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Automotive/1994 Mustang electrical question

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Expert: - 7/29/2007


Question
We have a 1994 Mustang GT convertable V8 and my 17 y/o son let the battery run down and tried to jumpstart it but got the terminals mixed up.  It runs now, but the speedometer, clock, cigarette lighters, radio, and after-market alarm system are not functioning.  Everything else is.  I cannot see that any fuses are blown (are they obvious when they are?) and the owner's manual is lost.  Can you tell me what I am likely dealing with and how much of a job this will be to repair?

Answer
Reversing polarity in this manner almost always damages any electronics not internally protected by diodes or other means, and many such devices are not equipped, partly to save pennies in production costs and also to create another sale to the unwary.  

In general, semi-conductor junctions will open much faster than will thermal fuses under fault conditions; in fact, the fastest-acting fuses (Designed to protect rectifiers) often are based on semi-conductor technology!  Nonetheless, one might occasionally be lucky.  The fact that the lighter does not work does suggest that a fuse was blown anyway; note that the lighter itself would suffer no ill effects due to reversed current flow, but the other circuits sharing that fuse would have caused that fuse to open, one hopes due to the action of a reverse-polarity-shunting  diode in one of the electronic devices.  If a protection diode was to blame, it is likely that replacing an open fuse will bring most or all of the devices on this circuit back to life.  If not, well ... !  Anyway, do check your fuse panel.

A "blown" fuse can be recognised by an open conductor in or on the fuse, depending on its design, discolouration due to burning at the point of opening and, if glass enclosed, a "bloom" created by condensing metal vapour on the inside of the glass envelope.  If the fuse is of such an enclosed design that its state is not visibly obvious, an ohmmeter or 12V test light can be used to check for continuity.  

Replace any blown fuses with new ones of the correct rating identified on the the holder, or in the replacement manual you should seek; this information might also be provided to you from any helpful automotive electrical shop.

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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