Fusible Links - 101

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Warthog

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With all the electrical problems people are having with the CUCV starter and Glowplug systems lately, I thought I would post some info.

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This is from www.Whiteproducts.com. It is located in their FAQ section.



What is a Fusible Link?
A fusible link is a short piece of insulated low-voltage cable within an automotive wiring harness that is designed to protect the harness in applications where a fuse is unsuitable. In an extreme current overload situation, the conductor within the link is melted while the ensuing flame and spark is contained within the link's insulation.

What are the ratings for fusible links?
Fusible links are not rated in amps like fuses because each installation is unique and designed to meet specific circuit protection requirements.

What size replacement fusible link should be used ?
The automotive service industry recommends using the same gauge and length as the blown fusible link after the cause of failure is corrected.

How do you use a parallel connector?
Insert the stripped end of a fusible link and the stripped end of the cable being protected into a parallel connector as shown, and crimp. A parallel connector should always be protected with electrical tape or heat-shrink tubing.

Parallel-Connection.jpg

This is a picture of the two types of wire connected together. The fusible link is the orange colored wire.


What size fusible link should be used in a new installation?
The suitability of a fusible link in a new application can be determined only by a qualified harness engineer with full knowledge of the circuit protection requirements, the installation and operating conditions, and the safety and liability aspects. We cannot make specific recommendations.

Are there any general guidelines for choosing a suitable fusible link?
Typically, a given harness segment is protected by fusible link that is four gauge numbers smaller. A 14-gauge wire would be protected by an 18-gauge fusible link. A 6-gauge wire would be protected by a 10-gauge link, and so on. Odd number wire gauge sizes like 19, 15, 13 and 11 are counted when sizing a link. The length of a fusible link should not exceed 9".

Can a fusible link be used to replace a fuse that blows frequently?
In general, a fusible link should never be used to replace an automotive fuse unless authorized by a vehicle factory service bulletin. Safety and liability issues are involved.

Can a blown fusible link be replaced with a MAXI™ fuse?
While "MAXI" fuses have replaced most factory-installed fusible links in late model vehicles, they do not have the same performance characteristics as fusible links and should not be used to replace them unless specifically authorized by a vehicle factory service bulletin. Fusible links continue to be used in most starting circuit applications.

Can Type SXL primary wire be used to replace a fusible link?
SXL wire would work as an emergency replacement, but it would not have the required "Fusible Link" markings and the insulation would not be designed to contain flame and spark in the event of a circuit failure.

Where can I find more technical information on automotive fusible links?
Specifications relating to conductors, insulation, wire size, length, location termination, identification and testing are spelled out in SAE Specification J156, The Society of Automotive Engineers' web address is www.sae.org.
 
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Warthog

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Read the first five pages of the "Section 3 - Troubleshooting" of "Chapter 2 - Service and Troubleshooting Instructions" of the TM 9-2320-289-20 or TM 9-2320-289-34 Technical Manual.

It will tell you how to read the diagrams, what the abbreviations are, wire labels and sizes, symbols and fuse sizes.

And this is always good advice.................

"ALWAYS start with fully charged and load tested batteries.

c. When troubleshooting a malfunction:
(1) Ensure that both batteries are fully charged."

****** NOTE - The fusible link is NOT the barrel shaped cover for the connector. It is the length of special wire. The cover is just that a cover. ******

Here is a list of the CUCV Fusible Links taken from the Tech Manual TM 9-2320-289-34

Diagram - Wire Label - Location - Use - Size

Diagram E-1 - 1 RED-2Z - 24v POS Terminal Board - 24v to the Starter relay - 16ga
Diagram E-1 - 3 BLU-2B - 12v ENG WRG Harness Block - 12v feed from batteries- 12ga - supplies 12v to entire system

Diagram E-3 - 1 RED-2R - 12v ENG WRG Harness Block - 12v to ignition switch - 16ga
Diagram E-3 - 1 RED-2F - 12v ENG WRG Harness Block - 12v to headlight fuse - 16ga

Diagram E-7 - 3 BLU-2A - 24v POS Terminal Board - 24v to the Glowplug Resistor - 12ga
Diagram E-7 - .5 ORG-503B - Glow Plug Relay - 12v sense wire to GP Controller - 20ga
Diagram E-7 - .8 GRN-509(E-H) - Glowplugs - 18ga
Diagram E-7 - .8 GRN-503(E-H) - Glowplugs - 18ga

Diagram E-9 - .5 ORN-2K - 24v POS Term BD - 24v to GEN2 exciter circuit - 20ga
Diagram E-9 - 3 BLU-2L - 24v POS Term BD - 24v to GEN2 positive terminal - 12ga
Diagram E-9 - 3 BLU-2B - 12v ENG WRG Harness Block - 12v feed from batteries - 12ga - relisting
Diagram E-9 - 3 BLU-2H - 12v ENG WRG Harness Block - 12v feed to GEN2 negative terminal - 12ga - This is where the "Isolated Ground" comes into play
Diagram E-9 - 3 BLU-2M - 12v ENG WRG Harness Block - 12v feed to GEN1 Positive Terminal - 12ga
Diagram E-9 - .5 ONG-2P - 12v ENG WRG Harness Block - 12v feed to the GEN1 excitor circuit - 20 ga

Diagram E-11 - 1 RED-101A - 24v POS Terninal BD - 24v to the Diagnostic port - 16ga
Diagram E-11 - .5 ORG-106A - GEN2 - 24v to the Diagnostic port - 20ga

So to sum it up there are:

Totals

4 - 20ga orange
2 - 18ga green
4 - 16ga red
6 - 12ga blue

Locations

24v POSitive Terminal Block
1 - 20ga orange
2 - 16ga red
2 - 12ga blue

12v Terminal Block
1 - 20ga orange
2 - 16ga red
4 - 12ga blue

Glow Plug Relay
1 - 20ga orange

Glow Plugs
8 - 18ga green

GEN2
1 - 20ga orange

You never know what you will need so stock up before hand and you will be prepared for the worst.


