What's a fusible link, and where's it located?

[Jesse Jaymes]

Posted my woes in the 73-91 forum. Having major hearburn over a truck that is losing juice.

Brand new battery, and pulled alternator and had it checked 3 TIMES at 2 different stores. Comes back PASSED each time.

Checked connections, checked grounds. Can't find anything.

My over the phone Chevy guru said to check the positive cable first, as he believes that the terminal end can swell or become corrupted from corrosion inside the insulation and not allow the electric to return to the battery as easy as it comes out

Then he made mention of a fusible link?? We spoke of a buss type box near the AC unit on the firewall. There is a small 1" X 1.5" black something near the buss box. He says that this is the fusible link and may be bad and not allowing the energy to return to the battery?

Battery last about a day or so, then it's drained.

THought it was really odd that after I pulled the alternator this last time, cleaned everything I could and checked wires and connections and fired it up(barely) the volt meter read about 10. I ran it down the road for 2 miles and we were down to about 8.5. As it sat and idled, I decided to watch the gauge for a bit. Then I reved it up, and the volt gauge actually got LESS volts and went to just 8.

What else can it be?? I will replace the pos cable for good measure and a stab at it. This alternator does not have a wire that runs to the batt. It runs from pos terminal to starter, then back up to the alternator.

Any help?

I've googled a bit and found two other saps that had the same problem, but Yahoo or whoever never did get them an answer. Coincidentally, both were 1979 GMC K25s.

 

[apache]

Ive seen cables that look perfect create all kinds of starting problems,corrosion in the cable where you cant see it due to the covering. a decent battery should sit around 12.5 to 12.8 volts,when its running taking a reading across the battery terminals you should see 14+ volts,not which terminals you have but with it running check the terminal of the batt and also the cable lead off the terminal,and if possible at the end at the starter,any differences will indicate a bad cable end or bad cable. Also with the truck not running and all parasitic loads un fused or disconected like the stereo memory and no dome light on,no key in the ignition with buzzer or anything and disconnect the positive cable then touch it to the positive terminal,if you get a spark theres a short somewhere drawing the battery down,time to start searching for a short. A fusable link is a section of special wire basically designed to burn or disconnect much like fuse. If its burned it will not make a closed circut. Regarding the lower voltage when revved up,it takes more energy to run the ignition at speed than idle. Id check the bundle of wires comming up from below by the starter up over the intake that run behind the alternator,seems a bad place on some where wear and burn thru can happen.
Also keep in mind new can mean nothing but shiny but still be defective. Do a load test on the battery and see if a cell is shorted,Ive seen new batteries off the shelf that have shorted plates.
Sorry for the jumble of info,kind how my brain works after to few beers

 

[K5dreamer]

1. remove alternator, take to advance auto parts and have tested for free. If bad, buy new one. If good, move on to step 2.

2. remove battery, take to advance auto parts and have tested for free. If bad, buy new one. If good, move on to step 3.

3. Now we get to the fun stuff, if your battery, and your alternator are good, you have what is refered to as a parasitic draw. there is no easy way to do this, so i apologize in advance.

4. Buy a battery kill switch. hook it up to the positive terminal of your battery. take a voltmeter (multimeter) and use aligator clips or some other method to attach the leads to the meter to either side of the battery kill switch. set it to read amps.

IMPORTANT!!!!!!!!!!!!! turn everything off, ignition, radio, lights, dome lights, anything that will pull current.

turn the switch to "off" or "open" in order to open the circuit, this forces all the amperage to be drawn through the multimeter. With it set on AMPS you should see a reading of "0.00". If you have numbers here, youve got a big short somewhere, go strait to step 6. If you see "0.00" go to step 5.

5. set multimeter to milliamps. and read. you should see numbers here. a draw of 10-15 milliamps is normal for a newer car, on a 1979 im guessing it should be 5-10milliamps. If its within this safe range, you can stop, you dont have a draw due to a short, go to step 7. If you have an above expected draw of 20 or more milliamps, go to step 6.

