84 K5 Blazer, melted wire on junction block - need a little direction.

[FormulaZR]

Hello all, I'm trying to help a friend with his 1984 K5 Blazer. It is a 6.2 diesel 4x4, but it is a Silverado - not a military model. I believe most of the wiring is similar to a gasoline.

Anyway, I've attached a picture of a wire on the junction block that melted. It begins as a single wire and then goes to two wires with a factory splice. What do these 2 wires go to and what are likely causes of the melt? Fusible link I believe?

From what I've been able to read without seeing any pictures of similar wires, I believe these go to the alternator? When the wire let go, he said the truck died and most of the dash/interior lost power. He twisted the wires together to get it a mile or so home and said the alternator was charging more than normal. I am sort of suspecting a bad alternator/voltage regulator. It is not being driven as pictured, just getting ready to start wire testing - but again, I'd just like some direction.

[bd]

The compromised link is 16 gauge and 6" long.  One of the two red wires protected by the link runs to the output post of the alternator, the other runs through the firewall bulkhead connector then splits to the ignition switch and fuse block.  Replace the 16-gauge link, but before reconnecting it to the junction block insert an incandescent test light between the link and junction block stud.  If the truck has an engine compartment lamp, disconnect it.  Make sure that everything is switched off and the cab doors and glovebox are shut.  Assuming the truck doesn't have a factory clock or aftermarket head unit, the test light should not glow.  If it glows, disconnect the charge lead from the alternator output post to see if the test light extinguishes.

[FormulaZR]

Thank you, to replace the fusible link - do we need to solder it in, use butt connectors (that doesn't seem like the correct answer), or is there another method?

Also, any insight on what we should be looking at for the cause of this in the first place?

[bd]

How to make a Fusible Link

You will diagnose for a short circuit drawing in excess of ~35 amps.  If the test light glows full brightness during the previously described checks, you will need to determine whether a hard failure actually exists or if an appliance was inadvertently left on.  To do this without risking damage to the new fuse link procure a 20-amp, auto reset circuit breaker (image) and attach a couple of 12" test leads terminated with alligator clips.  Connect the circuit breaker between B+ and the fusible link.  If there is a short, the circuit breaker will cyclically trip and reset.  Disconnect power and trace the red wires through the engine compartment and under the dash looking for pinches, cuts or chafes to ground.  If the test light glows full brightness when installed between the alternator output post and the charge lead with the ignition switched off and engine not running, then the alternator maybe internally shorted.

[75gmck25]

It could be a wiring short that is still there, and needs to be fixed, or it could be deterioration of the fusible link over time 

I made a mistake with jumper cables and hooked them up backward, and immediately saw that fusible link start to smoke.  The truck still worked, but the link was apparently partially compromised.   Later on I was driving on the highway at a good RPM and pulled out the headlight switch.  That burst of amperage to the lights made the fusible link burn through completely and I lost all power.

I bought a GM fusible link at the store, which is designed to protect 10 gauge wire with a 14 gauge fusible link.  I spliced it into the wiring and have never had trouble since.  I believe this is the link I used, and they stocked it at Advance Auto Parts.
https://www.amazon.com/Dorman-Help-85620-Gauge-Fusible/dp/B000COD0TW

Bruce

[bd]

I bought a GM fusible link at the store, which is designed to protect 10 gauge wire with a 14 gauge fusible link.

If you are endorsing a change to a greater capacity fusible link, this is bad advice.  It's akin to placing a penny under a screw-in fuse.  Upgrading a fusible link without appropriately upgrading the associated wiring is inviting disaster.  The vehicle could incur electrical fire and burn to the ground.  The potential for loss and liability is too great.  Twelve gauge wire is protected by 16-gauge fusible link.  The industry standard exists for good reason.  The proper solution would have been to discover why the fusible link failed (e.g., poor connection, physical damage to the link such as broken wire strands, or excessive current draw causing the fusible link to fulfill its design purpose and protect the vehicle from fire) and correct the problem. 

[75gmck25]

It is possible that your truck is wired differently, but my main power feed from the alternator is 10 gauge, and GM used  a 14 gauge fusible link.  I assumed that was the wire that had fried, but now that I look at the picture more closely, it appears to be the smaller gauge wire that fried.

On the same junction I also have a 12 gauge wire with 16 gauge fusible link that feeds the fuse panel.  If that is the wire you fried, you would use a 16 gauge fusible link.

Bruce

[bd]

Thank you for clarifying.  Although there are similarities from year-to-year, designs deviated in certain aspects as electrical demand increased over the life of the body style.  The best practice is to refer to the specific wiring manual for the year/model of vehicle under discussion.  This is also why it is so important for an OP to announce which year/model vehicle (s)he is questioning.

[FormulaZR]

Update to this. Finally got a multimeter hooked up. Alternator appears to be putting out the normal ~13.5v, but the wire has 22.5v on it. Of course as soon as I pull the 2 wire plug on the alternator to see if the voltage stays, the truck dies. Likely just a bad regulator in the alternator?

[bd]

Alternator appears to be putting out the normal ~13.5v, but the wire has 22.5v on it.

What are you trying to say?  Normal voltage regulator threshold is 14.2 - 14.5 VDC.  What was the current output of the alternator when you measured 13.5 volts?  Which wire measured 22.5 volts?  The engine shouldn't die just by disconnecting the regulator connector.