Blown fusible link

[MagnumTN]

My 1985 D150, 318 Magnum, was having non start issues. After testing and/or replacing all the components in the system, starter, relay, ignition control switch, ballast resistor, it finally turned out to be a bad ICU. Since replacing it the truck is now starting, but is not charging the battery. It was recommended to me that I replace the 3 fusible links located on the drivers side wheel well with fuses, which I did. There are 3 fusible links designated by wire color, black, dark blue and orange. I replaced the black one with a 20 amp fuse and the other two with 15 amp fuses. As soon as I started the truck, the 20 amp fuse blew. I subsequently replaced the 20 amp with first a 25 amp, then a 30 amp fuse. Both of those also blew. At this point I'm not sure how to proceed. Should I replace it with a larger fuse, which would require a larger fuse holder, or is the problem somewhere else? Any advice on this issue would be greatly appreciated. Thanks in advance for your help.

 

[reek]

you have to figure out what caused your fusible link to fail. clearly there is a significant current overload in your circuit. the fusible link is typically about 4 gauges smaller than your harness wiring. if it burned up/blew, then there was an overload through that circuit. I was meant as a sacrificial part to burn up instead of your wiring harness. putting a thicker wire with a large amp fuse in the place of a fusible link will partially defeat the purpose of the link.

 

[JohnnyMac]

Like @reek said, if you keep increasing the fuse size, eventually your wiring will become the fuse... I'm guessing that whatever it's protecting has a direct short to ground or I suppose it could just normally draw more amps than your fuse is rated for, but you need to find out what that fuse size should be and what exactly it is protecting? A long, tedious way to try to track down a direct short is to pull all of your accessory fuses then start the truck and insert one fuse at a time until the offending fuse blows. That should narrow down where to start troubleshooting.

The quick way to fix it is to replace the fuse with a piece of rolled up tin foil and then look for smoke. This way can be expensive and although I've done it, I don't recommend it...

 

[MagnumTN]

you have to figure out what caused your fusible link to fail. clearly there is a significant current overload in your circuit. the fusible link is typically about 4 gauges smaller than your harness wiring. if it burned up/blew, then there was an overload through that circuit. I was meant as a sacrificial part to burn up instead of your wiring harness. putting a thicker wire with a large amp fuse in the place of a fusible link will partially defeat the purpose of the link.

Thanks for your response. I understand the purpose of the fusible link but what I can't find out is what the amperage of that one is. I've located them on the wiring diagrams in a Haynes manual and on a Bishko CD manual but unfortunately neither lists the amperage. Obviously, I don't want to exceed it's rating as it would defeat the purpose of the fusible link. Any suggestion on where I might find that information?

 

[MagnumTN]

Like @reek said, if you keep increasing the fuse size, eventually your wiring will become the fuse... I'm guessing that whatever it's protecting has a direct short to ground or I suppose it could just normally draw more amps than your fuse is rated for, but you need to find out what that fuse size should be and what exactly it is protecting? A long, tedious way to try to track down a direct short is to pull all of your accessory fuses then start the truck and insert one fuse at a time until the offending fuse blows. That should narrow down where to start troubleshooting.

The quick way to fix it is to replace the fuse with a piece of rolled up tin foil and then look for smoke. This way can be expensive and although I've done it, I don't recommend it...

No, I don't think I'll be using any tin foil as a diagnostic tool. I could easily imagine myself setting the whole truck on fire. I did some more testing today. The fusible link that is blowing is the 12 gauge black wire. When the engine is off I read 0 volts on one side, the firewall side, and 12 volts on the battery side. When I start the engine I still get 12 volts on the battery side but 50 volts on the other side, which explains why it keeps blowing. Since this link is 12 gauge, to my thinking that 50 volts is excessive for that size wire. I have a 50 amp fusible link and by comparison, that wire looks to be about 6 gauge. I'm at a loss. At least with a full charge on the battery I'll be able to get it to a mechanic if I can't get it figured out. Thanks.

 

[JohnnyMac]

The fusible link that is blowing is the 12 gauge black wire. When the engine is off I read 0 volts on one side, the firewall side, and 12 volts on the battery side. When I start the engine I still get 12 volts on the battery side but 50 volts on the other side, which explains why it keeps blowing.

So, when you're reading that voltage, I'm assuming it's while the fuse is out because it's blowing yah? I'm not sure how you'd get 50 volts unless it's because your alternator is unloaded. If the alternator is spinning free and doesn't have the battery's load on it, it can't regulate itself and might put out 50v unloaded, but if you hook up the battery to it, it would "feel the load" and output 14ish volts. That being said, if that is 12g wire, then it's only rated for somewhere around 12-14 amps so I don't think that is the output of the alternator, but could be indicative of what the alternator is putting out. Sorry if I'm thinking out loud here, but if your alternator or voltage regulator are bad, that could be what initially fried your ECU (ICU?) so definitely don't use the tin foil test or you'll probably fry it again. I would pull the alternator and take it to an auto parts store that will check it for free. I'm not familiar with that truck's charging system and whether the voltage regulator is built into the alternator, the ECU or if it's a singular component, but if your alternator is good, then look at the voltage regulator. On a side note, voltage won't pop the fuse, it's the current. A static spark off of your finger is in the neighborhood of 25000 volts, but of such low current it doesn't have any oomph behind it, but put a wire between the posts on your 12v battery and you can damn near arc weld with it..... Good luck and keep us informed.

 

[MagnumTN]

So, when you're reading that voltage, I'm assuming it's while the fuse is out because it's blowing yah? I'm not sure how you'd get 50 volts unless it's because your alternator is unloaded. If the alternator is spinning free and doesn't have the battery's load on it, it can't regulate itself and might put out 50v unloaded, but if you hook up the battery to it, it would "feel the load" and output 14ish volts. That being said, if that is 12g wire, then it's only rated for somewhere around 12-14 amps so I don't think that is the output of the alternator, but could be indicative of what the alternator is putting out. Sorry if I'm thinking out loud here, but if your alternator or voltage regulator are bad, that could be what initially fried your ECU (ICU?) so definitely don't use the tin foil test or you'll probably fry it again. I would pull the alternator and take it to an auto parts store that will check it for free. I'm not familiar with that truck's charging system and whether the voltage regulator is built into the alternator, the ECU or if it's a singular component, but if your alternator is good, then look at the voltage regulator. On a side note, voltage won't pop the fuse, it's the current. A static spark off of your finger is in the neighborhood of 25000 volts, but of such low current it doesn't have any oomph behind it, but put a wire between the posts on your 12v battery and you can damn near arc weld with it..... Good luck and keep us informed.

Yes, the fuse was out when I tested the voltage. I've already had the alternator tested and it tested good. I've also replaced the voltage regulator. It is a separate component, mounted on the firewall. I'll get back into the manual today and see if I can test the output from the voltage regulator. I also have the old one and can test it as well. That was my initial thought, that the voltage regulator was bad, but after replacing it, I was stumped. At least now that I've replaced the ICU I can start it, and with a full charge on the battery, can get it to a mechanic if need be. I'm a descent mechanic, but suck at diagnostics. Tell me what's wrong and I can usually fix it. Thanks for the help.

 

[JohnnyMac]

Good luck my friend. I'll be interested to hear what you find.