Why would this happen? - Page 1
Forum section: electronics / general

author: fostersimported
author_rank: Newbie
timestamp: October 11, 2010, 07:17:52 pm
content: Hello, allThis is my first post but I have following the blog for some time. I need some ideas why this would happen to a Car Fuse.I have a 9 year old VW Golf never had any problems until this.I smelt a strange electrical burning smell and found that the 25amp fuse for the Demister/heated rear window and door mirrors had melted started to burn.The Current draw from the system is 16.5Amps and it had been running for about 2 hours. The demister system has be on for much longer periods of time before with out any problems. The fuse has not blown.Any advice on what to check.Many ThanksDon

author: jahonen
author_rank: Super Contributor
timestamp: October 11, 2010, 07:22:49 pm
content: It seems like a bad contact in fuse holder. High contact resistance heats the fuse. Try to check that, although I guess that the holder is also damaged.Regards,Janne

author: fostersimported
author_rank: Newbie
timestamp: October 11, 2010, 07:30:30 pm
content: Yes the fuse box is Damaged, I need to get it replaced I, but it is going to be a big job.

author: sonicj
author_rank: Frequent Contributor
timestamp: October 11, 2010, 09:30:14 pm
content: my advice would firstly be to avoid using that circuit until repairs are made. if the fuse box is otherwise ok, you can route that circuit through a standalone fuseholder and save yourself the trouble & expense of replacing the entire box.it might be tough to determine the condition of the fuse panel without actually seeing it in person.-sj

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 11, 2010, 11:53:17 pm
content: is the 16A++ current normal?

author: Zero999
author_rank: Super Contributor
timestamp: October 12, 2010, 08:06:30 am
content: It's a 25A fuse so 16.5A sounds pretty normal.Yes, the holder is bad. Maybe you could replace the holder temporarily but I'd recommend replacing the box in the long run because it's likely it will happen again as the other holders will be a similar age and condition and it's a fire hazard.

author: Nermash
author_rank: Frequent Contributor
timestamp: October 12, 2010, 08:11:48 am
content: Quote from: fostersimported on October 11, 2010, 07:17:52 pmThere's your problem

author: DJPhil
author_rank: Frequent Contributor
timestamp: October 12, 2010, 09:27:24 am
content: Tried to post this earlier but I was having internet trouble.I'm not sure this will help, but I found some wiring diagrams for a 2001 Golf. They're not the easiest thing to understand, but they might come in handy now or later. Search for 'heated rear window' and you'll find a switch, light, and heating element, nothing fancy.My guesswork: Assume the following . . .1) the element is working at over 12A (13.8A when the car's running)2) and the car's electrical system is 12V (13.8V when the car's running)3) The switch just puts the whole battery voltage into the element (very likely, as it's the cheapest). . . then there's less than an ohm of resistance in the circuit!That does seem a bit odd, but consider that as the element warms up the resistance rises until it reaches a steady temperature, much like a light bulb. The easiest way for the fuse to blow is for the wire between the switch and the element to get shorted to the chassis (ground in a car). This would usually blow the fuse, but if it's a short near the back of the car then you've got that extra wire and connectors driving the resistance up (remember it only takes one ohm to make things behave weird), so it might not conduct enough current to completely blow the fuse. If it's very wet and dirty on the back window there might be a low resistance connection to ground that conducts after the element temperature rises. If this is the case then you've got two resistances in parallel, and the variable resistance in the element would make it self biasing, or if it was unstable perhaps even oscillating. How weird!Most of that's fairly unlikely I guess, but it just shows how much odd stuff can happen with such a simple circuit. Here's what I'd do:Contact a VW dealership (no matter where you bought it from) and see if there's a safety TSB for this issue. This assumes you're in the US, but there's probably something similar everywhere.The NHTSA in the US has a search tool, and quick scan shows nothing directly related you'll want to check with a dealership to be sure. This means that it's a known issue related to safety, and they'll almost always repair the problem for free. Best of all worlds! Always check this before shelling out if you think it might be useful, it's saved me a lot of money in the past. Even if it's not a freebie, the techs there might be kind enough to answer a few questions that will help you fix it on your own if you ask nicely. I saved myself a few hundred dollars once by simply asking, "What the heck could even cause such a thing?" Good techies will problem solve a rhetorical question by reflex.Check all the connections in the system, and if you can manage it all the wire, for signs of overheating, moisture, dried fluid, dirt, or shorting to ground. Sometimes the problem jumps right out at you, but even if it doesn't you'll feel better knowing it's probably 'not that'. It was probably a bad connection in the fuse (or maybe a bad fuse) as Jahonen said, but it doesn't hurt to check. I guess the real concern now is finding a replacement fuse panel, and my suggestion there would be a junkyard. You can check the other cars for signs of similar problems as you go, and maybe figure out more about your own troubles. I once pulled four vacuum distribution busses, which are simply plastic tubes with outlets of different sizes, before finding a good replacement. They'd all cracked in the same place, and further investigation showed that old motor mounts caused the motor to torque hard enough to yank on the short hose. In cold weather the plastic was brittle enough to crack. Two of the three bad ones I found in the junkyard came from cars that were in front end collisions (a big vacuum leak makes power brakes worthless, and stopping distance multiplies several times). If any of you have late 80s Ford Thunderbirds, be advised!I hope that helps, I did ramble on a bit.