6. time to go to the fuse box. start pulling fuses one at a time, while keeping an eye on the multimeter, chances are youll pull one fuse, and the draw will drop to acceptable levels. time to look at schematics, find out what wiring is on the circuit with the fuse you pulled, and start tracking the harness looking for problems.

7. you dont have a parasitic draw, you have a short somewhere related to the running condition of the car. either a shorted ignition switch, and possibly a bad alternator.

if youre not getting a charge while the engine is running, you either have an open circuit on the power or ground to the battery, or a bad alternator, those would be my two guesses.

 

[Lunatic]

Sounds almost spot on to an issue I had.

There are two fusible links down at the starter solenoid. They look like normal wire, with a plastic capping about 1-1/2" long over them. One has "16" on it and the other "14" (Their link ratings)

Check there - Sounds to me as if your alternator isnt getting the signal to charge - Which comes from the hot off your solenoid. If its blown, no signal, no charge.

This was on my 86 GMC, 350

Good luck.

 

[smalltruckbigcid]

Sounds almost spot on to an issue I had.

There are two fusible links down at the starter solenoid. They look like normal wire, with a plastic capping about 1-1/2" long over them. One has "16" on it and the other "14" (Their link ratings)

Check there - Sounds to me as if your alternator isnt getting the signal to charge - Which comes from the hot off your solenoid. If its blown, no signal, no charge.

This was on my 86 GMC, 350

Good luck.

thats a great place to start

 

[Thunder]

Sounds to me like the Fuseable link too. As said, check the wires down by the starter you will see them. The Alt wire is the thicker one.
If the FL is blown/ bad ALt will not charge. You should be getting 12V all the time to the big terminal on the Alternator.

 

[Jesse Jaymes]

Many thanks thus far. At work today. Called my preferred parts place and they'd heard of fusible links, but didn't describe what I had pictured in my head.

They stated that they needed to know what size the links were (gauge?).

He stated that they were only a few inches long, and made it sound like it was to be spliced in place? That's the part that confused me. I kinda understand the concept, in that the wire or link, inside the insulated wire coating, may be blown or burned apart, allowing the entire "wire" to be stretched when pulled apart?

My good mechanic (who was too spendy to use for the cam/intake swap) made it sound like the fusible link was on the firewall, going from the busse type box towards the ignition side?

Appologize for being vauge, but I really was certain that it was an alternator and never did get under the truck and jack with the starter. Ran out of light and was a thunderstorm last night by the time I got any education on the fusible link. Plus the truck is at my GFs house, 40 miles away. So I will try and grab a fusible link and see if I can get it figured out and up and running tonight after work.

 

[lak2004]

I have a fusible link spot on the drivers side firewall with a bunch of stuff on it. There usually is also one by the starter as mentioned. It is a thin strip of metal that burns up just like a fuse would. It is sometimes a pain in the ass to find.

 

[Lunatic]

I went and found my old thread on this.....

Here is a pic of two fusible links:

The wires in the pic are about 10 gauge. The cap numbers represent the fusing link gauge - smaller 14 and 16 gauge - The piece intended to blow.

A fusible link is basically a piece of wire cut in half, with a small wire soldered back in where the cut was - If the rated wire becomes over loaded, the smaller "link" cant handle the overload and fries, saving components and the remaining circuit. Simple.

The thing is however - They dont just fry.

You have a dead short somewhere. Once you find the dead link, you can start tracing the circuit for the short. In my case, I had just put my 700R4 back up two weeks prior, and two weeks later, the insulation on the wire that I crushed between the block and bell housing finally grounded. POOF - there went the fusible link.

And, yeah lak2004 - fusible links are everywhere.



Good luck

 

[ChrisPerry]

I had a 80 k5 that had a similar issue. It was the power tap on the DS firewall. The block that all of the pos wires hook to. It melted inside and caused an intermitant short. I think that might be that Bus box you were talking about.