Ignition fuse keeps blowing after start

[silverchev08@gmail.com]

Had to have the truck towed the other day because it just died. Come to find out a wire that comes from the starter to ignition burnt up. Installed in-line fuse. Thought everything was great. Every time I start the truck up it blows the fuse and the truck will run rough . Once you turn it off you have to replace the fuse to start it again. Trace back all the wires and don't see anything burnt up. Wonder if it's grounding out or if there's a bad wire somewhere.

[bd]

So you replaced a fusible link with a fuse?  When does the fuse blow?  While cranking?  Only after it starts and runs for a few moments?   

Temporarily disconnect the B+ (charge) lead from the alternator and retest.  Remove the "fuse" and install a proper fusible link that is correctly matched to the harness.

[silverchev08@gmail.com]

Thanks bd.....Yes. There was a single fusible link connected to a power block at the firewall. Was trying to troubleshoot why the truck was completely dead and found that this fusible link had burnt up. Not sure how this happened but it provides power to the entire ignition system and column components. This was replaced with a 30a in line fuse. Yes. After starting the truck the fuse blows and the truck runs poorly. Like the timing is affected or something. I assume whatever the problem was that caused the fusible link to burn up still exists. Also I went ahead and replaced the alternator because it was not charging the battery adequately.

[silverchev08@gmail.com]

Btw..... Truck is a 85 GMC K20. And yes the fusible link burned out not sure how or even where to get a replacement. What do you think could've caused this?

[VileZambonie]

You can't replace a fusible link with a regular fuse. Disconnect the battery negative and trace the wiring from that fusible link onward. Often the wire insulation is cracked and exposed to ground etc. That would be a quick and easy thing to check before you go too far into testing. We call this preliminary checks/visual inspections.

[bd]

Heed this ^^^^.

What amperage alternator did you install?  Did the fusible link that melted at the firewall junction block splice directly to two 12-gauge red wires? 

Preassembled fusible links in a range of gauges are available at many auto parts stores.  Read through How to make a Fusible Link for pointers on replacement.  The gauge of a fusible link is very specific to the wire size it protects - see the correlation table at the bottom of the linked thread and follow it explicitly.

[silverchev08@gmail.com]

The fusible link burnt up with the old alternator that had been on there for many years. I installed the same amperage identical newer alternator and I'm having same issue. I looked all through the wiring and didn't see anything.Gonna check other parts of the wires.I will Crop out the in-line fuse with the fusible link.It's just weird thatwith a new fuse the truck starts up great and then runs crappyafter the fuse blowslike it's affected the timing or something.

[bd]

The 30-amp fuse blows because it cannot carry the 60-plus-amp charging current output from the alternator.  When the fuse blows, the ignition system becomes isolated from the battery but continues to function from power supplied by the alternator.  Simultaneously with the blowing of the fuse, alternator output voltage feedback to the regulator is severed and alternator output becomes wacky.

Since you already performed your preliminary inspection, replace the fusible link and move forward from there to determine whether symptoms continue.

[silverchev08@gmail.com]

Well I haven't found any burnt wires or anything so should I just go ahead and crop out the in-line fuse with a new fusible link and hope for the best? Not sure What caused the link to burn up in the first place. it may have just been a bad fusible link.

[VileZambonie]

it may have just been a bad fusible link

A bad fusible link would be one that allowed your truck to burn to the ground. If the fusible link blew, there's a reason. Follow bd's recommendation and replace it if you already inspected everything.

[silverchev08@gmail.com]

I mean I looked at everything I didn't see anything out of the ordinary no burnt wires or anything crazy. Could've been a small short in the wire that did it and I'm sure once I put a new fusible link if it does it again I guess that means I have bigger problems. But that's what I'm worried about. I mean really nothing overtly obvious was why the link burned up. I'm gonna look again tomorrow and see and then just put the new fusible link in and go from there I guess.

[75gmck25]

There should be at least two wires with fusible links connected to the starter post.  They both run up into a protective metal tube that is bolted on the back of the engine, with the wires emerging just behind the valve cover.  The tube protects them from engine exhaust heat.  The tube is also a PIA to work around and bolt on next to the exhaust manifold, so someone may have removed it and not put it back on.

1st wire - You can remove the alternator charge wire from the starter and just run a new charge wire directly over from the positive battery terminal to the BAT terminal on the alternator.  I found a replacement charge wire on a 88+ GM truck in pick and pull, and it was a larger gauge (8 gauge?) with the correct fusible link.  The stock GM setup in 88+ ran it to a power bus on the firewall, but I ran it directly over to my positive battery post.  I have a 94 amp 12si, and it works fine. 

If you also find the OEM power bus/strip on the junkyard truck, its a good item to pull.  It has about 6 screw terminals across, so its a great power point.

The 2nd wire with fusible link is the one for power to the fuse block.  It takes a lot of abuse because of exhaust heat.  Its probably a 10 gauge wire with a 14 gauge fusible link.  If its giving you problems I would buy all new wire and a fusible link, and make sure the wire has the proper heat rating.  The cheap wire you buy in little reels from Autozone or other strip-mall parts stores will not hold up.  You need something with an insulation heat rating equivalent to OEM wire.  Then make sure its runs smoothly through the protective tube and over to the power junction next to the brake booster.

Bruce

[silverchev08@gmail.com]

The metal drive has been long removed probably when the last owner overhauled the engine. I wound up ruining the wires in heat proof sheathing similar to that of header exhaust wrap. The second wire is the one I need to replace at the fuse block.

[silverchev08@gmail.com]

Meant running the wires in heat sheathing, not ruining. Also replaced the fuse with fusable link. Found two wires rubbing that shorted out also. Cropped out bad sections and soldered. Charged battery and truck started up. Ran for a bit no issues. So far so good!