it's lacking in many respects. I was kind of hoping somebody might have seen this before or if this was a problem that somebody had a clue as to a specific place to look other than where we've looked already or if somebody knew the more likely places to look.
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No, the switch doesn't complete the ground circuit. According to the diagram, it is in the hot line and supplies 12V to the lamps.
The blown fuse is simply parted, no evidence of major current flow or of any violent situation.
(This is the guy with the problem.)
And there is essentially NO movement of the car. The fuse blows with the vehicle simply sitting quietly.
Bob
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Bob wrote:

As Dave mentioned - a VERY common failure point for the boot-lid (trunk lid) wiring is the section that connects the lid to the body of the car. People have had cars catch fire because of failure of this section. The fix is to open up the bundle of wires and find where the insulation has worn through, then tape it up.. or replace that section of the harness.
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I've not got a full diagram for the E34, but assumed it was the same as the interior lights - and the door switches for those do just provide a ground. However, it matters not if the problem still exists with the switch disconnected.

Right.
If you have the circuit diagram you'll just need to check what else that fuse feeds (if anything) and check all the wiring. It's still likely to be a short somewhere - and this wouldn't necessarily show a 'violent' situation due to the resistance of the cable run.
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No, the switch is in the hot lead. The lamps go to ground. At least the Bentley book says so.
I guess I'll pull apart the harness and look for trouble there. But I am confounded because nothing touched the car and the fuse went. The dome lights worked fine and then poof.
To make it more of a mystery, I reached up and flexed the harness quite a bit before this latest fuse blowing and it felt fine. So maybe the problem isn't in the part that flexes. I measured the current draw while flexing the harness and there was no sign of an intermittent. So if flexing it manually doesn't cause the short, why would it short while sitting quietly?
There seems to be no other load on that fuse except one wire that says it goes to the burglar alarm, which I don't have on my car.
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Bob wrote:

Could there be some metallic debris in the fuse box? If the fuse box is in the engine compartment, then there could be some slight movement due to the engine cooling down or boot/door closure.
Andrew
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Andrew Morton wrote:

car around town or on the freeway, park it, come back and drive some more. Then I leave the car for the night with everything just fine, and in the morning the fuse is blown. Nobody near the car, no motion.
It's almost as if there is a timer somewhere that comes up and has a short in it. It can go for days or weeks without trouble or it can blow in a day. I sincerely believe that there is no correlation between the motion of the car and the short.
One thing that has never happened - drive the car, shut it off, and fuse blown. Never. Always when the car has been sitting by itself.
That's why I feel that tearing apart the harness won't yield the answer.
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I'm thinking that there's a weak diode in the lights-off circuit, that is shorting to ground.
FloydR
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andFloyd Rogers wrote:

what that means...
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I don't have a wiring diagram - it's probably in one of the numerous electronic boxes, possibly in the engine compartment or under the back seat - you'd be amazed at what's under the back seat of an E34 (or many other BMW models).
A diode can be though of as a solid-state relay. Notice how the lights turn off on a timer some time after you close the door? That's the timer turning off the diode that controls the lights.
Oh, for the olden days of real switches and relays.
FloydR
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Who asked about motion? I dunno dude. You have a car there that has all these complex systems that do somehting when you activate them.
And you have one thing in your car that's supposed to go off at some asyncronous autonomic point in time. And a fuse keeps mysteriously blowing? Uh...
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The other thing to look for is some defect with the fuse box. I've seen one where fuse 13 had it's contacts vaporized slightly from excessive fool pump draw such that the fuse coul dbe good but sometimes no current flowed through it. Sneaky.
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Ah, I was waiting for that bit. I had this dome light that kept blowing fuses. Nothing was amiss electcially other than this. All stock, except for that one wire that went to an old phone I don't have, how could it be a problem I've lived with this for 7 years.
It was, of course the problem. The meticulous gnomes of zurich that assembled our cars are not of the same ilk as the malicous 17 year old earning $12/hr installing radios and burgeler alarms. He doesn't give a shit about your car frankly and there's a high probability he has done something to make you cringe if you saw it. Or can now find it.
If it were me I'd first excise that alarm completely. Who knows what you'll find in the process?
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