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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / January 2007How warm should fusible links get?
On the '94 Safari/Astro there is a connector with 4 fusible links coming out of it that plugs into a distribution block on the firewall behind the battery. The C and D link contacts got hot enough to melt the plastic around them and the C circuit opened, causing the truck to stall and a bunch of accessories not to work. I just replaced the connector, splicing in two of the fusible links that came with the replacement and transferring the two undamaged ones to the new connector body. Everything works again, but I have some questions: Is it normal for fusible links to become warm under heavy load? How warm? With the engine running, headlights, dome lights, reading lamps, and blower on, it took about 5 minutes before I could sense that the C and D fusible links were warm to the touch. After 10 minutes it was a little warmer but still not anything I'd be immediately worried about. Still, a couple of hours driving on a hot night with the headlights and other accessories on could be another matter? Should I be concerned? Also, are their any special tools, probes, shunts, or other tricks that would make checking current draw through different fuses easier? Thanks, Mike D. > Also, are their any special tools, probes, shunts, or other tricks that > would make checking current draw through different fuses easier? Well, all that comes to mind is getting a good digital multimeter, measuring the resistance of the link, then measuring the voltage across the link, then calculating the current. > Thanks, > > Mike D. When I see that, it implies a bad connection causing an arc and heat. The fuse links will warm up because they are a couple sizes smaller than the circuit they are protecting, that's why they have a rubber coating usually. Hot enough to melt is bad news. How is the heat now after it has been taken apart and hopefully cleaned? The voltage drop across the running connection can tell a lot also. Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) > On the '94 Safari/Astro there is a connector with 4 fusible links coming > out of it that plugs into a distribution block on the firewall behind [quoted text clipped - 25 lines] > > Mike D. > When I see that, it implies a bad connection causing an arc and heat. Yes, the terminals were the source of the heat. Either fatigue or borderline overload (or borderline specifications by the designers) caused them to finally get hot and melt the adjoining plastic, damaging one circuit and causing another to disconnect, which is when the problem became visible. > The fuse links will warm up because they are a couple sizes smaller than > the circuit they are protecting, that's why they have a rubber coating > usually. Hot enough to melt is bad news. > > How is the heat now after it has been taken apart and hopefully cleaned? Hopefully that's the good news. I replaced the damaged connector and spliced in two of the four fusible links that came with the replacement. So everything is now clean and sound. The two links that failed get just a little bit warm under a full load.for 10 minutes. I'm a little uneasy about what would happen over a long night-time drive with lights and blower fan running, though. So I'm asking what kind of heating others think I can expect from a properly performing fusible link, versus one that's running slightly too hot and is going to fail again. If it's the latter, I need to see what's pulling too much. > The voltage drop across the running connection can tell a lot also. The new connections seem to be running cool. Until I see otherwise, I'm more worried by the heat being generated in the link and conducted to the terminals. If some heat is normal then I'll just keep a close eye on it for a few days and check the connector often for excess heat buildup. If they're never supposed to be noticably warm, then I have some troubleshooting to do on the load side. Thanks for responding Mike. Any input would be appreciated. Be well, Mike D. ... > > I just replaced the connector, splicing in two of the fusible links that > > came with the replacement and transferring the two undamaged ones to the > > new connector body. ... > > Is it normal for fusible links to become warm under heavy load? How warm? > > [quoted text clipped - 6 lines] > > > > Should I be concerned? >On the '94 Safari/Astro there is a connector with 4 fusible links coming >out of it that plugs into a distribution block on the firewall behind [quoted text clipped - 25 lines] > >Mike D. They are supposed to be four gages smaller then the wire size they are protecting so at full circuit load I would expect them to get warm. > mdevour wrote: > >With the engine running, headlights, dome lights, reading lamps, and [quoted text clipped - 6 lines] > They are supposed to be four gages smaller then the wire size they are > protecting so at full circuit load I would expect them to get warm. Thank you, Ashton. I've driven on several local trips now with lights and fan and other loads. So far I haven't felt anything more than the faint warmth in the C and D circuits' links and none in the connector body. So it looks like the job is done for now. I'll just check that connector from time to time over the next few months. Be well, Mike D. > They are supposed to be four gages smaller then the wire size they are > protecting so at full circuit load I would expect them to get warm. Dumb question; why don't they just use regular fuses like God intended? The wire with the rubber insulation can take a fair bunch of power surges without failing. Surging is bad for normal fuses, they crystallize and fail with time. Lots of circuits that come after the fuse link wire do have normal fuses to blow. It takes a major short or something like reversing the battery cables in a boost to cook them usually. Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) >> They are supposed to be four gages smaller then the wire size they are >> protecting so at full circuit load I would expect them to get warm. > > Dumb question; why don't they just use regular fuses like God intended? >> They are supposed to be four gages smaller then the wire size they are >> protecting so at full circuit load I would expect them to get warm. > >Dumb question; why don't they just use regular fuses like God intended? In addition to what Mike said, they do make a fusible link that's more like a fuse - it has a holder and element and would therefore be much easier to replace. But if you think about it from the automakers view, the cheap 4 gages smaller fusible link, is very easy to install and, well, cheap. And for the most part they will last the life of the vehicle. > On the '94 Safari/Astro there is a connector with 4 fusible links coming > out of it that plugs into a distribution block on the firewall behind > the battery. The C and D link contacts got hot enough to melt the > plastic around them and the C circuit opened, causing the truck to stall > and a bunch of accessories not to work. Did you determine why the contacts got so hot they melted the plastic? > I just replaced the connector, splicing in two of the fusible links that > came with the replacement and transferring the two undamaged ones to the [quoted text clipped - 3 lines] > > Is it normal for fusible links to become warm under heavy load? How warm? I would worry more about what caused the connection to get so hot (not warm) that it melted the plastic. > With the engine running, headlights, dome lights, reading lamps, and > blower on, it took about 5 minutes before I could sense that the C and D [quoted text clipped - 11 lines] > > Mike D. > Did you determine why the contacts got so hot they melted the plastic? > > I would worry more about what caused the connection to get so hot (not > warm) that it melted the plastic. All I can figure out, John, is that after 12 years one or both of the C and D circuit terminals got a little tired and started getting hot due to increased contact resistance. It probably produced a runaway that took out the contact next to it. With fresh contacts there is no noticable heating in the connector. I will have to keep tabs on it as we get toward summer and things run hotter, or if we take a run to Florida. <grin> I also don't know how long the problem took to finally stop the vehicle. The connector may have been damaged by something done to it long before I bought it, and only took this long to finally become obvious. Thanks for your help. Mike D. |
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