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Automotive/Car Horn replacement blows fuse

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Expert: - 8/26/2008


Question
I am trying to replace my original car horns on my 2000 Dodge Dakota.  I have hooked up wires to the new horns, which then work.  But when I place them in the spot and screw them in where the others were my 20 amp fuse blows. I am using the original screw that held the first horns.  I have placed electrical tape on the back of the new horns so the metal will not touch on the frame, but still the fuse blows.

Answer
The most likely possibility is that you did not pay heed to polarity or colour code when connecting the wires to the horns.  It would appear that you connected the supply wire (supplying 12VDC via the fuse and perhaps from your horn switch as well, depending on the system's design - see note below) to that terminal on one or both horns that connects to the horn case and thus to the vehicle chassis ground when the horns are mounted.

Disconnect the horns and then use a low-ohms range multimeter or ohmmeter to determine (i) if there is continuity from the terminals to the horn case and, if so, (ii) which of the terminals has the LOWEST resistance to the horn case.  Note that a low-ohms range meter will be needed because both connectors to the horn will exhibit quite low resistance, due to the horn coil being a high current coil that will itself be of very low resistance.  If a continuity path and resistance reading are found, then follow the step in the next paragraph.  If, however, both terminals offer very high (MegOhm range) or infinite resistance to the case, skip the next paragraph and proceed on to the NOTE below.

Similarly, the horn lead which is intended to provide the negative-side connection will be found to offer very low resistance to the vehicle chassis.  This will be the lead that should connect to the grounded terminal on the horn.

NOTE: Check the leads for the presence of VOLTAGE.  You may find that 12VDC is ALWAYS present on one supply lead to each horn.  On the other hand, you may find that 12VDC is present ONLY WHEN THE HORN SWITCH IS ACTIVATED.

If one lead to each horn supplies 12V AT ALL TIMES (provided the fuse is good), then your horn system is the design type in which one terminal is connected to the supply and the other goes to vehicle chassis ground by way of the horn switch; closing that switch completes the negative-side path and operates the horn.  These horns must ALWAYS have two terminals, and NEITHER should be connected to the horn case.

If one of the leads supplies 12V ONLY WHEN THE HORN SWITCH IS PRESSED (provided the fuse is good), then your horn system is the design type in which the negative side of the horn is always grounded to the chassis and the horn operates when the horn switch passes 12V to the horn positive-side terminal.  Most newer horns of this type have both negative terminals, but many older ones will have the positive terminal only, and will rely on the physical mounting of the horn to the chassis to provide the negative side connection.

The overall evidence suggests that your horn system is of the second type, and that you have connected the switched supply lead to the horn's negative terminal.

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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