Automotive/Blown Fusible Link

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Expert: - 6/2/2008


Question
Mr. Kenward, I have a 1993 Nissan Sentra XE. The fusible link on the positve battery cable just started catching on
fire. This happened after I installed a new alternator. What would cause this to happen? Any insight would be greatly appreciated. Thank You, Josh.

Answer
Any fusible link which goes as far as to "catch fire" would certainly suggest a high-level fault current, suggesting in turn that a high-energy low-internal-resistance source - your battery - has been connected across a similarly very low resistance load, effectively a "short" circuit ... in this case, essentially a "bolted" fault!

Inasmuch as this happened just after the installation of a new alternator, I would have to wonder if:

(1) the connector that was to go to the alternator's positive output post inadvertently went to the alternator or vehicle chassis instead or, alternatively,

(2) the rectifier diodes in the alternator had accidently been forward biased at some point, resulting in their failure.  The typical failure mode for over-stressed semi-conductors is to fail into a short-circuited condition, ie: diodes become "bi-odes".

Depending on the general condition of the electrical system, particularly the main bus circuit, there could be other avenues for failure, proximity to the body or chassis to bad or burned insulation on a main lead, for example.  From this vantage point, one cannot see whether this might be a problem or not, but can only suggest that one does a systematic check of all parts of the power circuit.  Some problems cause other problems and, given that fault-level currents have flowed on your main bus, diligence would dictate that it all should now be verified as sound before closing off the problem.

Regards ... EGK

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Ernest (Ernie) Kenward

Expertise

The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!

Experience

A key skill in my work and hobby pursuits both is STRATEGIC TROUBLESHOOTING. I am a senior instructor in Electrical Engineering Technology at a leading Canadian polytechnic, my areas being Electrical Power and Industrial Control, electrical and electronics design and manufacturing, and AutoCAD and related CAD/CAE software - plus equipment problem-solving and new equipment design and prototyping. Hobby-wise, I have 30-plus years of experience in auto restoration, mostly in electrical and mechanical systems. Ongoing projects include a 1959 Edsel Corsair, my 1978 Ford E250 class-B motorhome conversion, and the care and upkeep of my Mercedes 300CD. My vehicles become engineering test beds for electrical and mechanical upgrades as ideas present themselves. This includes the design and production of circuit boards to restore or enhance features for which no OEM replacement parts are obtainable, or where better specifications or reliability can be had via newer concepts. Regarding the E250 RV conversion, I designed and continue to revise a custom power distribution system, managed by a Programmable Controller (PLC); this has made most revisions as easy as uploading new firmware as I develop it. The "mini" PLC is a powerful device for custom automotive control systems. One good example (there are many) would be the Moeller "Easy Relay"; these offer a wealth of control, monitoring and variable-and-status display options for such projects. A good example project which has worked well is that one for my RV noted above, which has been on the job - revised in firmware only - for a decade now. It is a load management and charging control system to avoid the sulfation-induced early failure that often befalls deep-cycle batteries used in RV power applications. The battery installed in 2003 lasted long enough to more tnan pay for the PLC that contributed to its longer life ... and the PLC will be there for the next battery as well!

Organizations
IEEE - senior member ... past WCC Student Activities; SME - senior member ... past chair, greater Vancouver chapter chair 318; Edsel Owners' Club - have served in various capacities on chapter executive during seventies; have been Power and Driveline resource on the Edsel Owners' Club "E-team" for more than a decade.

Education/Credentials
Graduate of UBC

Awards and Honors
Certificates of appreciation from IEEE and SME for work in student and chapter activities

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