Battery - Alternator Fusible Link

When things go wrong.

On other Pages
Fusible links are time dependent fuses.  Very high current from a short will burn a fusible link quickly.  Moderately high current will only melt the link when it gets hot enough.  The length of time depends on the current and size of the link.

Fusible Link Purpose
Fusible Link In Action
Complete Charging System Wiring Diagrams

Understanding the Charging System Ammeter

'67 Barracuda Main Page
'85 Grand Wagoneer Main Page
Charging & Light System Wiring Diagrams
Headlights & Fog lights

Tips on Wiring (CJ Forum)

Credits: This page expands on explanations by "NachoRT74" on others moparts and ifsja forums

Purpose of the Fusible Link.

If the battery shorts, all of the battery power will be trying to get to ground. 
Fusible link (or links on some models) will protect the rest of the wiring from damage if there is a short before any fuses or circuit breakers.
Unlike fuses and circuit breakers, fusible can handle normal high current loads, such as running ignition and headlights and other equipment for short periods of time. 
When subject to high current for over time periods, the link should overheat and fail before the attached wires and connectors do.
If there is a dead short, the battery current is high enough to melt the fusible link quickly.

Fusible link


  Red arrows represent current.  Heavier arrows represent more current. 

Damaged Insulation on a Wire Close to Battery

Bare Conductor touches chassis.  Battery current goes to ground at contact.
Ammeter shows nothing; Or, if alternator is running, may briefly show maximum charging.

Link Melted.
Evidence of overheating likely at connectors.

Fusible Link worked

Example 2: Internal Short in Alternator

Shown here the short is in the output circuit of the alternator.
Ammeter will show maximum discharge!

Alternator Short
Fusible link should have protected against major damage.  Expect evidence of overheating at all connections.  Any places that were loose or oxididized will be worse.

Fusible Link burned


Fusible Link and Continuous Equipment Current

Problems are likely when the charging circuit is used to carry high current continously.  Such use may or may not cause the fusible link to melt first.  If there is more resistance at other points, they will fail first.   This can happen on a stock system by continuously or repeatedly charging a low battery.  More info on this here.

Another cause of continuous high loads on the charging circuit is the addition of circuits to the battery or starter solenoid.
This creates a long path from the main power source, the alternator.  It also goes through many connections.  The charging circuit was not designed to carry this much power.  Thats why the fusible link can be 14 or 16 gage.   Over time the heat of this additional current makes each connection worse.  The fusible link may not fail first.
The bulkhead connection on the alternator side will be carrying the largest load.   However, all the connections will be at risk.
Furthermore, as voltage drops, the equipment will want more current, which in turn creates more heat.

Example of Power flow with new circuits added at starter solenoid. Circuit Amperage
In this example, 15 amp could be from:
  Lights on relays,  9 amps
  Electric Fans, 6 amp minimum

 20 amps assumes 5 amps for charging.
 25 amps from alternator includes:
     2.5 amps for field circuit
     2.5 amps for ignition coil
Some modern amperage loads at
Electric Winches are special consideration.
  They can draw 200 to 400 amps in use.
  An alternator can not supply that much current. It
will have to come from the battery.  Then the alternator
will have to recharge the battery.


This Website:
Understanding Charging Circuits with Ammeter
'67 Barracuda Main Page
'85 Grand Wagoneer Main Page

    Advice and information provided for discussion and to be helpful.  Neither myself or Nacho can be liable for your decisions or workmanship. You need to make your own decisions, and know about or discover the risks before barging ahead on any endevour.

    The information and pictures on this page are free to use and share by all.  You may not sell anything that I or Nacho has written or illustrated.

Nov 2017