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Automotive/1988 Chevy Silverado Won't Start

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Expert: - 8/23/2007


Question
I recently replaced the battery in my truck, but failed to notice that the previous owners had reversed the battery and cable positions during a rebuild.  Luckily I had installed a disconnect switch so it was only connected in reverse a short time.  Now it turns over, the radio, turn signals, and headlights work, but it will not start.  Where should I start looking for the problem?

Answer
Yikes!  I hate it when predecessors leave a legacy of "undocumented features", especially when they go against an expected standard!  You have my sympathy.

It takes only milliseconds to fry a diode or other semiconductor junctions, especially given that the available fault current from an automotive battery can be upward from 1000 A (at its terminals), due to the battery's internal resistance being so low.  The time your disconnect switch was closed would be a virtual eternity compared with this!

If you are lucky, the vital electronics of your ignition system or ECU would have been protected by a shunting reverse-polarised diode across the supply input, in series with a reasonably fast-acting fuse in series between the input and the shunting diode.  If that was the case, then the reversed diode would have immediately started conducting upon the application of the reversed voltage, effectively shorting the supply, thus keeping the level of that voltage from rising significantly.  The diode cannot be expected to sustain this shunt action for long however, as the high short-circuit current drawn would cause its temperature to rise very high and very quickly.  That is where the fuse comes in; if all works according to the design plan, the fuse will open before or just as the diode fails, in either event shutting off the reversed voltage before the protected electronics is damaged.

(Some may ask: Why not protect the electronics by instead  connecting the diode in SERIES with the supply, polarised to allow forward "correct" current to pass, while blocking any reversed "incorrect" current from getting through should the supply be reversed?  The answer is that, generally, diodes will drop some voltage, and this will increase with increasing forward current.  In low voltage systems such as automotive systems, this drop will waste energy and cause regulation problems.)

Again, I hope that your engine electronics have some sort of reverse-polarity protection such as that described above.  Fixing it involves replacing the fuse and likely the diode as well.  In most cases, this will be a board-level repair, and that may pose a problem, since most automotive shops will simply replace boards or entire modules.

What you need to do is find a friendly automotive technician, technologist or engineer ... or, if you have the inclination and some small-scale solder skills, you may want to dig into this yourself.  If you find that diode and fuse combo, the repair can be quite easy if you are a careful worker.

Hope this goes in your favour ... EGK

PS Afterthought:  Almost forgot ... once you do get your trusk started, check the voltage across your battery terminals and turn on various electrical loads.

With luck, the voltage will rise to and maintain 14.2 volts or so, dropping to about 13.8 volts after an initial few minutes of battery equalisation.

If you are not so lucky, the voltage will be lower, and mush lower with increasing load; it may not even rise at all above the stand-alone battery voltage of 12.5 - 12.8 volts.  Low voltage or no charging voltage at all would indicate blown alternator diodes.  That, unfortunately, is a very likely outcome of battery reversal.

The fix involves either replacing the alternator, or buying a rebuild kit for it; kits generally include a new rectifier stack, bearings and brushes.  Both alternatives are fairly cheap.

Cheers ... 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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