Topic: What is a "Fusible Link"?

[Oz]

Sent: 2/9/2005 7:36 AM

" These are wires that have a box shaped insulation on them.   Inside these boxes are a smaller gauge wire that is put there to melt and interrupt the circuit if there is a problem. " (rshaw500)
 
THANK YOU SO MUCH for this explanation, Rick!  I've heard this term over and over again and have been scratching my head about what a fusible link is for over 2 years.
 
Now, my questions are:
Where are these links typically found?
What are the critical ones to be aware of for fault diagnosis?
What ones are located in areas susceptible to damage by engine heat or other sources and what has anyone done to move or protect them?
Are fusible links represented by a special symbol in the wiring schematics?

[denisondc]

Sent: 2/9/2005 8:14 AM

       The one fusible link on the early Dodge motorhome chassis, is about 8 inches long, and is connected at one end to the big terminal on the starter relay, its other end is a connector.  Very handy for replacement, assuming you have found and eliminated the short circuit first.  It is a decent distance from the exhaust system on  mine - dont know about the 440 exhaust.   
    On most modern cars there can be 6 to 10 fusible links, grouped together, and feeding off of one splice from inside a molded rubber item, to the various circuits.  These are buried amongst the other wires under the hood beside the motor, between the firewall and the battery.   Typically a fusible link will be 4 American Wire Gauge numbers smaller than the circuit it is protecting.  A circuit with a 12 awg wire would be protected by a 16 awg fusible link.  The wire  insulation is made of something called hypalon, which will melt but not easily burn - this is for fire protection. 
   Fusible links are labelled as such on the wiring diagrams.   
    I carry a spare one I got on ebay, but I also carry the tools to try and learn what cause the link to blow in the first place.

[HeavyHaulTrucker]

Sent: 2/9/2005 8:37 PM

Mark, on the 77 models, there are two fusible links.  Both come off of the large terminal on the starter relay.  One supplies current to the flasher circuit, and controls the turn signals & emergency flashers.  The other is fed from the alternator output, supplying current directly to the battery when the engine is running; when the engine is being started, it completes the circuit for starting.  If it blows, the only way you can start the engine is to manually jump the starter relay; however, even though the engine will run, there is no charge supplied to the battery.  This is the one that is critical for fault diagnosis.  The other one will not affect the operation of anything but the flashers.
 
Fusible links are available at any auto parts store for $3 to $5; look in the electrical aisle.
 
As denison said, though, you must find the cause of the fusible link blowing or you are just wasting your money replacing it.
 
John

[DaveVA78Chieftain]

Please note that where there was only one or 2 fusible links on a Dodge MH chassis, the GM P-30 and John Deere (Oshkosh) chassis have several links.  P-30 has them at the starter solenoid and the junction block on the forward bulkhead (outside underhood access typically on passenger side).  John Deere has them near in the area of the radiator by the ignition module and voltage regulator.
Fusible links are simply wires that are 2 guages smaller than the circuit they are protecting.  They will blow out before the circuit wires will.  They are also covered with non-flammable insulation to prevent fires from occuring if they do open up.  However, being a smaller gauge wire they can also act as a resistance which results in heat which can weaken them with time.  These devices can cause one to scratch your head in frustration trying to figure out why the voltage is so low at the regular fuse block OR cause the alternator to overcharge because the regulator does not sense the proper battery voltage.  They are a simple prtection device just understand that heat does affect them with age.
 
Dave