author: PetrosA
author_rank: Frequent Contributor
timestamp: October 12, 2010, 11:22:19 am
content: Janne is correct, you have a bad (damaged or loose) contact in the fuseholder. This will create sparking which eventually leads to a "glow arc" and cause the damage you see to the fuse and fuseholder.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 12, 2010, 03:52:22 pm
content: doing a bit ofCSI, looking at the physical of the damage, the heat comes from the left side of the contact (top picture), the right is clean. so highly probably janne is correct about loose contact (left one). if its shorted 12V to gnd, the middle (fuse) part will blow the heat first. comparing top and bottom picture, i can see the fuse is still intact, pls do continuity check on the contacts (clean them first with sandpaper or something) and pls confirm.edited: confirmed! i didnt catch your last line earlier.

author: fostersimported
author_rank: Newbie
timestamp: October 12, 2010, 06:29:12 pm
content: Well Thanks for all the advice, I will put some pictures of the fuse box up tomorrow. I should replace the complete fuse box but by the looks of things it is hard wired into the car. I would have to cut all the cables and rejoin them.I will post pictures and get your input.ThanksDon

author: orbiter
author_rank: Frequent Contributor
timestamp: October 13, 2010, 12:05:04 am
content: I'm not sure of the exact electrical layout of a golf but if it's not a fuse contact problem.. what used to happen a lot on hatchback and estates models a few years ago (especially vehicles that didn't have newer copper boot contact pads for the heated rear window etc) was that you'd find a little flexible rubber sleeve between the tailgate and the vehicles main body. This rubber sleeve contained the wiring for the heated rear screen. What would happen is that over years of tailgate use, the wires inside the sleeve would degrade and break causing either a failure of operation of the heated screen etc, or a short against adjacent broken wires thus blowing the fuse...An auto electricain would be able to track this down for you quite easily, however if your competent around cars you could check for the basics yourself and if it is something simple, fix it too.. To check for the above just lift the tailgate and have a look for a flexible rubber sleeve between the tailgate and the main body of the vehicle containing wires (normally close to the hinges.) If you find a sleeve containing wires give it a bend & a wiggle at various points, you can sometimes feel if the internal wires are broken this way. Failing that unplug one end of the rubber tube and pull it back a little (be carfull not to strip or bare any wires) just enough to check if the wires are in good order or not.As you mention your door mirrors you may also have to check the rubber wiring sleeves between the doors and body for faults/breaks there, however I'd check the tailgate sleeve first if you have one.

author: FreeThinker
author_rank: Frequent Contributor
timestamp: October 13, 2010, 11:58:33 am
content: Deffo a bad connection on the fuse/fuseholder.cut the cables out and fit an in line fuse, check for damage on the adjacent fuse holders and keep an eye on it for a few days.Could well be somone has replaced the fuse in the past and not pushed it fully home, electrical faults are one of the main causes of car fires so take care.

author: bearman
author_rank: Regular Contributor
timestamp: October 16, 2010, 10:02:14 pm
content: I had a similar situation with my fishing boat and a trolling motor fuse. It is a 12 volt system but it can draw up to 30 amps at full motor speed. The fuse and holder melted down completely before the fuse finally blew. I contribute this issue to corrosion due to a wet environment and no maintenace done on the fuse or holder for several years. The boat sits outside in the cold winters so there is condenstaion is an additional factor also.My fix was easy though. This was an in line fuse holder so I just replaced it with the heaviest gauge fuse holder I could find and then filled the holder with electrolytic grease to stop corrosion from killing it again. I cleaned the fuse prongs also before placing the fuse in the holder. 3 years and counting, no problems with it again. I do inspect it now though.B

author: sonicj
author_rank: Frequent Contributor
timestamp: October 16, 2010, 10:23:27 pm
content: corrosion xthe grease works too, its just messy....