 
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Dave Kay

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Thanks for the great link--- in fact that dealer has tons of stuff I can use to fix my hillbilly-hacked GMC cviv. That truck is the C3500 fleet-type model and has A LOT of the same electricals as M1008. Looking in the old GM official manuals wasn't always a help because lots of stuff in the electrical schematics didn't match up--- because those were all for the civiv models. Anyway, thanks again Warthog, your electrical knowledge and advice is priceless!
 

cpf240

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$30 at Radio Shack will get you a decent multimeter, also get extra fuses for it while you are there. Trying to troubleshoot electrical issues without a multimeter is like trying to use a magnifying glass in the dark :cookoo:
Or keep an eye on the Harbor Freight ads, as they have a "free with purchase" coupon for a multimeter every now and then.
 

Warthog

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How do you tell if a fusible link is bad? Obviously melted rubber is an indicator but if that is not visible what do yo look for?
You can gently tug on the link wire. If it stretches then it is burnt. Also if the outside housing is split, burnt, brittle, it is most likely burnt.

A fusible link can be partially/mostly burnt and still conduct a small amount of current. This can give you a false reading with a ohm meter. A test light is better as it will put a load on the wire. If the light is bright the link should be good. A dim light will tell you there is an issue.
 
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stranger75

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I was able to find 12 gauge fusible link at a local NAPA. It comes in a 10 ft roll and is NAPA part #785856.

I'll humbly admit that prior to this experience, I did not know what the fusible link looked like. I had read about them and understood their function, but had never burned one or had to replace one. In fact, based on the picture Warthog posted in the Fusible links 101 sticky, I thought the link was much smaller and actually concealed by the black plastic barrel. Now I understand that the fusible link is the actual length of wire connected to the 12V terminal block.


The black plastic connector is basically a cover for the factory installed "butt connector", although it is actually crimped and soldered together.

I am about to replace the links and will report when finished.
 

Warthog

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I was able to find 12 gauge fusible link at a local NAPA. It comes in a 10 ft roll and is NAPA part #785856.

I'll humbly admit that prior to this experience, I did not know what the fusible link looked like. I had read about them and understood their function, but had never burned one or had to replace one. In fact, based on the picture Warthog posted in the Fusible links 101 sticky, I thought the link was much smaller and actually concealed by the black plastic barrel. Now I understand that the fusible link is the actual length of wire connected to the 12V terminal block.


The black plastic connector is basically a cover for the factory installed "butt connector", although it is actually crimped and soldered together.

I am about to replace the links and will report when finished.
Good information. I need to add more info to this thread. Thanks for reminding me.
 

Warthog

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I guess it is time to update this thread. Time sure gets away from me.

Today's lesson is on how to replace your burnt fusible link.

I keep the majority of my electrical supplies in my old lunch box. I have had this box for over 30 years. Goes back to my oil patch days when I was just a young punk and I knew everything

IMG_1953.jpgIMG_1954.jpg

The items I will be using are: Fusible link (I bought a "kit" off of fleabay), butt connectors, knife, wire crimpers, wire strippers, shrink tubing and a heat source



IMG_1957.jpg

You can buy butt connectors that do not have the plastic housing but you have to buy them in lots of 100 and they have to be ordered. I don't like to wait so I make my own. I just go to the parts store and buy a pack of connectors and remove the plastic. YMMV

I start with my knife, crimpers and the connectors

IMG_1958.jpgIMG_1959.jpg

Next comes the cutting and removing of the plastic housing. This is only one way of doing it. You may have other ways. Be very careful with the knife

IMG_1960.jpgIMG_1961.jpgIMG_1962.jpgIMG_1968.jpg
 
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Warthog

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Next you need to prep the new fusible link wire. The wire itself has a special rubber coating and will be inprinted with the type of wire and the wire size.

The package of wire I bought has a metric conversion chart for the different sizes. Some parts house will sell just the wire or they may have a kit with the wire and connectors all setup. It all depends on the store itself. I have found that most of the premade are the smaller size (16-18ga) and I usually need a 12ga.

I like to have all the pieces before hand and just make my own.

IMG_1968.jpgIMG_1974.jpgIMG_1973A.jpgIMG_1969.jpg

Today we are making a replacement link for a 18 gauge link. This is a link I picked up from a parts truck at the local Pull-A-Part.

You can tell the size of wire you are working with as it will be embossed into the plastic housing. This one happens to say 18. Just a reminder, the plastic housing is just a cover for the connector It is NOT the fusible link.

IMG_1963.jpgIMG_1966.jpg

So you cut off the old link at the plastic housing, crimp on a butt connector, install some shrink tubing, install the fusible link wire and add a ring connector.

When you have everything in place you heat up the shrink tubing. Then you reconnect the wires.

IMG_1970.jpgIMG_1971.jpg

Here are a handful of good fusible links I picked up at the Pull-A-Part. Only cost me a buck. You never know when you might need one.

IMG_1975.jpg
 
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Recovry4x4

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If I may add to this. The stepdown butt connectors are insulated but I knocked the connector out of the insulator and used heat shrink as an added measure.
 

av8rnik

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This post saved me this weekend. Blew the fusible link going from the 24 terminal bar to the starter relay under the dash. Once i figured out it was the link, i got it replaced and good as new. THanks!
 
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