Automotive/03' chevy silverado

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Expert: - 2/6/2009


Question
I am starting to have problems with the speedometer in my truck. It drops to 0 after driving for a bit, no matter how fast or slow I am going. However, when I shut off the ignition and restart the truck it will work for awhile more then continue the cycle. What could be my problem?

Answer
I have no details on your truck, so will answer generally as follows:

The assumption is that you have an electronic speedometer, given the very recent vintage of your truck.  If such is the case, then this problem would have the hallmark of being a thermally-induced intermittent connection in the speed-sensing circuit.  This may well be a circuit-board problem or perhaps even a problem with a failing semi-conductor device such as a driver transistor.  

Whatever the specific device may be, the problem is indicative of a circuit path being present at ambient temperatures but opening once the affected device or circuit board junction temperature rises due to the heat of operation.

In the case of circuit boards, the problem may be as simple as a cold solder joint, or one that has been subject to physical stress to the point where it cracked.  Cold joints were more common in the days of hand assembly, and are rare with boards that have been wave-soldered, but they are not unknown.  Stress fractures are more common, especially in vehicular applications.

Finding the problem can be "fun" owing to the intermittent nature of the problem (and to Murphy's Law!), but can be facilitated by the use of "freeze mist" to cool suspect joints and devices to see it the problem can be generated - or cleared - when the tested device is subjected to controlled changes in temperature.

What you do beyond that point would depend on your experience.  An electronics guy may resolder the device, or change out any "flaky" semiconductor device.  A board swapper might buy and install a new circuit board ... probably $$ or $$$!  

Too bad - the original idea by which electronics in vehicles was to open a new world of inexpensive repair was that a new or reconditioned board could field replace a defective board, and this defective out-take would be repaired in a central electronics shop and then put back into inventory as a requalified board.  Aside from keeping costs down (due to the real production costs of electronic circuits being quite cheap), this time-honoured practice in dealing with industrial electronics boards offered the added advantage of identifying and tracking design defects so as to revise the designs so as to eliminate them.

Unfortunately, the automotive industry at large appears not to have seen this benefit, nor has the home appliance nor A/V nor other consumer industries.  With the designed life cycles of many of those latter products falling to little more than their warranty periods, one needs only one guess to figure out why!

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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