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Automotive/87 chevy blazer fuse issue

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Expert: - 9/26/2007


Question
my fuse that controls the horn/radio/overhead light go out almost instantly when a fresh one is inserted. is there anything i can do before i start shopping around for a mechanic?

Answer
A fuse that opens almost instantly upon restoring power to a circuit is responding to a fault-level current, and this is indicative of a "short circuit": an abnormally low shunt resistance downstream of the fuse, drawing current well in excess of the current for which the circuit was designed and wired.  In effect, the fuse is doing its job!

The short circuit most likely is one to the vehicle's chassis "ground" (I prefer to call it 0V common due to the true "earthy" meaning of ground in most electrical codes!)  and can exist in any part of that bus supplying your horn, radio or dome lights.

Your best place to start in locating the fault would be a visual inspection of the wire leaving the fuse terminal, through to whichever harness it becomes part of, and all the way to each of the powered devices: your dome light switches and light sockets, the radio, and the horn and its switch.  Look for wires made bare and in contact with the body or chassis.  Chafing caused by movement in proximity to a metal edge, or burned insulation caused be too-close-contact with exhaust inlet pipes, headers or manifolds are common causes of short circuits.

If nothing is visible by that approach, the next step to isolating the fault so as to locate it calls for a process of elimination.  Leaving the fuse out and using an ohmmeter with a low-ohms range to do a continuity check from the load-side fuse terminal to the vehicle common while turning on and off the dome lights and the radio could identify one of those two circuits or devices as being the problem if, for example, the measured resistance drops essentially to zero ohms when one is turned on while the other is not; the device NOT causing the resistance to drop would NOT be the location of the short circuit.

If the resistance is low and stays steadily low no matter what you do with the horn, dome lights or radio switches, then start looking for harness connectors for each of these, open them - one at a time - in turn.  If the resistance goes up when any one connector is opened, that connector is the portal to the shorted circuit.

For any of this, a wiring diagram would be very helpful.

If you do start shopping for help, your best choice would be an auto-electric specialist.

Happy hunting and good luck ... 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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