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Automotive/f150 gauges peg when i try to start

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Expert: - 11/10/2007


Question
2000 f150 4.6l...when i turn the key all the gauges peg to full then come back to normal..the starter will turn over half turn then the gauges peg again..battery charged full..no clicking from relay just stops turning over and gauges go peg to full..

Answer
My E250 is much older than your F150, so I have no electrical diagrams for your vehicle, one which likely relies on signals from the vehicle microcontroller to drive the gauges.  

The older Ford trucks (and cars) provide a regulated plus-five volts DC to the gauge bus, and each gauge then goes to the vehicle chassis and battery negative ("ground", if you wish) by way of resistive sensors.  The operating characteristic of most of these sensors is to reduce resistance as the measured variable increases, thus a full or full-scale-deflection (FSD) indication would be at lowest resistance.

In a system such as described above, the only scenario by which ALL the gauges would go FSD, or even hit the peg, would be if the 12-to-5-volt regulator somehow got bypassed, putting twelve volts on the gauge bus.

I will have to think on this one a bit ... the problem does intrigue me - IF, in fact, it really IS a gauge problem, and not just a transient effect of the voltage being pulled way way down by a bogged-down starting motor, and having an adverse effect on the vehicle controller.  That IS possible, because the resistance to chassis ground of a high-torque DC motor is VERY low, and is the only thing limiting motor current when it is not turning and therefore not producing back-EMF.

I will see what I can find in the next few days, but please do keep me posted as to what you might find.  In the time being, why not check out the battery and starter cables, along with their terminations, and the starter contactor itself.  It could be that something as simple as a high-resistance stranded cable joint into a clamp-type battery connector is the cause of a severe voltage drop and of the other problems.

Regards ... 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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