when key is twisted fully to start, no voltage detected on the purple wire and wire is continuous to the firewall with no stops along the way. I did not check at the neutral safety switch, but if it was positive at the switch it would have to come from the yellow wire.

 

Most likely, something shorted "B" to ground such as the alternator and when that link blew then 12v on "E" no longer makes it to the ignition switch for the start circuit. Rig up a fused wire jumper using say 12 gauge and a 10A fuse, cut out the burnt link on "B", insert this jumper and connect the battery. If the fuse blows, disconnect the alternator on "D" and repeat. If the fuse still blows leave the alternator disconnected and cut "E" and repeat. This assumes that you have already followed these wires and ruled out simple wire shorts.

George

 

can that also serve as a permanent replacement instead of a fusible link once I figure it all out? If so, I would just need to correctly size the wire gauge and fuse amperage.

 

You can buy fusible links for a proper replacement. I dont know how the reaction time and transient behavior compares to a std issue fuse. The amperage value may also be quite high. The repair has to be done right because of the current carrying level on that wire. If its the big alternator post then your looking upwards of 100 amps. Thats why we dont want to start the truck during the experiment.


George

 

when you say 100amps, is that during a short or during normal usage? I would think you would need a battery sized cable to handle a load like that.

for fusible links, what would be a good site to find them? How do you know how much “fuse rating” you need?

to do the the testing you mentioned earlier in post #32 above, if I don’t have a spare 10 amp fuse laying around, can I make my own fusible link with say 12 gauge wire with a short section of 18 gauge wire in line?

 

Most std alternators are rated for up to 100 amps. They normally put out much less than that of course. I am referring to the largest red cable on the alternator. Fusible links are a special type of wire thats meant to overheat and break but not catch on fire. A 12 gauge fusible link is not just a piece of std issue 12 gauge stranded wire. Standard wire will not behave the same way. I know that O'Reillys and Amazon has them A fuse is better because a short will blow right away. A fusible link will let the fault cook longer.


George

 

ok, so I cut out fusible link “B” (which was fried in 2 spots) and then made an in line wire with the same gauge as the red wire “D” coming from the single terminal of the alternator . I actually used that pesky old black wire after disconnecting it from the trailer electric brake circuit. I didn’t have any 10 amp fuses, so I just used the 20 amp in line fuse that was already in the pesky black wire.

when the broken fusible link was in place, wire D had no 12 V. When the pesky black wire was used as the inline fuse connecting D to the junction block and the battery cable connected, there was 12V (actually 11.95V) between the single terminal on the alternator f to which “D” connects) and the negative battery post.


I am not sure if I have done it correctly

 

OK so the issue that blew the link is either resolved, intermittent, or was associated with the trailer brake circuit. Now probe "E" as close to the firewall as you can get and see if you find 12v. If you do start turning things on but dont start it yet. Make sure to include the ignition switch to the run position. What gauge wire is "B"? That will give us a clue on the fuse value you need before we try to start the truck.


George

 

I will go out and run those tests.

I don’t think the black trailer wire would have done anything because it would not have caused current to flow from the junction block to the alternator (B/D wire).

Here are photos of the fusible link

 

ok, pics and additional comment in the post above.

As for the test, I only had that one 20A fuse in the black wire connected to the junction block (acting in lieu of B), so I disconnected that black wire from wire D, and connected it to wire E. Even without the ignition turned to run, it was 12v and did not change when switched to run.

But with the junction block always hot, both this year and the previous test should always show 12V, no?